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Batteries – present and future challenges
Annika Ahlberg Tidblad1, Helena Berg2, Kristina Edström3, Patrik Johansson4, and Aleksandar Matic4
1. Scania CV AB – Materials Technology, Hybrid and Electronics, SE 151 87 Södertälje, Sweden
2. AB Libergreen 3. Department of Chemistry – Ångström Laboratory, Uppsala University,
Box 538, SE 751 21 Uppsala, Sweden 4. Department of Applied Physics, Chalmers University of Technology, SE-
412 96 Göteborg, Sweden
Swedish Hybrid Vehicle Centre 10-2015
Batteries – present and future challenges
In order to find long-term battery solutions for electric vehicles, both the
present and future challenges have to be reviewed. In two complementary
reports, this project sheds light on both aspects by identifying the gaps
between the battery packs and vehicle requirements, and reviewing research
trends of emerging battery technologies in the 2025 perspective.
The first part of the project relates to the current regulations for batteries and
the on-going discussion for the development of future regulations and how this
will influence the present available battery cells and the vehicle requirements.
The gap analysis is based on legislation, scientific publications and vehicle
requirements, both heavy-duty vehicle and passenger car requirements. On-
going research trends are identified to analyse if the gaps can be closed in the
near future. The title for the part is: “White spots on the future battery map
induced ty the development of vehicle regulation”.
The main identified issues are the on-going discussions about the risks with
electrolyte leakage and whether the organic solvents used in the battery cells
could be harmful for battery users in any way. Toxicology shows typical
solvents such as EC, DMC and EMC to be harmful even at low amounts. Beside
the discussion about toxicity, the worry about thermal propagation is a main
concern where relevant test methodology is lacking. The outburst of fire and
explosions are as serious issues as the worry for toxicological effects. The
report suggests future research directions towards solid state batteries or at
least the use of gel type electrolytes to diminish the risk for electrolyte leakage.
The second part of the project; “Emerging Battery Technologies towards 2025”,
reviews research trends to identify how these technologies may fit to different
vehicles and vehicle requirements in terms of performance, weight, volume,
and cost. The main question answered is whether there are any potential post-
Li technologies to replace the Li-ion technology in electric vehicle applications
by 2025. Overall, this study indicates that until 2025, any huge improvements
in the performance of automotive batteries are highly unlikely as there are no
game-changing technologies approaching the consumer market today.
In more detail, higher energy density and higher power capable electrode
materials promise to significantly lower the battery cost by reducing the
amount of material and the number of cells needed for the entire battery pack.
In order to utilise the very attractive energy densities of some of the emerging
technologies, however, very low C-rates must be used. Again, from a pack
perspective, more cells in parallel are then needed to fulfill the performance
requirements. Work is needed to develop new materials and also electrode
couples that offer a significant improvement in energy and power over today’s
technologies.
The cell voltage will also play an important role for the cost: cells having 2 V
lower nominal voltage will result in a battery pack 75% more expensive.
Therefore, the complete battery pack must be evaluated when comparing
emerging battery technologies. As a consequence, cells of lower cell voltage
must be significantly less expensive to produce in order to be competitive at
pack level.
The main research and development needed is related to the next generation
Li-ion batteries operating at high voltage levels (5V). Moreover, cells having
anodes of Si or metallic lithium will be the most attractive solutions for electric
vehicles by 2025 and therefore research should be strengthened for these
concepts. Also, efforts must include the development of novel electrolyte
formulations and additives to form a stable solid electrolyte interphase or even
more efficient solid state electrolytes for improved abuse tolerance, longer life,
low temperature operation, low toxicity and fast charge capability. For power
demanding applications, the most attractive solution by 2025 will be
asymmetrical super capacitors.
Pack-level innovations should focus on technology to reduce the weight and
the cost of thermal management systems, structural and safety components,
and system electronics. Currently, these “non-active” components of a battery
increase the volume, weight, and cost of the finished product. Approaches to
reduce the sizes of these inactive components in the cell and battery should be
pursued. The cost reduction potential is highest for the pack components; a
potential of ca. 75% resulting in a total cost reduction of the battery pack by
about 55% by 2020.
Swedish Electric & Hybrid Vehicle Centre Chalmers University of Technology Hörsalsvägen 11, level 5 SE-412 96 Göteborg Phone: +46 (0) 31 772 10 00 www.hybridfordonscentrum.se
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White spots on the future battery map
induced by the development of vehicle
regulation
Annika Ahlberg Tidblad1 and Kristina Edström2 1. Scania CV AB – Materials Technology, Hybrid and Electronics, SE 151 87
Södertälje, Sweden
2. Department of Chemistry – Ångström Laboratory, Uppsala University, Box 538, SE 751 21 Uppsala, Sweden
Swedish Hybrid Vehicle Centre 10-2015
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Content
1. Introduction
2. Regulatory Landscape
2.1 Battery Directive 2.2 Reach 2.3 UN Recommendation on the Transport of Dangerous Goods –
Model Regulations 2.4 UNECE Vehicle regulations
2.4.1 UNECE R100_02 2.4.2 EVS-GTR 2.4.3 Proposals for future GTR
3. Li-ion batteries (LiBs)
4. Regulatory concerns that will impact the LiB market and future technology developments
4.1 Introduction 4.1.1 Metals in Li-ion batteries 4.1.2 Organic Solvents in Li-ion batteries 4.1.3 Concerns with hazardous chemical substances 4.1.4 Leakage 4.1.5 Venting 4.1.6 Environmental protection – recycling
4.2 Concern with uncontrolled propagation of single cell failure 4.3 Concern with fire hazard 4.4 Concern with performance and durability
5. Research directions as a consequence of the regulations
6. Concluding remarks
Acknowledgements
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Abbreviations BMS Battery Management System C-rate The rate of battery cycling EUROBAT Association of European Automotive and Industrial Battery Manufacturers EU European Union EV Electric Vehicle EVS Electric Vehicle Safety GTR Global Technical Regulation (UNECE 1998 Vehicle Agreement) HEV Hybrid Electic Vehicle LiFePO4 Lithium iron phosphate LiB Lithium-ion battery OEM Original Equipment Manufacturer PAC Protective Action Criteria for chemicals PHEV Plug-in Hybrid Electric Vehicle R Regulation (UNECE 1958 Vehicle Agreement) REACH Registration, Evaluation, Authorization and Restriction of Chemicals REESS Rechargeable Electric Energy Storage System SEI Solid Electrolyte Interphase SIB Sodium Ion Battery UN United Nations UNECE United Nations Economic Commission for Europe WP.29 World Forum for Harmonization of Vehicle Regulation xEV Generic term for electric vehicles including all levels of electric drive
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1. Introduction This is one of two studies of the future needs for batteries with the goal to exemplify what research efforts are needed to reach by 2030. While this report focuses on current legislation as well the ongoing discussions forming the legislation for the future and how this will influence the use of batteries in different kinds of applications, the other report by Helena Berg, Aleksandar Matic and Patrik Johansson elaborates on different possible chemistries and their potential of becoming the leading batteries 2025 in vehicle applications. By separating the study into these two complementary reports we hope to cover the most important future research directions in a 15 year perspective. The total volume of all kinds of batteries that entered the European Union per year are, approximately 800.000 tons of automotive batteries, 190.000 tons of industrial batteries, and 160.000 tons of consumer batteries [1]. These data are from 2009 and all prognostics point at increased levels of battery use. In Figure 1 the global battery market for 2009 is depicted. This study will first give an overview of the regulatory landscape; including existing and near-future legislation applicable for automotive batteries and cells, followed by an overview of the lithium ion battery (LiB) technology, as this is predicted to be the predominant battery technology for electric vehicle (EV) application in a foreseeable future. Finally we discuss foreseeable regulatory impact and the key areas of concern regarding LiB technology from a regulatory standpoint on LiB, focusing on automotive application, and draw up a map of future research directions that can address the existing technology gaps identified by the regulatory landscape.
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Figure 1. Global battery market 2009 [2]
2. Regulatory landscape 2.1 Battery Directive EU legislation is formulated in the Batteries Directive 2006/66/EC [3]. This Directive intends to “contribute to the protection, preservation and improvement of the quality of the environment by minimizing the negative impact of batteries and accumulators and waste batteries and accumulators. It also ensures the smooth functioning of the internal market by harmonizing requirements with regards the placing of batteries and accumulators on the market. With some exceptions, it applies to all batteries and accumulators, independent of their chemical nature, size or design”. In practice this means that the “Directive regulates the marketing of batteries containing some hazardous substances, defines measures to establish schemes aiming at high level of collection and recycling, and fixes targets for collection and recycling activities.” This includes instructions on labeling of batteries and how batteries have to be removed from equipment at disposal. Presently The Battery Directive restricts content of mercury (<5 ppm) and cadmium (<20 ppm) in portable batteries. There are exemptions from these limits for specific applications but these are reviewed on a regular basis to determine if the exemption should be revoked due to technology advances on the market. Marking requirements apply to batteries containing higher concentrations of mercury and cadmium than the specified limits as well as batteries containing <40 ppm lead. The collection and recycling targets are progressive to stimulate the market to continually develop more efficient processes.
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The Battery Directive also aims to improve the environmental performance of all operators involved in the life cycle of batteries and accumulators, e.g. producers, distributors and end-users and, in particular, those operators directly involved in the treatment and recycling of waste batteries and accumulators. Producers of batteries and accumulators and producers of other products incorporating a battery or accumulator are given responsibility for the waste management of batteries and accumulators that they place on the market. This description of legislation comes from EUROBAT, an industry organization promoting the interests of the European industrial, automotive and special battery manufacturers within EU [4]. The mail goal for the EU Battery Directive is to minimize the negative impact of batteries on the environment and improve their overall environmental performance. Technical development towards improving the environmental performance of batteries is encouraged. However, in recognition of the chemical nature of batteries and ingoing substances, all batteries should be collected. The Directive also ensures the Member states to take care of storage and recycling of spent batteries in an appropriate way which is fit for the purpose. The progress of the areas covered by the Directive has to be reported regularly. In this Directive automotive batteries are defined as SLI (starting, lighting, ignition) whereas hybrid and electric vehicle traction batteries are treated as industrial batteries. The Battery Directive acts as a framework law, foreseeing further legislation in the field of batteries.[3] The fast development of new battery technologies, e.g. the vast family of rechargeable lithium batteries that currently find their ways into more and more applications, give at hand that a more thorough review and deliberation of what substances may need regulation is expected in a foreseeable future. The lithium based chemistries do not contain materials where mercury, cadmium or lead are predominant, nor common contaminants. However, there are multiple other substances contained in these batteries with the potential to cause adverse effects on the environment. There are also secondary legislation on batteries formulated after 2006. Here is a list of what they contain: Regulations about the calculation of recycling efficiencies of the recycling process of waste batteries and accumulators (Commission Regulation (EU) No 493/2012 of 11 June 2012) [5]; This regulation states that the target for collection rates should be at least 45% by September 26, 2016. The targets are
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defined in terms of average weight. The target is 50% for all other batteries than lead-acid (target 65%) and nickel-cadmium (75%). . Rules regarding the capacity labeling of portable secondary (rechargeable) and automotive batteries and accumulators (Commission Regulation (EU) No 1103/2010); [6] A questionnaire for the Member States reports on the implementation of the Directive 2006/66/EC (Commission Decision 2009/851/EC); [7] Requirements for registration of producers of batteries and accumulators in accordance to the Directive 2006/66/EC of European Parliament and of the Council (Commission Decision 2009/603/EC); [8] There are, however, not only legislation for environmental and safety concerns but also regulations about the market of batteries. Some examples are given here: A common methodology for the calculation of the annual sales of portable batteries and accumulators to end-users (Commission Decision 2008/763/EC); [9] How to place batteries and accumulators on the market (Directive 2008/103/EC [10] of the European Parliament and of the Council of 19 November 2008 amending Directive 2006/66/EC on batteries and accumulators); [3] Implementing power conferred on the Commission (Directive 2008/12/EC of the European Parliament and of the Council of 11 March 2008 [11] amending Directive 2006/66/EC on batteries and accumulators and waste batteries and accumulators); [3] In addition to adopting the above legislation, the Battery Directive sets the following deadlines for implementation:
26/09/2009: article 12 requirement for producers or third parties to set up schemes for treating and recycling waste batteries and for all collected batteries to undergo treatment
o 26/11/2011: Recycling of batteries must meet the recycling efficiencies listed in Annex III Part B
26/09/2009: Capacity marking rules must be followed (article 21) 26/09/2010: Review of nickel cadmium exemption for cordless power
tools. Due to the advancement of, for example rechargeable LiB technologies, the exemption for use of nickel cadmium batteries in
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cordless power tools will be revoked for products placed on the market after 2015.
26/03/2010: Calculate recycling efficiency 5th full calendar year after entry into force: 1st calculation of collection
rates by Member States 2012: Collection Rate of 25% of portable batteries 2016: Collection Rate of 45% of portable batteries
o Member States must report to commission every year (before end June) on progress of collection
26/09/2012: First national implementation report (article 22), final due date 26 June 2013
2015: Commission must review the implementation and impact of the Directive (article 23)
2.2 REACH [12,13] REACH stands for Registration, Evaluation, Authorization and Restriction of Chemicals. It is a European Union regulation dated from 18 December 2006. REACH regulates what kinds of substances that can be used in products on the European markets. The regulation restricts the use of some substances that that are currently used as active materials in industrial and automotive batteries and accumulators, but the list also includes substances that may be included in future battery technology products. The substances are collected in Article 58 § 1 (e) and Article 58 § 2 of Regulation (EC) No 1907/2006 (REACH). 2.3 UN Recommendation on the Transport of Dangerous Goods – Model Regulations All the regulatory activities described above are focused on environmental
protection and waste stream control comprising all different kinds of batteries
and accumulators, regardless of application. However, with the growing market
of primary and secondary lithium batteries, there is an increasing concern from
the industry and the authorities aimed at safety related issues. Transportation
and handling of lithium batteries during transportation is one aspect that
surfaced due to repeated incidents involving consignments of lithium batteries
during transportation. International regulation for all modes of transport, land,
sea and air, are based on the UN Recommendations on the Transport of
Dangerous Goods – Model Regulations (ST/SG/AC.10/1/Rev.18) [14]. The
Model Regulations are translated into transportation legislation comprising
detailed instructions for packing, marking, documentation and testing based on
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mode of transport: ADR/RID for land transport (road and rail, respectively),
IMDG Code for sea transport and IATA/ICAO for air transport. Except for the
IATA/ICAO, which is revised on a yearly basis, the other regulations are revised
every other year. The packaging instructions are based on how the cell or
battery is transported, which is indicated by the UN identity code:
UN3090 Lithium primary cells and batteries (incl. Li metal and Li alloy)
UN3091 Lithium primary cells and batteries in or with equipment
UN3480 Lithium ion cells and batteries (incl. Li-ion polymer)
UN3481 Lithium ion cells and batteries in or with equipment
UN3171 Battery propelled vehicle
There are different packing instructions for cells/batteries depending if they are
new, damaged/defective or waste for disposal/recycling.
All primary and secondary lithium cells and batteries must be tested and
approved against UN 38.3 (ST/SG/AC.10/11/Rev.5) [15]. Testing is performed
hierarchically, so that cells are tested first, and, thereafter, any cell assembly
subjected to transportation must also be tested and found compliant. The tests
are representative of conditions that can occur during normal transport and
handling:
T1: Altitude simulation
T2: Thermal test
T3: Vibration
T4: Shock
T5: External short circuit
T6: Impact/crush
T7: Overcharge
T8: Forced discharge
All legislation mentioned so far, is generally applicable and not specific for
vehicle application.
2.4 UNECE Vehicle regulations
As with transport regulation, which is global, international vehicle regulation is
also handled under the UNECE umbrella [16]. There are two different
categories of international vehicle regulation: the “R” regulations that are
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developed under the 1958 Vehicle Regulation Agreement and the “GTR”
regulations that are developed under the 1998 Vehicle Regulation Agreement.
The geographic applicability of the “R” and the “GTR” regulations, respectively,
depends on the contracting parties, i.e. countries, that have signed the
respective agreements. The two regulatory systems are related in the sense
that they affect each other. When there is both an “R” and a “GTR” regulation
on the same technology area, they have to harmonize and not imply
contradictory requirements. Additionally, regulations should as far as possible
be technology agnostic and not impose undue design restrictions. The benefit
of having global regulations is of course that it replaces national regulations in
the countries of the contracting parties, thus removing market barriers for
OEMs by introducing the same technical requirements on multiple markets. “R”
and “GTR” regulations are developed through collaboration between
authorities and market stakeholders, e.g. vehicle manufacturers, suppliers to
the vehicle industry and Testing Services.
The introduction of electrified vehicles, xEV (HEV, PHEV, BEV), using LiB on the
market has raised concern about safety performance of the rechargeable
electric energy storage system (REESS). The reason is the high energy density of
the LiB, both in terms of gravimetric and volumetric energy capacity, and the
history of LiB related incidents that have occurred in other product segments,
notably consumer products (laptops and cell phones) as well as severe
incidents with LiB during transportation, resulting in significant material
damage and/or personal injury.
2.4.1 UNECE R100_02
REESS safety performance is regulated by UNECE R100_02 (1958 Vehicle
Agreement) [17]. This regulation is applicable within EU, Japan, Korea and a
diversity of countries in Asia and Africa and contains requirements for electrical
safety of xEV, independent of the battery technology used in the REESS. It is
very challenging to develop requirements that are applicable across battery
chemistries and technology solutions. R100_02 [18] replaces R100_01 [19] and
is currently voluntary for vehicle type approval, but becomes mandatory for
vehicles seeking type approval from July 2016. The scope of the regulation is
safety of the high voltage traction battery system in use, that means during
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normal operation. The tests are designed to evaluate typical environmental
conditions and potential hazardous scenarios that can occur during normal use
of the vehicle:
Vibration
Thermal shock and cycling
Mechanical impact
Mechanical integrity
Short term ground fire exposure
External short circuit
Overcharge protection
Overdischarge protection
Overtemperature protection
R100_02 applies to both passenger vehicles and heavy duty vehicles, but
includes some exemptions from specific requirements for heavy duty vehicles
due to technical differences as well as fundamental differences in the overall
regulatory practice for passenger vehicles vs. heavy vehicles.
2.4.2 EVS-GTR
An EVS-GTR for improved vehicle safety is currently in progress with USA,
China, EU, Japan and Korea as co-sponsors for the regulation, Figure 2. A final
draft is to be submitted to UN ECE for vote at the end of 2015, however it is
expected to be delayed by at least 1 year. The scope of the EVS-GTR has been
expanded compared to R100_02 in that the GTR also considers charging and
post-crash scenarios. Initially, the EVS-GTR was intended for passenger vehicles
only, but applicability to heavy duty vehicles is being considered. Since there
are some fundamental differences in technology solutions between heavy duty
vehicles and passenger vehicles, such as charging strategies and crash
management, inclusion of heavy duty vehicles in a passenger vehicle regulation
is not straight forward and requires a number of difficult technical
deliberations.
Test proposals considered for the EVS-GTR include the in-use tests from
R100_02, sometimes with modifications and additional requirements, plus a
number of additional tests, for example:
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Water protection test
Long term external fire exposure
Protection against unintentionsal exposure of humans to high voltage
post-crash
Protection against uncontrolled thermal propagation
Additionally, the EVS-GTR is considering a number of new acceptance criteria
related to emissions of hazardous chemical substances and thermal events.
Figure 2 The makings of and Electrical Vehicle Safety Global Technical
Regulations (EVS-GTR). The GTR merges the national regulations of the co-
sponsors with existing UN ECE regulations and new requirements for electric
vehicle safety.
2.4.3 Proposals for future GTR
During 2015, UNECE WP.29 has initiated prestudies to investigate the need for
and feasibility of developing new GTRs for xEV on the following topics,
suggested by different countries:
Battery Performance and Durability (USA and Canada)
xEV battery recycling
Determining power of EVs (Germany and Korea)
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Method of stating energy consumption of xEVs (China)
If the prestudies support development of GTR regulation on any of the above
topic, these activities are projected to will in 2016.
3. Li-ion Batteries (LiBs)
To better understand why all the above described regulations and discussions about new legal measures are so concerned with the cell chemistries we will give a short description of what a LiB is, its cell contents and principally how the battery works. The LiB is a whole family of different possible materials that can be selected according to where the battery will be used [20, 21, 22, 23]. There is also a complementary group of lithium-based batteries considered as future possibilities – known as “beyond lithium” – such as the lithium-sulfur and lithium-oxygen batteries [24, 25, 26, 27, 28, 29]. They are not going to be considered here since they are too far away from the market. However, if they would enter the market it will be the same regulations that handle their safety, transportation and market aspects as for LiBs. Another more realistic (from a market perspective) beyond lithium chemistry is the sodium ion battery (SiB) [30, 31, 32]. A SiB is based on the same principle of intercalation/insertion electrodes as LiBs but with the mobile ion being the larger sodium ion making the choices of electrode and electrolyte materials vast but not as large as for the smaller lithium ion in LiBs. In general a battery converts chemically stored energy to electrical. In a LiB the stored energy can be calculated in how much lithium that can be hosted in the atomic structures of cathode and anode materials. This is described as the lithium ions being intercalated or inserted into the electrodes. It is the cathode which is the bottle neck for how much total energy (how much lithium that can be inserted) that a LiB can deliver. There is therefore a constant hunt for better cathode materials. A commercial LiB consists most often of a graphite negative electrode, a positive electrode of a transition metal oxide or a phosphate and an electrolyte composed of an organic solvent containing a lithium salt (or a mixture of lithium salts) and additives to enhance stability and safety. The positive and negative electrodes are not short circuited due to the electrolyte which is soaked into a porous polymer separator between the electrodes, which allows the transport of ionic charge carriers but prevents electrical contact. An
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example of a typical LiB is shown in Figure 3. When the battery is made it is in a discharged state. The battery must start by being charged which means that the lithium ions must be transported through the electrolyte and intercalated into the graphite electrode. The battery is now ready to be used. If the cathode material is an oxide the potential of the battery is larger than if the cathode is lithium iron phosphate. During the first charge there is also a reaction with the (at these potentials close to that of lithium metal) electrolyte taking place on the graphite surface. A so called SEI (Solid Electrolyte Interphase) is formed. This SEI is about 20 nm thick and protects the graphite from being destroyed during the battery cycling. The role of this SEI for battery safety and the battery lifetime is extensively discussed [33, 34, 35, 36, 37].
Figure 3. Schematic diagram of a LiB composed of intercalation electrodes. Graphite is to the left and an insertion cathode to the right.
Most LiBs operate in a temperature window of -15°C to + 60C. An interesting exception is the Bolloré Blue car found on the streets of Paris where the Li-battery has a metallic lithium negative electrode, a polymer electrolyte,
LiFePO4 as the positive electrode and operates at +80 C. One of the limitations is the low ionic and electric conductivity in the battery components. This will limit how fast a battery can be charged. Due to its
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operational principles, LiBs are hence not as fast in building up charge as, for example, supercapacitors. There are several text books on LiBs that give details of the chemistry taking place at cell level. One recent with focus on automotives has been authored by Helena Berg [38].
Figure 4. Energy vs. Power for different battery and capacitor systems [39]. The Ragone plot in Figure 4 shows that the energy density of a LiB is still not high enough to meet the needs for EVs while both power and energy density already meet the requirements for Plug-in Hybrid Electric Vehicles (PHEV).
4. Regulatory concerns that will impact the LiB market and future technology developments
4.1 Introduction In this report we only discuss substances that are included in LiBs and in batteries beyond LiBs included in Article 58 § 1 (e) and Article 58 § 2 of REACH. All existing battery technologies contain some unwanted substances, from environmental and toxic considerations. These substances may be the substances making up the active materials in the electrochemical cell, in which case their presence is purposeful, and therefore it can be difficult to phase out these substances without affecting the energy storage properties or electrical
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performance of the battery. In other cases, the hazardous substances are introduced into the cell as contaminants to the main constituents. In this case, raw material sourcing and production process parameters are the main determinants for the quantity of these substances in the cell. Removing trace contaminants of heavy metal, for example, may not have any detrimental effect on the performance of the battery, but the technical challenges or costs involved may be prohibiting. Traditional, mercury, cadmium and lead have been considered as the most problematic heavy metals in batteries, which is why these substances are regulated by the Battery Directive [3]. Technology advances have made it possible to almost completely eliminate the amounts of mercury, cadmium and lead in alkaline primary and rechargeable batteries. However, these metals are not the primary environmental concern in LiB. These batteries contain other types of electrode materials as well as organic solvent electrolytes with highly reactive salts, PF6 is currently the prevalent conductive salt in LiB on the market. Examples of heavy metals that occur in LiB include nickel, cobalt, copper, chromium, vanadium and a vast amount of different species at very low concentration, for example thallium and iridium. With the introduction of large scale LiBs on the market, in xEVs and in various types of large scale stationary applications, there has been an increasing focus on the potential hazards of the organic solvents and salts that can be found in the electrolyte. How toxic are they? What happens if a battery cell or battery pack starts to leak electrolyte or if the battery gets pressurized and starts venting electrolyte fumes into the surrounding? What substances can be expected to be present and what is the range of vapor pressures expected in commercially available cells? How will the substances spread from the source and what is a realistic exposure hazard for occupants inside the vehicle as well as for people residing outside of the vehicle? If the electrolyte fumes ignite and start to burn, what chemical species are expected in the fire smoke and what factors influence formation of HF, CO, polyaromatic carbons and other toxic substances under these conditions? Since the exact electrolyte composition is a well-guarded secret by the cell manufacturers, how can generic safety regulations be developed that can handle the wide variety of LiBs presently on the market as well as those expected within a 10-15 year time-frame?
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4.1.1 Metals in Li-ion batteries Historically, the only metals of concern for batteries have been mercury, cadmium and lead. With the introduction of LiB, this map is changing. The metals and metallic contaminants relevant for LiB are different, but not necessarily less problematic from an environmental point of view. Additionally, the wide variety of possible cell chemistries of LiB compared to lead-acid and rechargeable alkaline battery technologies imply that efficient recycling processes are more difficult to develop. The first task that needs to be done is to determine which, if any, metals that need to be regulated. The next step is to determine which, if any substances that needs to be restricted, what would constitute acceptable levels as well as universally suitable methods to detect and quantify the presence of the restricted metals.
4.1.2 Organic solvents in Li-ion batteries Electrolyte solvent chemistry in LiB is a complex area of study. Whereas there are a limited number of common organic solvents making up the bulk of the electrolyte, typically carbonates such as ethylene carbonate and dimethyl carbonate, there are multiple additional constituents present in the electrolyte in order to achieve the required performance of the particular cell chemistry and design. These additives can be film forming to prevent corrosion reactions between the electrolyte and electrode surface. They can be molecules trapping the anion from decomposing during the cycling or they can be flame retardents for improved safety or redox shuttles to prevent over-charging or over-discharging of the battery. Some examples of efficient additives are described in ref. 40, 41 and 42. There are also examples of solid electrolytes such as polymer electrolytes [43, 44] and lithium-conducting ceramics [45, 46]. These are yet less commonly used due to their lower ionic conductivities and in some case the compounds are brittle. In this respect it is also important to mention ionic liquids that might be an alternative due to their high thermal stabilities but still cost is an issue and another drawback is a lower ionic conductivity [47].
4.1.3 Concern with hazardous chemical substances From a regulatory perspective, concern is focused on potential toxic effects of electrolyte constituents and reaction products leaking or venting out of the battery into the environment. The toxicology assessment of substances
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reported to be used as electrolyte solvents and additives is still at an early stage and the tendency from the authorities is to be extremely cautious about allowing scenarios where the vehicle occupants and general public may be exposed. Present day LiB electrolytes are complex mixtures of solvents, conductive salt and various additives. The recipes are well-guarded secrets and it is nearly impossible to gain detailed knowledge of all constituents and their concentrations in the solution. The high flammability and propensity to generate fire upon release at elevated temperatures is another reason for concern. The decomposition reactions of the electrolyte is oxygen forming, which further increases fire hazards and make any fires challenging to extinguish, especially if the cathode material is an oxide which will add fuel to the fires. In order to overcome the concerns, electrolyte research is essential. There are two possible routes to satisfy the immediate concerns: solid electrolytes that contain no liquid components capable of escaping from the cell and liquid electrolytes made from substances that are well documented as non-toxic, non-volatile and not flammable. Additive research aimed at solving existing performance and durability issues would further complicate the electrolyte chemistry and there is often very little information about what additives are present or the amounts added. Hence, although new additives may benefit the cell and battery performance, this line of research runs a risk of not meeting the concerns expressed by regulating authorities about potential health effects to vehicle users, first and second responders or the general public surrounding a vehicle where there is a battery incident involving release of electrolyte from the cells. In the regulatory context, electrolyte release is defined by two separate processes: leakage and venting, depending on the origin and nature of release.
4.1.4 Leakage Leakage is generally considered to be electrolyte release in liquid form, for example as a result of tab weld failure or if there is a crack or rupture on the battery cell casing or in the cell venting devise through which electrolyte can seep out. The organic solvents currently used in LiB electrolytes are volatile and flammable, the potential occurrence of leakage is causing concern. One of the risks considered is that of igniting the escaped electrolyte and thereby releasing enough heat energy by combustion to initiate a thermal event that may propagate into a more serious incident. The second hazard is that of exposure of the vehicle occupants to toxic substances. Currently, there are differing opinions about the actual toxic effect caused by both acute and long term
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exposure to electrolytes and electrolyte fumes, and there is a risk of assuming an extremely precautionary approach to when making toxicological assessments in order to avoid adverse effects on the vehicle occupants as well as road rescue workers and first responders. However, overestimating toxicity resulting in very restrictive requirements on automotive battery electrolytes may jeopardize the future of LiB in xEV applications and it should be noted that there are no known reported field incidents of harmful situations involving electrolyte leakage or venting affecting vehicle occupants. The realistic toxic hazard situation depends on the volume and concentration of the species released as well as the actual exposure parameters including the duration of exposure and the effective uptake of the chemical species by the human body. The likelihood of exposure is another parameter that needs to be considered as well as a fair comparison with the risks posed by competing propulsion technologies. LiB are generally considered leakage free as they belong to a sealed battery technology, designed to have no exchange of materials with the environment during normal operating conditions. Leakage will, under this assumption only happen as a consequence of a severe cell or battery failure, and it is therefore considered very unlikely to happen. Additionally, electrolyte is a costly cell component and therefore the amount of electrolyte fill into the cells is precisely dimensioned to provide sufficient wetting of the electroactive materials. Hence, aside from the initial charge-discharge cycles, there is no free electrolyte present inside the cell. The reason that small amounts of excess electrolyte can be observed when opening fresh cells is that electrolyte absorption into the materials is a stepwise process which also involves formation and stabilization of the SEI layers on the electrodes. The possible initial overfill condition is intended to prevent drying out inside the cell leading to premature ageing; irreversible capacity and power loss. The amounts of overfill is proportional to the cell size, but is limited to the milliliter range for current automotive cells on the market, and partly reflects the overall engineering quality of the manufacturer. Premium brand cells typically require less overfill than more basic LiB cells. Regulatory efforts targeting controlling any effects of electrolyte leakage may become unproportionally stringent and limiting for the technology. The risk assessment needs to be balanced in terms of predictions of realistic likelihood of leakage and a realistic approach to assess the potential toxicological effects on humans. Advances in electrolyte and cell research that reduces the quantity of electrolyte needed in order to wet the active materials and ensure long
21
operational lifetimes is one possible approach to addressing the concerns. Another approach is to look for other types of functional electrolyte systems that can replace the organic solutions, e.g. solid electrolytes or other types of conductive liquids that are compatible with the electroactive materials on the electrodes and stable at the required cell voltages.
4.1.5 Venting Venting is an event associated with electrolyte leakage, but which also implies that additional chemical processes are occurring inside the cell causing build up of gaseous constituents from electrolyte and electrode decomposition reactions. If this happens to the extent that there is significant gaseous evolution, LiB are designed to vent in a controlled manner to prevent further more severe situations from occurring, e.g. explosion. When the cell vents, several species are released including but not limited to, CO2, CO, H2, O2, hydrocarbons and HF. Figure 5 shows typical gas constituents formed during excessive heating and combustion of common LiB electrolytes. The gas also contains a fine aerosol of electrolyte solution. The relative amounts of the various constituents depend on the conditions in the cell prior to and at the time of venting. If the fumes should catch fire, it is nearly impossible to extinguish due to the formation of oxygen as a reaction product, enabling combustion even when external oxygen sources are eliminated.
The venting fumes are very hot and constitute a risk of attaining burns in addition to potential toxic effects. The general strategy for managing this risk is to ensure that the vehicle design does not allow venting fumes to enter into the passenger compartments of the car. There have been proposals to enforce strict control of venting gas, such as on-board gas monitoring and a very restrictive view on venting as an acceptable failure mode for cells. However, this may drive cell manufacturers to construct cells that vent at higher temperatures and pressures, thus leading to increased risk of severe incidents from other failure paths.
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Figure 5 Gaseous species formed on extreme heating and combustion of a
typical Li ion battery electrolyte mixture [48].
4.1.6 Environmental protection – recycling
The research trend towards inexpensive and common materials in LiB to bring down the total costs of the energy storage system in automotives creates an added challenge in itself. A primary driver for recycling is that there is a secondary market willing to buy the recycled material. If the economic incentive is not naturally in place in the market, then the entire cost of recycling needs to be financed by other sources.
4.2 Concern with uncontrolled propagation of single cell failure Following the reports of spectacular LiB failures where a thermal event has cascaded and propagated from a single cell event into a multi-cell or even a whole battery thermal runaway, there is concern of the consequences if this was to happen to a xEV battery. For one, the automotive LiBs comprise more cells and have higher voltage and power specifications than other LiBs that the general public comes into contact with. This is regarded as a potential risk of causing human and material injury. Second, the automotive industry typically mitigate the risk by robust system engineering and design including strategies for handling potential risk situations. However, from a regulatory standpoint it
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is desirable to impose a test with acceptance criteria, and this is not an easy task, considering the variety of design solutions developed by different manufacturers, who are reluctant to share detailed information about proprietary system safety and performance features. This is a source of controversy between the industry and the regulating bodies, as it is challenging to develop a test procedure that does not require manipulation or modification of the REESS in such a way that the safety design features that the manufacturers have put in place to mitigate risks are not compromised. Although propagation is a possibility, especially in the event of an internal short circuit inside a cell in the pack, the likelihood for occurrence has been deemed low by the automotive industry. The BMS structure of the automotive REESS limits normal operations well within the defined voltage, current and temperature limits of the cell chemistry. The position is that sufficient safety is ensured provided that the system can contain any incident and prevent it from spreading beyond the LiB. Traditional ways of assessing internal short circuits on cell level have proven to be far too aggressive to be representative of a field event in a xEV pack. Various thermal propagation initiating methods have been considered without success in finding a viable solution to this problem. These include:
1. Nail penetration 2. Nail prick (incomplete penetration) 3. Blunt nail crushing 4. Inserting an external heater into the pack 5. Overcharge 6. Inserting defined “contaminating” Ni particle into a cell 7. Local chemical explosive 8. Extended external fire exposure
The first 6 methods are known from academic research and from cell and battery safety standards development, but they are not suitable for regulatory safety assessment. Standards are often used to assess general battery behavior and investigate the limits of a technology. The acceptance criteria for the test are defined by the parties performing the test based on the purpose of the investigation. Regulatory acceptance criteria must be fulfilled in order for a vehicle to be approved for the market. If the assessment method accelerates the risk scenario too much, then there is a danger of excluding viable vehicle designs from the market based on flawed test methodology. Hence, the equivalency to the envisioned real life event must be ensured. None of the
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methods listed above meet this requirement. In all cases, there is an acceleration of the triggering event beyond that of what is expected in the case of a spontaneous internal short circuit field failure triggered in a cell. For example, in several cases, it is difficult to limit the trigger to a single cell. Electrical heaters and external heat sources apply heat for too long to be representative of an internal single-cell thermal failure. A majority of the methods also involve manipulating the REESS physically and/or disabling necessary safety prevention features intentionally designed into the battery in order to mitigate thermal events. This is unacceptable as it is likely to affect the outcome of the test and poses questions about the equivalency of the test result to real life conditions. A preferable LiB development solution would, of course, be realizing a cell design and chemistry that is immune to propagating thermal events. However, research towards more reliable, balanced and realistic test methods is also necessary.
4.3 Concern with fire hazard As mentioned earlier, the currently used electrolytes in LiB are highly flammable and imply a risk of fire. The ignition temperatures are fairly low
(~450-470 C). During venting, electrolyte ejection at high pressure may result in flames extending far from the battery or vehicle. Fires are documented to be difficult to extinguish due to the self-sustaining ability to provide oxygen for combustion by cell internal reactions involving decomposition of the electrolyte.
4.4 Concern with pPerformance and dDurability The most recent regulatory propositions include methods for setting requirements on performance and durability of the battery system. The basis for this line of regulatory activities is environmental protection. The large scale LiB systems used in vehicles would impact greatly on the quantity of waste batteries in case the realized life expectancy does not reach the expected life time of the vehicle, which would mean introducing vast quantities of heavy metals and other substances into existing battery waste streams. The other part of the regulation proposal is to look into methods of determining the efficiency of the electrical power system including:
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1. Method of stating energy consumption 2. Determining power of EVs
The challenge for the automotive industry is the dependence of performance and durability on usage patterns and uncontrolled external parameters as well as the design of the battery system. Performance in this sense has traditionally been viewed as a concern between the automotive makers and their clients, handled by warranties and other market drivers. However, the introduction of new vehicle technology seems to stimulate rule making authorities to investigate new approaches to apply the mandate of regulation. One possible explanation may be the global concern in the market about the actual life time of LiB REESS in vehicles. The technical specifications indicate that the original REESS should last the entire life time of the vehicle, but if this life time expectancy is possible to reach with existing battery technology is still debated and has yet to be proven from normal operational use. Regulation can be considered as a means of transferring the “risks” of not meeting targeted performance from the users to the manufacturers and forcing the industry to prioritize the development of efficient and durable battery solutions for future vehicle applications.
5. Research directions addressing regulatory concerns There are several aspects ranging from LiB cell content to a battery pack of LiB cells that have to be taken into account when describing future research directions. We base our suggestions mainly on chapter 4 and the environmental and hazardous discussions which form the regulatory considerations today and what it seems in the future. We see several important topics to consider:
1) Research towards more reliable, balanced and realistic test methods Test methods at battery pack level are performed and developed by companies. However, a better scientific description how different ways of testing batteries will impact the life time and safety of a battery is still lacking. Methods for testing the safety of cells not only when they are freshly produced need to be developed. Most battery cell failures take place after some hundred cycles rather at their pristine state. Additionally, methods to assess safety on a system level are required since single cell test results are not representative of the REESS system or vehicle safety performance. This should not be surprising, since the battery pack and REESS system contain several layers of safety
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functionality not available on cell level. There are also indication that size itself plays a part in the test outcome, for example, fire testing of single cells and cell assemblies comprising different amounts of cells [49] indicate that heat release rates of single cells are much higher than for multi-cell test objects. This seems to be due to shielding effects as well as the greater mass of the multi-cell units. The system level tests have to be non-intrusive so as to eliminate the risk of flawed test results due to manipulated test objects and not real safety failures of the system.
2) Research towards more stable SEI on high capacity anodes
The main environmental issues regarding anode materials used today, which is mainly a carbon (graphite) material and a cellulose or polymer-based binder, is the energy needed for highly purifying the graphite and the large amount of water necessary for electrode fabrication. The safety issues are related to the SEI formed during cycling of a LiB. This SEI is formed due to thermodynamically instable organic solvents but it protects the graphite from unwanted side reactions at the same time as it consumes lithium during its formation. When heating a LiB cell the first reaction will be that the SEI will react by emitting some heat. It is not a large amount but could be large enough for heating the electrolyte and then the cathode material. Normally there are protections within the cell that can take care of the heat generated. It can be a separator that melts and shuts down the cell or other ways of handling this issue. On a REESS level, there is active or passive cooling in place, regulated by the BMS, that kicks in and lowers the temperature even before critical temperatures are reached. The BMS will also limit the current flow through the battery pack to control further thermal evolution or, in the event of a severe temperature increase, disconnect and disable the battery. There are several research directions regarding the negative electrode. One is to incorporate silicon in the graphite to increase the amount of storage. Another important direction is to stabilize the SEI and increase its temperature stability. This can be done by effective thin coatings or by the use of solid electrolytes.
3) Research towards more stable high capacity cathodes One reason for using lithium iron phosphate instead of transition metal oxide cathodes is the thermal stability. The trend is, however, to increase the volumetric capacity of the total LiB cell and then a high capacity cathode is a
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necessary route to take. This implicitly means that some oxide must be used. During a thermal runaway the oxygen in the oxide will add extra fuel to the fire. If a high capacity cathode will be used, the research direction for the full cell development should be to make the whole cell more thermally stable by improved cell design and novel electrolytes. A solid electrolyte with high thermal non-leaking properties will be beneficial also for a highly reactive electrode material.
4) Electrolytes As discussed earlier in this report the research on additives such as film formers, flame retardants and redox shuttles will not be enough to stabilize a liquid organic solvent electrolyte. The future research trends will in this respect be a turn back to solid electrolytes such as polymers or even ceramic inorganic ionically conducting membranes. If the battery cells could be allowed to operate at slightly higher temperatures than room temperature then the drawbacks of low conductivities could be hampered. New deposition techniques for inorganic membranes are also supporting the development of thinner ceramic membranes and plasticizers that can make them less brittle. We forsee that this probably will be the most important research directions the coming years to hinder a negative development for LiBs in terms of regulatory constraints due to fear of leakage and venting of battery cells. Inherently non-toxic and non-flammable liquid electrolytes would also meet the requirements of the market. However, this would most likely mean needing to move away from the present organic solvents and conductive salt solutions. Research directions involving ionic liquids are ongoing, and may turn out to be a viable electrolyte choice from a safety standpoint. However, as with solid electrolytes, ionic liquids typically require higher operating temperatures in order to achieve reasonable conductivity.
6. Concluding remarks This report has had the aim to discuss the current regulatory developments that influence the future market and use of LiBs for automotives. Based on the ongoing discussion about the environmental and health issues concerning liquid electrolytes used today we see that important research directions are to make larger efforts in finding thermally stable solid non-brittle electrolytes with high ionic conductivities for LIBs to be undisputable as energy storage solutions for the automotives of the future.
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Swedish Electric & Hybrid Vehicle Centre Chalmers University of Technology Hörsalsvägen 11, level 5 SE-412 96 Göteborg Phone: +46 (0) 31 772 10 00 www.hybridfordonscentrum.se
Emerging battery technologies towards 2025
Helena Berg, AB Libergreen Aleksandar Matic, Chalmers Patrik Johansson, Chalmers
Göteborg, May 2015.
2
Executive Summary Higher energy and higher power capable electrode materials promise to significantly lower the battery cost by reducing the amount of material and the number of cells needed for the entire battery pack. The cell voltage will also play an important role for the cost: cells having 2 V lower nominal voltage will result in a battery pack 75% more expensive. Therefore, the complete battery pack must be evaluated when comparing emerging battery technologies. As a consequence, cells of lower cell voltage must be significantly less expensive to produce in order to be competitive at pack level. In order to utilise the very attractive energy densities of some of the emerging technologies very low C-‐rates must be used. Again, from a pack perspective, more cells in parallel are then needed to fulfil the performance requirements. Work is needed to develop new materials and also electrode couples that offer a significant improvement in energy and power over today’s technologies. The main question to be answered by this study is whether there are any potential post-‐Li technologies to replace the Li-‐ion technology in electric vehicle applications by 2025. The main research and development needed is related to the next generation Li-‐ion batteries operating at high voltage levels (5V). Moreover, cells having anodes of Si or metallic lithium will be the most attractive solutions for electric vehicles by 2025 and therefore research should be strengthen for these concepts. Also, efforts must include the development of novel electrolyte formulations and additives to form a stable solid electrolyte interphase for improved abuse tolerance, longer life, low temperature operation, and fast charge capability. For power demanding applications, the most attractive solution by 2025 will be asymmetrical super capacitors. Beyond 2025 the preferred technologies are Na-‐ion, Li-‐S, and Mg, mainly due to cost reduction potentials of the cells (Na-‐ion and Li-‐S) and two-‐electron redox reactions (Mg). Research efforts should be increased to find the most optimal solutions for scaling up cells for vehicle applications. Pack-‐level innovations should focus on technology to reduce the weight and the cost of thermal management systems, structural and safety components, and system electronics. Currently, these “non-‐active” components of a battery increase the volume, weight, and cost of the finished product. Approaches to reduce the sizes of these inactive components in the cell and battery should be pursued. The cost reduction potential is highest for the pack components; a potential of ca. 75% resulting in a total cost reduction of the battery pack by about 55% by 2020. Overall, this study indicates that until 2025, any huge improvements in the performance of automotive batteries are highly unlikely as there are no game-‐changing technologies approaching the consumer market today.
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1 Introduction 4 1.1 Battery requirements – high level picture 5 1.2 Boundary conditions of the study 7
2 Emerging battery technologies – Research trends 8 2.1 Next generation Li-ion 8 2.2 Solid state Li-metal 10 2.3 Na-ion 15 2.4 Mg 20 2.5 Li-S 22 2.6 Li-oxygen 28 2.7 Organic concepts 33 2.8 Asymmetrical super capacitors 34
3 Emerging battery technologies – Vehicle implications 38 3.2 Voltage 39 3.3 Energy and Power density from cell to pack/system 40 3.4 Cost trends 42 3.5 Pro’s and con’s 44 3.6 Conclusions and proposed actions 45
4 References 47
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1 Introduction In order to obtain electric vehicles with longer all-‐electric driving range, the search for batteries with higher energy density is one of the key issues. Hybrid electric vehicles rely on batteries having high power capabilities. Energy and power are two properties, which are not possible to optimally combine in one single cell and therefore the cell selection is a critical step in the development of electrified vehicles. It is, however, not only energy and power that is important for long lasting batteries with high performance over the entire life of the vehicle; durability, safety, and cost are other factors of interest. Today, most vehicle manufacturers are using Li-‐ion batteries, and a lot of HEVs are also produced using NiMH batteries. New improved electrochemical active materials will enhance battery performance. The cells will be further optimised along with improved production processes; from raw materials to complete cells. Moreover, the understanding of ageing mechanisms to prolong the life and/or use more of the energy will most likely be the main issue for the vehicle and battery pack manufacturers to enhance both calendar and cycle life. The production of active materials, cells, and modules/packs will be further improved to reduce the cost, and to increase the robustness, capacity and safety. There are some general routes to improve the performance, life, and cost of battery cells and packs; summarised in Table 1. Table 1. Improvement routes of battery cells and packs. Cell level Pack level Energy High-‐voltage/high-‐capacity
materials (electrodes and electrolytes)
Low-‐weight balance of plant components, Control strategies
Power Electrode design (e.g. 3D design), Utilisation of high-‐rate electrode materials
Cell-‐to-‐cell connections, Control strategies, Thermal management
Life Understanding of degradation mechanisms
Thermal management, Control strategies
Safety Electrolyte (salts, solvents and additives), Separators, Electrode coatings
Thermal management, Control strategies, Housing, Electronics, Vehicle integration
Cost Standardised cell formats, Use of low-‐cost raw materials and production processes.
Modularisation, Standardised electrical components, Selection of optimal cell for specific vehicles
Emerging battery technologies with new functional materials and/or concepts can also be the route for enhanced battery performance, especially as the (theoretical) energy density of many emerging technologies is very attractive. To be an attractive technology for electric vehicles these emerging technologies will have to show improved
5
performance on the battery pack level and equal or better cost in combination with long life. As an example, the Department of Energy (DoE), US, has set a high-‐level road map for batteries for electric vehicles pointing out examples of next generation Li-‐ion concepts and some emerging post-‐Li technologies. Their focus is on volumetric improvements:
-‐ half of today’s volume by utilising graphite/high-‐voltage cathodes (theoretical energy 560 Wh/kg and 1700 Wh/L)
-‐ a third of today’s volume by Si-‐based anodes/high-‐voltage cathodes (theoretical energy 880 Wh/kg and 3700 Wh/L)
-‐ a tenth of today’s volume by Li-‐metal anodes/high-‐voltage cathodes (theoretical energy 990 Wh/kg and 3000 Wh/L) and by Li-‐S or Li-‐O (theoretical energy 3000 Wh/kg and >3000 Wh/L)
Clearly there are limitations with this simple view; the theoretical values are for cells and not the corresponding battery packs, and moreover, especially important from a vehicle perspective, the power capabilities are not included. Nevertheless, the figures provide some concrete basis for how (large) improvements can be made. The aim of the present study is to summarise the status of emerging battery technologies and their route towards 2025 for vehicle implementation. Based on a few basic important parameters for electric vehicles – energy, power, and cost – the trends of the emerging technologies are reviewed and summarised. Advantages and challenges for the different technologies are compiled and recommendations and proposed actions are given. As there are mainly research and laboratory-‐scale cells available today, the review is directed towards cell materials, while merely the foreseeable implications for battery packs are given. Indeed, an attractive cell performance might be less attractive when looking at the final vehicle installation. The main question to be answered: Are there any potential post-‐Li technologies to replace, or complement, the Li-‐ion technology in electric vehicle applications by 2025?
1.1 Battery requirements – high level picture Vehicle targets in terms of all-‐electric driving range and fuel saving potentials exist for almost all regions and vehicle manufacturers. The targets are often of short-‐term as well as long-‐term character. In this study the 2025 perspective is of interest, but includes technologies beyond 2025. Targets and goals set by authorities, industry, and universities on vehicle level, battery pack level, and cell level are reviewed indirectly. In the present study the following performance parameters are used for the comparison of the different emerging battery technologies: energy and power, voltage levels and profiles.
1.1.1 Energy and Power Energy and power densities are the commonly used characteristics employed for the comparison. Figure 1 summarises some publicly available battery pack targets from Japan (METI), the US, and EU.
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Figure 1. Energy and power targets for a battery pack for various needs from Japan, the US, and EU. Furthermore, depending on type of electric vehicle, the power to energy ratio varies widely. Figure 2 indicates the ratios for different types of electric passenger cars [1]. As can be seen the cells suitable for these applications are either power or energy optimised. For comparison, three buses are added; BYD EV with a 320 km range, Volvo PHEV with a 10 km all-‐electric range, and Volvo full hybrid. The trend for the buses is in line with the trend for passenger cars, but most likely it is not the same cells that are the most suitable for both the passenger cars and the buses.
Figure 2. Power-‐to-‐energy ratios for various types of electric passenger cars [1]. For comparison three buses have been added as references (stars).
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The power capability of the cells is one of the most important factors during the cell selection process. Energy optimised cells are associated with low C-‐rates and power optimised cells are not suitable for energy demanding usage. The electrode and cell design highly affect the power capabilities. To understand the full potential of emerging battery technologies in terms of power capabilities is difficult since most of the technologies are still at the research stage, i.e. no automotive optimised cells are available. Indications are, however, available as to whether or not the emerging technologies will be suitable for use at high C-‐rates.
1.1.2 Voltage levels and profiles For full HEVs and towards further degrees of electrification the cell voltage level is not critical for the total battery pack voltage, but will affect the number of cells needed. In case of lower voltage levels, i.e. 14/24/48 V, however, the cell selection can be critical for the overall battery pack performance. The battery voltage will put constraints on other parts of the electrical drive system in electric vehicles. Voltage hysteresis, i.e. the difference between the voltage profiles during charge and discharge of the battery, will most likely affect the power electronics. Moreover, voltage hysteresis can be a source of severe losses, especially during brake energy recuperation.
1.2 Boundary conditions of the study To cover all ongoing activities within the field of rechargeable batteries is impossible within the limited scope of this project. Therefore, some boundaries have been set. Only electrical rechargeable battery technologies of interest in electrified vehicles (from micro-‐HEV to full EV) and concepts being discussed in the vehicle context are described and discussed. Cells and cell materials are the main objectives. From cell data implications on battery pack are obtained and trends summarised. The possible improvements of the Li-‐ion battery technology are also treated, but not at the same detail level as for the emerging battery technologies. For most cases, no commercial automotive cells are available at the market and therefore the battery pack performance will only be indicative. Moreover, the cost of the emerging battery technologies are rough estimates since the production of various materials will be the dominant cost driver for new technologies for high-‐volume production of Li-‐ion cells, the material cost can be in the range of 50-‐60% [1]. Furthermore, the life cycle and recycling perspectives are not included.
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2 Emerging battery technologies – Research trends In the following sections the emerging battery technologies are described and evaluated for 2025. The implication of the basics of the different technologies for vehicle installation is treated in Chapter 3. While many of the emerging technologies are studied extensively at an academic level, only limited vehicle relevant data is available, which is the focus of this report. First, however, the improvement routes for the next generation Li-‐ion technology is summarised to provide an appropriate perspective/base-‐line.
2.1 Next generation Li-ion The Li-‐ion battery technology will be further improved by advancing the performance of materials, designs, and processes aiming at the performance and the cost of Li-‐ion batteries. Specific areas of improvements include high voltage cathodes, high-‐energy anodes (e.g. anodes based on Si or Sn), high voltage and non-‐flammable electrolytes, novel processing technologies, high energy and low cost electrode designs, etc.
2.1.1 Research trends Cathodes: The capacity of an active electrode material can be increased by: i) increasing the average electrode potential, ii) increasing the number of electrons involved in the redox reactions, and iii) decreasing the molecular weight per mole electrons exchanged. For the next generation Li-‐ion batteries mainly the first route is in focus, even if the two latter are being investigated. The research and development on advanced cathodes is primarily focused on the Li-‐Mn rich oxide materials of general formula xLi2MnO3⋅(1-‐x)LiMO2 (M=Ni, Mn, Co), the 5V spinel materials (e.g. LiMn1.5Ni0.5O4), and Ni-‐rich NMC materials charged to higher voltages. The Li-‐Mn rich materials have the potential to give cells of rather high energy density, about 300 Wh/kg [2]. To charge cells utilising the ‘traditional’ NMC to higher voltage levels, for example to 4.6 V instead of 4.2 V, would improve the energy density by about 20%. The durability of such a voltage increase has, however, to be secured. The electrolyte stability and the structural disordering occurring during cycling of the cathode material are issues to be understood. The use of high-‐voltage spinel materials is mainly limited by the instability of the electrolyte at these voltage levels. Surface coatings, electrolyte purity, additives to create a more stable SEI, and additive/binder free alternatives are routes forward. Other routes are doping of the NMC materials and to increase the stability of inactive components (like current collector, binder, and conductive additive) at high voltages. The issues of Li-‐Mn rich materials are primarily voltage fade, high impedance especially at low state of charge, metal dissolution, and low electrode density. By increasing the voltage level both increased and improved capacity will be achieved. Approaches to enabling higher voltage operation include varying the material composition within the particles (for example the outer material being more stable against the electrolyte), coatings, metal substitutions, and electrolyte additives that form a protective coating on the cathode particles.
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Other examples of materials strategies being pursued include high voltage phosphates, such as Li manganese phosphate (LiMnPO4) and Li cobalt phosphates (LiCoPO4). The LiMnPO4 material has a potential of about 0.5 V higher vs. Li/Li+ than LFP, which results in a specific energy density increase of about 15 %. An even higher potential is achieved by LiCoPO4: 4.8 V vs. Li/Li+, resulting in about 40% higher energy density. The rate capabilities of these two materials are poor, however, and LiMnPO4 is unstable when charged (due to the Jahn-‐Teller effect of the MnIII dominance in the structure). Moreover, the ionic and electronic conductivities, and thereby the power performance, of both LiMnPO4 and LiCoPO4 are even considerably lower than those of LFP, making the composite electrodes much less energy dense. Cathode materials based on silicate chemistry are also promising: Li2MSiO4, M=Fe, Mn, Co or a mixture thereof. The Li2MSiO4 materials are of interest due to their (in theory) ability to electrochemically extract two lithium ions, i.e. double the capacity. Li2FeSiO4 has a theoretical capacity of about 330 mAh/g. These materials generally exhibit high temperature stability due to the strongly bound oxygen atoms in the SiO4 polyanion, but the overall performance is highly dependent on the production process. It is possible to extract more lithium from the structure and thereby increase the capacity by means of substitution by other transition metals. The most obvious choice is Mn by virtue of its MnII and MnIV oxidation states. A substitution of about 20% results in theoretical capacities exceeding 200 mAh/g. However, to date researchers have not succeeded in reversibly extracting the second Li from these materials and high voltage operation will also be a challenge due to the lack of stable electrolytes. The distinct changes in voltage during cell operation would be highly favourable in the development of robust control strategies. Another group of potential cathode materials are the fluorophosphates (LiMPO4F) and the fluorosulphates (LiMSO4F), both often with potentials higher than 4.5 V vs. Li/Li+. Transition metals of 3d character are favoured: M=V, Co, Fe, Ti, Mn, etc. Even if the molecular weight of the framework units are higher than for example NMC, thus resulting in a reduced energy density, the polyanionic materials exhibit very stable framework structures and a wide range of substitution possibilities. The high electrode potentials and fast lithium diffusion, especially in the case of M=V, are the main advantages, while a general shortcoming is low electric conductivities. An example of an electrochemically active material that can be used either as positive or negative electrode is LiVPO4F. This material exhibits two plateaus: at 1.8 V and 4.2 V vs. Li/Li+, resulting in a cell voltage of 2.4 V. The reversible capacity at both plateaus are rather similar and in the range of 150 mAh/g. Anodes: The main “next generation” anode technologies to be pursued will be alloy based, predominantly silicon and tin based anodes. Silicon based alloys are one of the most interesting anodes concepts in terms of high capacity. The challenge is the large volume expansion during the alloying reactions with lithium. Research to improve Si-‐based anodes includes: Cu foam current collectors to enable better utilization of Si nano-‐particles, Si nano-‐wires directly deposited on current collectors, a variety of nano-‐structured and nano-‐porous Si materials, and a new group of electrically conducting binders for use in Si anodes. All these routes have the potential to achieve materials with more than 1000 mAh/g, based on half-‐cell experiments. The challenge is to tackle the
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SEI stability and higher loadings that will make the electrode structures more relevant to commercial batteries. Other alloy-‐based anodes are also under the development. One example is a Si/graphene composite material developed by Argonne National Lab [3]. Independent tests have shown from full-‐cell measurements (with advanced cathode and electrolyte materials) 525 Wh/kg and specific anode capacity of 1250 mAh/g [4]. On a more exploratory front, some research into conversion reaction materials (e.g. CoO, Fe2O3, and CuF) is performed. These materials provide high capacity (often more than 600 mAh/g) [5]. However, the issues with these materials include: poor kinetics, poor capacity retention on cycling (often due to metal agglomeration), large irreversible capacity loss, and large voltage hysteresis. Electrolytes: Current electrolytes, typically 1M LiPF6 in 1:1 EC/DMC, provide good performance and stability within limited voltage and temperature ranges. However, the solvents are highly flammable and typically have a high vapour pressure, which causes them to gas at elevated temperatures, building up pressure within cells over time. Also, the LiPF6 salt is known to react almost instantly with water, producing HF, which in turn attacks nearly all elements of the cell. This reaction, along with the instability of LiPF6 above ~80°C, leads in part to the challenges in Li-‐ion cells’ high temperature capability. Work on new electrolytes and additives is focused on one or more of the possible improvement areas of high voltage stability, high temperature stability, low temperature operation, abuse tolerance, lower cost, and possibly longer life through SEI stabilisation. Research areas include: flame retardant liquid electrolytes, single ion conductor electrolytes, new salts providing better high temperature stability (for example LiTFSI), and electrolytes that enable much lower temperature operation. However, one of the main challenges is to find electrolytes with improved high voltage stability; an issue for example handled by use of additives incl. ionic liquids. Separators: Current focus is on developing separators that provide enhanced abuse tolerance, better high voltage stability, and improved low temperature operation. Some of the technologies being developed include a ceramic impregnated separator that shows much improved low temperature performance and greatly increased high temperature melt integrity. The latter may be important to the avoidance of shorts during high temperature excursions that can occur when traditional separators shrink. Another is developing a separator and process to permit direct deposition onto anode and/or cathode sheets.
2.2 Solid state Li-metal The development of long-‐life-‐cycling lithium batteries was initially based on metallic lithium anodes together with suitable solid electrolytes. Metallic lithium has a very high electronegativity while possessing the lowest density amongst all metals, leading to its high specific capacity (3861 mAh/g) and has thus been considered to be the best candidate for rechargeable Li-‐battery anodes. The key for this battery technology is to find an electrolyte stable towards the metallic lithium anode and to inhibit Li-‐dendrite formation during cycling.
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Compared with liquid-‐electrolyte lithium batteries, the bottleneck of solid-‐state batteries is their poor performance under high power, resulting from the low ionic conductivity of the solid electrolyte, the electrode/electrolyte interfacial compatibility, and limited kinetics of the electrodes. Reasonable rate capabilities can, however, be achieved by utilising a thinner electrolyte, good interfaces, and fast kinetics of the electrodes. There are two general classes of materials used as electrolytes in all-‐solid state batteries: inorganic ceramics/glasses and organic polymers. The most obvious difference between these materials is their mechanical properties. The high elastic modulus of ceramics and glasses makes them more suitable for rigid battery designs, and the low elastic modulus of polymers is useful for flexible batteries. Polymers are also generally easier to process than ceramics, especially at lower temperatures, which reduces the production costs. On the other hand, ceramics are more suitable for high temperatures, high voltages, and aggressive environments. The anode and the electrolyte are the main issues for this type of battery technology. Therefore, the research trends for metallic lithium anodes and solid electrolytes will be given. The BlueCar by Bolloré is equipped with Li-‐metal polymer batteries. More than 2000 cars are running in the centre of Paris in the car-‐sharing programme Autolib’. The battery packs have an energy density of 100 Wh/kg or 100 Wh/L [6] and the cars have a driving range of 250 km on one charge with a C-‐rate of C/4.
2.2.1 Research trends Anode: The main issue using metallic lithium as anode is the dendrite formation on the lithium anode during repeated charge/discharge cycles, which can cause internal short circuiting and, thus, a severe safety concern. Low Coulombic efficiency is another issue facing the lithium electrode. It has been proposed that the continuous growth of the SEI on the lithium electrode surface under uneven current distributions and formation of irreversible “dead lithium” are responsible for the dendrite formation [7]. Stabilising the surface of the metallic lithium anode has been proposed both by mechanical and chemical means. To find a stable Li-‐anode is not only of interest for solid-‐state Li batteries, but also for the Li-‐S and Li-‐O2 battery technologies (see 2.5 and 2.6). The chemical stability of lithium on a metal substrate is also a major issue for the self-‐discharge behaviour. The chemical stability of deposited lithium has been studied by understanding the corrosion process [8] and mechanisms have been proposed [9]. To protect the metallic Li surface is another route forward. One example is to introduce an interfacial layer of hollow carbon nano-‐spheres. Stable cycling results using a current density of 1 mA/cm2 has been achieved with a capacity of 1 mAh/cm2 and a Coulombic efficiency of ca. 99% for more than 150 cycles [10]. Electrolyte additives may also play an important role in the stability. The effect of mechanical surface modification on the performance of lithium has been investigated by utilising micro-‐needle surface treatment techniques [11]. The mechanically modified surface was shown to improve the rate capability by 20% at a
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rate of 7C. Moreover, the cycling stability was increased by 200% showing 85% of the initial discharge capacity after 150 cycles, compared to untreated bare Li metal showing 85% of the initial discharge capacity after only 70 cycles. This technique has been suggested to suppress Li formation of high surface area Li during the Li deposition process, as preferred sites for controlled Li plating are generated. Stabilised Li-‐metal powder is another example, which has been evaluated mainly in Li-‐ion cells [12], where results indicate that the powder could be used as an independent source of lithium. Electrolyte: There are several types of possible solid-‐state electrolytes for all-‐solid state Li-‐batteries. In the following the ceramic and polymeric electrolytes are reviewed. A common issue for these electrolytes are the low ion conductivity at room temperature and therefore the need of elevated operational temperatures. Ceramic Many ceramic electrolytes have a voltage window beyond 5 V, and thus do not decompose under anodic current, such as Li10GeP2S12, [13] Li3PS4, [14] Li4SnS4, [15] Li7La3Zr2O12, [16] and amorphous lithium phosphorus oxynitride (LiPON) [17]. Furthermore, with a solid electrolyte, the concern of transition metal dissolution from the electrodes into the electrolyte is minimal. Compared with liquid carbonate-‐based electrolytes, most solid electrolytes are intrinsically non-‐flammable. Moreover, lithium metal is compatible with many solid electrolytes and is less likely to form dendrites during cycling because of the mechanical robustness of the solid electrolyte [18]. One widely studied type of solid electrolytes is the NASICON structured compounds: AxMM’(XO4)3 already identified in the late 60’s [19] (A= alkaline metal, Na originally, M=transition metal(s)). The structure is a three-‐dimensional network of interconnected conduction channels and two types of interstitial positions where conducting cations are distributed. The number of alkali cations (x) per structural formula AxMM’(XO4)3 can be adjusted depending on the oxidation states of the transition metals and the element X and the large interstitial space can accommodate up to 5 alkali cations per formula unit [20,21]. The structural and electrical properties of NASICON type compounds vary with the composition. By substitution of trivalent cations (e.g. Al, Cr, Ga, Fe, Sc, La) for Ti4+ in the octahedral sites, the Li ion conductivity in can be improved [22,23]. Adding B2O3 to LiTi2(PO4)3 ceramics has been investigated and the ionic conductivity at room temperature was significantly enhanced [24]. The boron oxide acts also as a sintering aid to reduce the grain size, while enhancing the contact within the solid electrolyte. At a larger B2O3 content the Li-‐ion diffusion is limited at the grain boundaries. NASICON-‐type compounds are stable with Li or Na metal electrodes only when reducible transition elements are absent. An example of NASICON compounds have, however, been reported without any reducing element [25]. However, this compound has a very low Li-‐ion conductivity (around 10−8 S/cm) at room temperature and is only stable above 50°C. The substitution of Ca for Zr in LiZr2(PO4)3 introduces additional Li+ ions and stabilizes the structure at room temperature. The room-‐temperature bulk Li-‐ion conductivity of Li1.2Zr1.9Ca0.1(PO4)3 approaches 1.2*10-‐4 S/cm [25]. A recent very complete overview of NASICON structures can be found in [26]. Another type of ceramic electrolytes is the LISICON compounds (Li2+2XZn1-‐XGeO4) [27,28]. The LISICON framework has a relatively low conductivity (about 10-‐6 S/m at
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room temperature). Furthermore, LISICON structures can be highly reactive with lithium metal and atmospheric CO2 and the conductivity decreases with time. The structure can, however, be improved by substituting some oxide ions by larger and more polarisable sulphide ions -‐ the thio-‐LISICON family [29,30]. A recently reported new lithium superionic conductor, Li10GeP2S12 in the thio-‐LISICON family has a three-‐dimensional framework structure and exhibits an extremely high lithium ionic conductivity of 1.2*10-‐2 S/cm at room temperature [31]. This represents the highest Li-‐ion conductivity achieved in a solid electrolyte, exceeding even those of liquid organic electrolytes. By ab initio calculations and MD simulations [32] the ion conductivity has been explained and the compound is in fact a metastable phase not stable towards reduction by lithium. The calculated band gap is 3.6 eV and the reported electrochemical stability window (higher than 5 V) is likely the result of a passivation phenomenon. Another family of Li-‐ion conducting oxides with garnet-‐related structure (general formula Li5La3M2O12 (M=Ta, Nb)) is currently being investigated [e.g. 33]. These oxides exhibit pure lithium ion conductivity and a wide electrochemical stability window. A lithium ion conductivity at room temperature of about 4*10-‐5 S/cm was obtained in barium doped samples, Li6La2BaTa2O12 [34,35]. Laboratory-‐scale cells made of LiNi0.5Mn1.5O4//LIPON//Li have been cycled between 5.1 V and 3.5 V vs. Li+/Li at rates of 5C, and a high Coulombic efficiency of 99.98% was achieved [36]. This indicates that the decomposition of the solid electrolyte is minimal. The charge loss over 1000 cycles for the tested cells were about 125 times smaller than in corresponding liquid-‐electrolyte cells. Polymeric Polymer electrolytes offer several advantages over ceramics, including good processability and flexibility, and exhibit several attractive properties like dimensional stability, safety and, in most cases, the ability to prevent lithium dendrite formation [37-‐39]. Furthermore, their ability to deform elastically and plastically are suitable properties for all-‐solid batteries, allowing efficient interfaces towards the electrodes and for volume changes taking place during cycling. For reviews of the early developments in this field please see [40-‐43]. The ion mobility is associated with local structural relaxations of the polymer. In solid polymer electrolytes, lithium salts are solvated by the polymer chains, while in others a solvent is added to form a polymer gel, but those require a mechanical support and will not be discussed here. A recent review can be found in [44]. The most commonly used polymer for lithium-‐ion conducting solid electrolytes is poly(ethylene oxide) (PEO), in which Li-‐salts are effectively dissolved. The conductivity of PEO with various lithium salts is of the order of 10-‐5 -‐ 10-‐6 S/cm at room temperature with the highest values observed for LiTFSI as the salt [38], but further improvement is needed for room temperature battery applications. The activation energy for lithium-‐ion conduction in PEO decreases with increasing temperature. The ionic conductivity of PEO is essentially due to transport in the amorphous regions, so the conductivity generally decreases with increasing degree of crystallization, but cation conductivity can be also observed in crystalline phases [45].
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PEO is stable towards metallic lithium and batteries employing PEO based electrolytes have been demonstrated to have capacities similar to batteries with the commonly used liquid electrolytes. One general issue with polymer electrolytes is that the anions have higher transference numbers than the cations due to ‘trapping’ of cations at the polymer chain [e.g. 46]. One strategy to enhance the transference number of the cations is to increase the volume and mass of the anion; why very bulky and heavy anions have been designed, but without definitive success. Adding ceramic particles – fillers, such as LiAlO2, alumina, titania, NASICON, or silica, in composite polymer electrolytes (CPE) leads to an increased ion conductivity, probably related to a decrease of crystallization [47-‐50]. The change of the ionic conductivity with the filler content is non-‐linear with a maximum in the range 5–15 wt% of filler, depending on the polymer matrix, the lithium salt used, and the nature of the filler. The effect of process conditions, type of fillers and size, has also been investigated [51]. The addition of ceramic particles can also improve the mechanical properties of the polymer, which is important in designing a polymer electrolyte, because most changes that increase conductivity are detrimental to the mechanical performance. Another route has been to investigate polymers within porous ceramic alumina, the reverse of the CPE [52,53]. In addition to modifying PEO in composites, alternative solid polymer electrolyte materials have been developed having high mechanical strength, such as acrylate-‐based electrolytes [38]. Examples of these electrolytes are poly(ethylene oxide)-‐methyl ether methacrylate (PEOMA) [54], polystyrene-‐block-‐poly(ethylene glycol) methyl ethyl methacrylate (PEGMA) [55]. Block copolymers have also been described as possible solid polymer electrolytes, where for example polystyrene blocks improve the mechanical behaviour of the membranes keeping acceptable lithium ion conductivity [56]. Different conduction mechanisms can dominate in different temperature ranges, and by mixing polymers the operating temperature of the electrolyte can be expanded. Li-‐ion conductivity can also be enhanced by forming lamellar structured block copolymers, where blocks can be ionophilic and the ionic conductivity increased at higher temperature [57]. A different approach is to fix the anion on the macromolecular chain. In these polymers the transference number of the lithium cation is expected to be unity so that polarisation losses due to anion migration can be avoided. One example is short side-‐chain perfluorinated sulphonic acid (Hyflon) ionomer, using lithium hydroxide in absence of organic solvent [58,59]. Another attractive electrolyte is Li-‐poly(4-‐styrenesulfonyl(trifluoromethylsulphonyl)imide) (PSTFSI), containing –SO2-‐N-‐-‐SO2-‐CF3 anions, attached to a polystyrene chain, which are associated with a lithium counter cation. An ionic conductivity of 10-‐6 S/cm has been measured at room temperatures for PEO/PSTFSI composites [60]. The interfacial instability between the electrode and electrolyte is a great challenge for solid-‐state batteries [13,15,61,62], and proper engineering of the electrode/electrolyte interfaces is ultimately required for acceptable cycling performance of most solid-‐state lithium batteries [13,61,63,64].
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2.3 Na-ion Based on the same basic principles as the Li-‐ion concept other types of metal-‐ion (Me-‐ion) concepts are possible. The Me-‐ion concepts of relevance depend on the electrochemical capacity and the operating voltages. Na-‐ion is one of the most attractive Me-‐ion candidates and the concept is comparable to the Li-‐ion concept in very many aspects; the voltage levels are in the same range and the energy density is comparable with Li-‐ion batteries and thus of interest for electric vehicle applications. Sodium is three times heavier than lithium (23 g/mol and 6.9 g/mol, respectively) and is 0.3 V less electropositive, so relatively high gravimetric and volumetric capacity penalties (ca. 15%) may have to be paid in moving from lithium to sodium batteries. Yet, the 0.3 V difference is based on the metals and not on the ‘true’ anode materials used. Moreover, the availability of sodium in the Earth’s crust is more than 1000 times higher than that of lithium, resulting in a more solid sustainability perspective and long term cost competitiveness for the Na-‐ion concept. Another advantage of Na-‐ion cells compared to Li-‐ion cells is the fact that Na does not form alloys with aluminium, hence aluminium can be used as current collectors for both electrodes, resulting in a lower total weight (and material cost) of the Na-‐ion cell compared to the Li-‐ion cell (avoid heavy and expensive copper). A 7% weight reduction of the cell can be expected. If the Na-‐ion technology could be achieved, early estimates predict a 30% cost decrease of the cell materials (incl. Cu to Al) with respect to Li-‐ion technology while ensuring sustainability [65]. Such a cost reduction also takes into account the possibility to develop cheaper sodium-‐based electrolytes. The principle of cell operation is the same as its Li-‐ion cousin: sodium ions are shuttled between the cathode and anode through a non-‐aqueous (or aqueous) electrolyte. During charge, sodium ions are extracted from the high voltage positive electrode, with a working potential around or above 3.0 V vs. Na/Na+ (see Figure 3 for potential electrode materials), and are inserted into the low voltage negative electrode, whose working potential is ideally lower than 1.0 V vs. Na/Na+ (Figure 3).
Figure 3. Electrode potential of some suitable active electrode materials for Na-‐ion cells [66].
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2.3.1 Research trends Cathode: Cathode materials of interest are primarily those having low activation polarisation for Na+ transport and small volume changes during insertion and extraction. The main groups of material for the cathode are, as for Li-‐ion cells, layered transition metal oxides and polyanionic framework structures. The layered compounds are often of NaxMO2 (x≤1) insertion character, where M is a transition metal, e.g. Mn, Co, Fe, or Cr, or a mixture thereof. The electrochemical properties of layered NaMnO2 have been showed that 0.8 Na can be reversibly cycled with good capacity retention, equivalent to a capacity of 200 mAh/g [67]. The voltage profiles exhibit pronounced stepwise processes indicative of structural transitions. Maybe the presently most promising cathode material, in terms of both sustainability and electrochemical performance, is Na2/3Mn1/2Fe1/2O2 [68]. In addition to the low cost of manganese and iron, the material is attributed with a high specific capacity of about 190 mAh/g and a specific energy over 520 Wh/kg based on half-‐cell measurements [68]. This is comparable to LiFePO4, which exhibits a practical cathode energy density of about 530 Wh/kg. Moreover, cell retains about 70% of its reversible capacity when the cycling rate is increased from C/20 to 1 C. The superior rate capability of Na2/3Mn1/2Fe1/2O2 compared to that of many other layered transition metal oxides is correlated to its smooth charge/discharge voltage profile, which suggests a lack of pronounced structural transitions during cycling. At higher potentials, the Na-‐based material is not subject to detrimental oxygen release, which is common for the layered LiMO2 materials. Furthermore, full-‐cell data using hard carbon anodes have shown capacities of 100 mAh/g for 150 cycles in the voltage range of 1.5-‐4 V at a rate of 0.5 C [69]. The capacity of such a cell is limited by irreversible processes associated with the carbon negative electrode that emerge from the formation of an SEI. Layered oxides utilising magnesium substitution (e.g. Na2/3Mn4/5Mg1/5O2) have shown discharge capacities of 150-‐220 mAh/g between 1.5 and 4.5 V with an excellent retention of capacity (>96%) [70,71]. A large fraction of this reversible capacity is associated with a well-‐defined voltage plateau at 4.2 V. In the search for cathode materials with stabilities acceptable for practical Na-‐ion cells, studies have been performed on the effect of Li substitution on the structural and electrochemical properties of a layered material, Nax[LiyNizMn1-‐y-‐z]O2 (0< x,y,z<1) [72]. A smooth voltage profile over the entire range of cycling was observed, which indicates sodium de/intercalation through a solid-‐solution process. Cells with capacity within the range of 115-‐200 mAh/g have been shown, when cycled between 2.0 and 4.4 V, with acceptable retention (91% after 50 cycles) and rate capability [72,73]. Despite the advantages that layered sodium transition metal oxides offer for electrochemical energy storage applications, their air sensitivity is a challenge. This is an important issue in terms of the reproducibility of results from research studies on these materials, in addition to concerns of storage and handling from a large-‐scale application point of view. Therefore, other types of cathode materials are also being investigated. The polyanionic framework structures, often containing PO4 groups, either alone or in combination with F, enables of low-‐energy Na+ migration pathways, possibilities of tuning the operating voltage by modifying the local environments, and favourable
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structures for a flat voltage response offer some crucial advantages. In addition, their robust covalent frameworks render them thermally stable and ensure impressive oxidative stability at high charging voltages. NaFePO4, unlike its lithium analogue olivine-‐type LiFePO4, normally crystallises in thermodynamically more stable structure (triphylite or maricite) without diffusion pathways for Na ions [74-‐76]. Electrochemically active olivine-‐type NaFePO4 has been prepared by low-‐temperature Li/Na exchange from LiFePO4. This electrochemically active NaFePO4 has shown about 100 mAh/g at 1C rate [77]. A more promising group of cathodes materials are the NASICON compounds, having an open 3D frameworks enabling fast conduction of Na ions. These compounds were initially explored as solid electrolytes [78] and only more recently as insertion materials. Amongst the various NASICON compounds, Na3V2(PO4)3 has emerged as an interesting candidate because of its impressive energy density (400 Wh/kg) and thermal stability in the charged state [79]. The corresponding voltage profile has two voltage plateaus (at 3.4 V and 1.6 V) corresponding to the V3+/V4+ and V2+/V3+ redox couples, respectively. Only the higher voltage couple is, however, suitable for a cathode material or the material could be used as both anode and cathode. The material has a poor electronic conductivity, and therefore nano-‐structured materials embedded in a matrix of, for example porous carbon or carbon nano-‐fibres, are needed to achieve the acceptable capacity at practical current rates [80,81]. Inclusion of fluorine atoms in the covalent polyanionic framework has been shown to improve the voltage of the active redox couple. One promising example is the fluorine containing NASICON analogue Na3V2(PO4)2F3 (NVPF) [82] due to its high average voltage of 3.9 V [83]. Other examples include Na2FePO4F and Na1.5VPO4.8F0.7. The former has a voltage of 2.90-‐3.05 V and a theoretical capacity of 120 mAh/g [84]. Despite pathways available for fast Na ion mobility, the electrochemical kinetics is not as favourable as in Na1.5VPO4.8F0.7, which also exhibits a layered structure. The latter compound has demonstrated the attractive cycling performance, with 95% and 84% capacity retention after 100 and 500 cycles, respectively, at 1C rate and negligible overpotential throughout the charge/discharge process [77]. Anode: Na-‐ion cells do not employ metallic sodium as anode. This is mainly due to the formation of dendrites and the safety issues related to the usage and handling of metallic sodium having a melting point of only 98 °C. Thus, the success of the Na-‐ion technology is strongly dependent on the development of safe and efficient anode materials. Hard carbons or metal oxide intercalation compounds are of main interest, but alloys or conversion reaction materials may be used. Graphite, as used in Li-‐ion cells, cannot be made to electrochemically incorporate Na+ ions into the host structure [85,86]. Therefore, disordered carbon materials, with diverse morphologies, microstructures and degrees of graphitisation, are used for Na-‐ion anodes. In hard carbon the Na+ ions are inserted into nano-‐pores between randomly stacked graphene layers and reversible capacities up to 300mAh/g have been achieved [87]. The promising performance of hard carbon as a negative material for Na-‐ion cells has been demonstrated in full cells using NaMn1/2Ni1/2O2 as the positive electrode [88] and NVPF [82]. Cycling Na-‐ion cells of hard carbon anodes at a high C-‐rates and/or to low state of charge levels may result in safety issues, however, since the insertion potential is very close to the sodium plating
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potential. To overcome these issues porous hard carbon prepared based on a silica templating approach has been reported [89]. The high porosity and advanced microstructure enhance the high rate capability, thereby resulting in a capacity of about 180mAh/g at a rate of C/5. Furthermore, insertion of sodium in expanded graphite has been reported [90,91]. The distance between the graphene layers strongly influences the reversible capacity and capacities up to 280mAh/g has been shown with a capacity retention of more than 70% after 2000 cycles, which suggests that the expanded graphite anodes are attractive from a durability perspective [90]. The insertion potential of these materials is a sloping curve from 1.5 V to 0 V with more than 80% of the capacity under 1 V. The higher voltage compared to hard carbon thus increases the safety at the expense of the energy density. As for Li-‐ion cells, anode materials based on oxides are of interest for Na-‐ion cells. Na2Ti3O7 exhibits a particularly low potential desirable for Na insertion [92]. The insertion of two additional sodium atoms occurs at a reversible plateau around 0.3 V vs. Na/Na+ and corresponds to about 180 mAh/g. Very slow rates (about C/25) have to be used to achieve this capacity and a composite electrode with 30% carbon black is necessary; a decreased energy density will result and the carbon black is also responsible for large irreversible capacity losses of the same order of magnitude as the reversible capacity observed on the first cycle [92]. The rate capability has recently been significantly improved by reduction of the particle size. Reversible capacities of 110 mAh/g at 4 C and 75 mAh/g at 5 C have been shown [93,94]. Another example of oxides as anode materials is Na0.66Li0.22Ti0.78O2, showing a reversible capacity of ca. 120 mAh/g at C/10 with an average voltage of 0.7 V vs. Na/Na+ and a rate capability of 75 mAh/g at 1 C and 75 % capacity retention after 1200 cycles [95]. The basically attractive performance is attributed to the very small volume change (0.8%) upon sodium insertion. Overall, the performance of these anodes is, however, not at present attractive for vehicle applications and further research is needed. Other types of negative electrode materials are different alloys, mainly based on Sn, Sb, or Sn/C. The large ionic radius of the Na+ ion is, however, expected to result in large volume changes upon formation of sodium alloys. With an average voltage of 0.3 V vs. Na/Na+ and a theoretical capacity of 790 mAh/g, Sn is a promising alloying candidate. Despite a volume expansion of 420%, composite electrodes of Sn powder with a polyacrylate binder show a reversible capacity of 500mAh/g over 20 cycles, though at a slow cycling rate [96]. A larger reversible capacity of about 600mAh/g over 160 cycles at a rate of C/10 with an average voltage of 0.8 V vs. Na/Na+ has been reported for microcrystalline Sb [97]. The rate capability of this material is attractive, showing a reversible capacity of 500mAh/g at 4 C rate. Moreover, Na insertion into amorphous P is attributed with a high capacity of 1500mAh/g during first cycles at C/10, with an average potential of 0.6 V vs. Na/Na+. About 1000mAh/g retains after 80 cycles, and at higher rates (1C) a reversible capacity of 1000mAh/g has been observed [98]. Electrolyte: With the respect to the electrolyte, Na-‐ion cells have the same demands as every other battery concept i.e. it must be stable in the whole voltage range in order to secure durability and safe usage. The electrolytes used in Na-‐ion cells are very similar to the ones used in Li-‐ion cells. Only non-‐aqueous electrolytes are considered due to the voltage levels needed for vehicle applications. All current non-‐aqueous electrolytes for
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Na-‐ion cells are based on carbonate solvents, such as ethylene carbonate (EC) and propylene carbonate (PC) because of their very high dielectric constants, large electrochemical stability windows, and low volatilities. PC has become the main solvent used for Na-‐ion cells and is the base of about 60% of the electrolyte formulations in the Na-‐ion battery literature, a notable difference compared to Li-‐ion batteries where the use of graphite anodes prohibits the application of PC. Nonetheless, pure PC based electrolytes seem to induce strong capacity fading and low Coulombic efficiencies for hard carbon negative electrode materials [88]. EC has been introduced as co-‐solvent and is becoming a key player since it promotes a more stable SEI also for hard carbon electrodes, likely related to the formation of ether functionalities upon reduction [99,100]. The electrochemical operation window for the Na-‐ion technology is approximately the same as for Li-‐ion and thus the range of suitable salt anions are similarly limited by their oxidation and reduction properties. Sodium salts (based on the anions PF6−, TFSI−, FSI−) are less toxic than their lithium counterparts. They are also easier (less costly) to obtain in their anhydrous state and easier to purify. The most commonly used salt is NaClO4. Even it the salt is difficult to dry, NaClO4, and in analogy with the Li-‐ion technology, other salts used and practically applicable are NaPF6, NaCF3SO3, NaFSI, NaTFSI, and NaBF4, with the first being more common, though the sensitivity of the PF6 anion to hydrolysis is an unsolved issue. From a safety perspective, the larger thermal stability of sodium salts as compared to lithium salts is expected to be an advantage [101]. Ionic liquid (IL) based electrolytes have also been considered for Na-‐ion batteries. Most research has focused on the electrolyte materials and not on battery performance directly. The main tracks are related to the possibilities to improve the cell safety based on the intrinsic properties of ILs. The conductivities, viscosities, and thermal properties have been studied, where the Na electrolytes exhibit a conductivity ca 1.2 mS/cm higher than the analogous lithium electrolytes [102]. Half-‐cell measurements using IL based electrolytes have been made as a proof-‐of-‐concept utilising a binary eutectic of NaFSI-‐KFSI exhibiting a conductivity of 3.3 mS/cm and a NaCrO2 electrode [103,104]. With an electrochemical stability window up to 5.2 V vs. Na/Na+ [105] and a stability vs. the aluminium current collector, this type of electrolyte is clearly interesting for future applications. The same electrolyte was also tested vs. an Sn-‐based alloy anode [106]. Utilising an IL based on NaTFSI in Pyr14TFSI has shown that the deposition of metallic sodium on a copper working electrode does not occur until a potential of -‐0.2 V vs. Na/Na+, which thus is a considerable safety advantage for work with low potential electrodes such as hard carbon [107]. Thus IL electrolytes can act profoundly different, but still the Na IL based electrolytes seem to act with some sincere prospect for further development, especially in terms of cycling stability. Cell performance: In 2003 Valence Technologies reported a 3.7 V Na-‐ion cell using NaVPO4F as the cathode and hard carbon as the anode [108]. The electrolyte used was 1 M NaClO4 in EC:DMC and the cell, tested at room temperature at C/10, showed a reversible capacity of 80 mAh/g, and the fade was unfortunately up to 50 % already after 30 cycles. Full-‐cells made of Na3V2(PO4)2F3//hard carbon and an electrolyte of 1M NaClO4 or NaPF6 in EC:PC:DMC have been shown to operate at 3.75 V and exhibit a theoretical energy density comparable to that of graphite//LiFePO4 Li-‐ion cells [99]. The cells tested
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showed a stable capacity of 97 mAh/(g cathode active material) for 120 cycles at C/5 and a Coulombic efficiency >98.5%. Some recent industrial R&D has also been disclosed to some extent, for example by Toyota. One openly distributed industrial report by Sumitomo disclosed the fabrication of NaFe0.4Mn0.3Ni0.3O2//hard carbon coin and laminated cells using 1 M NaPF6 in PC electrolyte exhibiting good cycle life and rate capability, although metrics to compare with similar lithium-‐ion cells were missing [109]. Moreover, this report also discussed heating and overcharging tests carried out, indicating a better performance than for comparable Li-‐ion cells, as 200% overcharge did only result in swelling without burst or ignition. Faradion, a UK-‐based company developing Na-‐ion cells, claim an energy density of their 18650-‐cells to be 126 Wh/kg and 343 Wh/L [110], roughly half of the Li-‐ion 18650-‐cells (C//NCA chemistry) produced by Panasonic for Tesla and about 30% more energy density than a 18650-‐cell made of C//LFP chemistry. Faradion has recently disclosed 3Ah Na-‐ion pouch cells using hard carbon and a layered oxide cathode (165 mAh/g) [111]. These are reported as comparable to Li-‐ion state of the art. Even if the Na-‐ion technology is still immature, it is clear that the Na-‐ion technology can compete with the Li-‐ion technology in several aspects; about the same capacity as Li-‐ion materials with the potential of lower raw material costs. With respect to safety there is no indication or scientific grounds to tell whether Na-‐ion batteries will be safer or not than Li-‐ion batteries, but preliminary accelerating rate calorimetry tests suggest that they will be at least as safe as Li-‐ion batteries [112,113]. Aside from the Na/S and ZEBRA high-‐temperature systems (not treated here), no commercialized non-‐aqueous Na-‐ion cells exist at present. There are large opportunities for research and development: cathodes, anodes, electrolytes, and half and full cells.
2.4 Mg Rechargeable Mg batteries have for a long time been considered as a highly promising technology. The theoretical capacity is related to the number of electrons involved in the redox reactions and therefore it is of interest to use multivalent ions to double or even triple the capacity. Thus, despite their larger atomic weights, magnesium and aluminium based concepts can be attractive because of their ability to exchange two and three electrons, respectively, compared to only one electron for lithium and sodium. The practical capacity in turn depends on the amount of reversible ions during the charge and discharge processes. The main issue is, however, to find durable materials for long-‐time cycling and at rates needed for vehicle applications. Magnesium possesses several characteristics that rank it as one of the most favourable metal anodes for high energy-‐density batteries. Due to its bivalency, its specific volumetric capacity is greater than 3800 mAh/cm3, higher than that for metallic Li (ca. 2050 mAh/cm3). Moreover, Mg is a benign and abundant metal in the Earth's crust. Despite its potential reactivity it is stable enough in ambient atmosphere for handling and electrode preparation processes. The first breakthrough was demonstrated in 1990 with the development of an anodically stable electrolyte (an ether solution containing Mg salts based on organo-‐borate or organo-‐aluminate anions) [114]. A decade later, the
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next breakthrough was achieved by the development of ethereal electrolytes containing Mg-‐haloalkyl aluminate complexes [115]. The main challenge of Mg based batteries and for all multivalent concepts, is not, however, on the anode side, but on the cathode side. The cathode requires materials that allow both several oxidation state steps and acceptable diffusion rates of the Mg2+ cation. The ideal material would be a compound based on transition metals having one reversible redox couple per inserted Mg2+, i.e. a two-‐electron reduction of the transition metal, e.g. vanadium, manganese, or titanium. The most promising materials are insertion compounds based on oxides or sulphides, due to their capacity and potential. Two main routes can be taken to achieve high-‐energy rechargeable magnesium batteries: i) relying on high capacity/low voltage Mg cathodes and ii) utilizing moderate capacity/high voltage Mg ion insertion cathodes. The latter will be limited by the maximum practical intercalation level attainable with Mg ions, which is estimated at 200–300 mAh/g. Several studies have concentrated on the development of cathode materials with higher capacities and voltage using complex electrolytes [116,117]. These cathodes are, however, limited to about 200 mAh/g and a 2 V operation voltage (vs. Mg). The most studied group of materials for the cathode is the Mg-‐based Chevrel phases MgxMo6T8, where T is S, Se, or a mixture thereof [118]. The structure consists of octahedrally coordinated Mo in a cubic framework of the anions S and/or Se. The Mo atoms exhibit variable oxidation states and the anion framework provides diffusion pathways in several directions and a variety of sites for the inserted Mg2+ ions. Up to four electrons can be sustained by the Mo6 clusters, resulting in a theoretical capacity of up to two Mg2+ ions per MgxMo6T8 unit. Factors affecting the ability of the material to incorporate Mg2+ are primarily the solid-‐state diffusion rates, which, if not favourable enough, will increase the electrode polarisation and may cause ion-‐trapping. Secondary factors are possible co-‐insertion of the electrolyte solvent and concomitant structural distortion or decomposition of the positive electrode material. Yet, many of these materials suffer from low electronic conductivity and blended materials may therefore be used. The Chevrel phase compounds enable rapid Mg2+ diffusion rates due to the large amount of vacant sites available in the structure and the diffusion rates can be further enhanced at elevated temperatures. The first successful magnesium battery prototypes used Mo6S8 cathodes and were able to sustain more than 500 cycles at a moderate rate with low capacity fading, though the specific capacity was rather low (ca. 60 mAh/g) [115]. These results have been improved by substitution of sulphur by selenium, resulting in increased magnesium ion mobility and decreased irreversible capacity, previously caused by magnesium trapping. Nonetheless, this comes to the expense of a decrease in operation potential and a lower electrochemical capacity: 88.8 mAh/g vs. 128.8 mAh/g. A compromise seems to have been found with Mo6S8-‐ySey (y=1.2) compounds [117]. There are some few other potential Mg cathode materials of interest. The main drawback is a lower reversible capacity, for example magnesium cobalt silicates [119]. In some cases, these materials are targeted to operate at higher potentials and enhance the energy density of the cells, which include amongst others, first principle calculations
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suggesting the suitability of the MgVPO4F compound [120] or experimental studies on transition metal oxides. The most significant reliable findings are based on V2O5 cathodes [121]. To attain a specific energy comparable to that of Li-‐ion batteries, further breakthroughs are required concerning the stability of the electrolyte towards the anode (>3 V), and for high specific energy cathode materials. Any breakthrough in these directions will have to demonstrate full compatibility of the electrolyte with both electrodes, as well as allowing fast and highly reversible magnesium electrodeposition and dissolution. In 2007 an electrolyte stable close to 3 V was demonstrated [117]. The main bottleneck for any further development of high potential materials for Mg batteries is indeed the absence of suitable electrolytes – with enough stability to truly test them in half or full cells and thereby develop a high potential magnesium based battery technology. Presently there are a few companies trying to develop rechargeable Mg batteries, including Sony, LG Chem, Honda, and Toyota, as part of their R&D efforts in the battery field. The American company Pellion Technologies [122] is fully devoted to the development of high energy-‐density Mg rechargeable batteries.
2.5 Li-S The reaction of sulphur to Li2S has a theoretical capacity of 1673 mAh/g and in combination with an anode of metallic lithium, Li-‐S batteries can reach gravimetric and volumetric energy densities of 2500 Wh/kg and 2800 Wh/L, respectively [123]. The cost of such a cell would thus be much less than a corresponding conventional Li-‐ion cell based on just the materials cost. The cell operates in the voltage range, which is more or less safe. In addition sulphur is an abundant element and non-‐toxic. Undoubtedly, all of these advantages make Li-‐S cells an attractive emerging battery technology. The main drawbacks are, however, the insulating nature of sulphur and polysulfide dissolution causing active sulphur loss, low power capabilities, and rapid capacity fading. In the charged states sulphur exists in the form of a large molecule, i.e. S8, in the cathode. The conversion to Li2S is a multi-‐step reaction. At discharge lithium ions from the anode react with the sulphur cathode and long-‐chain lithium polysulphides (Li2Sx, 4≤x≤8) are formed [124,125]. These intermediate products, generated at the initial stages, are soluble in the commonly used electrolytes. In the subsequent stages of discharging these long-‐chain polysulphides will turn into insoluble Li2S2 and finally Li2S [126]. A typical discharge and charge voltage profile for the first cycle of a Li-‐S cell is shown in Figure 4.
Figure 4. A typical voltage profile for the first cycle of a Li-‐S cell.
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Based on the reaction steps, the discharge process can be divided into four regions: at 2.2-‐2.3 V, Li2S8 is formed and dissolves into the electrolyte: S8 + 2Li → Li2S8, subsequently the dissolved Li2S8 transfers to short-‐chain poly-‐sulphides and a corresponding reduction of the cell voltage occurs; Li2S8 + 2Li → Li2S8-‐n + Li2Sn. After this insoluble Li2S2 or Li2S is formed at a second lower voltage plateau at 1.9-‐2.1 V, which contributes with the major capacity of a Li-‐S cell; 2Li2Sn + (2n-‐4)Li → nLi2S2 or Li2Sn + (2n-‐2)Li → nLi2S Hence the composition of the electrolyte changes with voltage and furthermore a redox shuttle mechanism is enabled leading to that the theoretical capacity can seldom be obtained in practice. Besides the electrochemical reactions, complicated disproportionation reactions of the poly-‐sulphides also take place in the electrolyte; all affected by the composition of the electrolyte and temperature. Based on above, the Li-‐S cell is indeed more of a liquid electrochemical system. Since both sulphur and its reduction products are non-‐conductive, the operation of a Li-‐S cell entirely depends on the dissolution of poly-‐sulphides. Reasonable specific capacity of about 800 mAh/g in the first cycle can remain at 510 mAh/g after 60 cycles when cycled at slow rates between 1.7 V and 2.8 V has been obtained [127]. Lithium-‐sulphur batteries have been studied for more than four decades, since the late 1960s [128]. In spite of the tremendous progress, however, there are few reports on lithium sulphur batteries with appreciable capacity performance up to 1000 cycles [129,130]. There are thus still challenges remaining thorny and unsolved. The first is associated to the insulating nature of sulphur and its electrochemical products that only allow ions and electrons to diffuse on their surfaces. Second, polysulphides as discharge intermediate products dissolve into the organic electrolyte, which reduces the amount of active cathode materials [126,128]. The dissolved polysulphides can also diffuse to the lithium anode driven by chemical potential and the concentration difference between the cathode and the anode, be reduced to Li2S and Li2S2, and deposit on the lithium anode [131], leading to undesired parasitic reactions. The last major problem for Li-‐S batteries is the large volume expansion of sulphur as high as ~80% during cycling. The cathode will be pulverised by the internal strain resulting, leading to loss of contact between the electrode and current collector and severe capacity fading. Thus, to ensure consistent cycling performance of Li-‐S systems over several hundreds or thousands of charge/discharge cycles as required in practical applications, all these three major problems need to be solved. Despite enormous developments accomplished, the commercialisation of this battery still has a long way to go, based on reviewing technological breakthroughs. For vehicle applications, the high energy density is attractive both in terms of weight and volume. The high capacity is also due to the involvement of two electrons in the redox reactions. The potential of low-‐cost cells due to the abundance of sulphur is also attractive from a vehicle perspective. The main drawback is the low voltage output of ca 1.8 V. Furthermore, the self-‐discharge rate is high, and there is a risk of H2S evolution. The insulating properties of the discharge products can lead to rapid ageing. Furthermore, from a safety perspective, the low melting point of S at ca. 115 °C has to be considered.
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2.5.1 Research trends Most research efforts are focusing on development of novel cathodes via various synthesis methods in order to make different nano-‐structured materials. In the following the research trends for the cathode and the electrolyte of Li-‐S cells will be given, while the Li anode is currently less researched. Cathode: Sulphur cathodes for high capacity and great cycling stability should feature: i) sufficient content of sulphur, ii) adequate conductivity (e.g. by adding conductive materials such as different kinds of carbon), iii) flexible structure to buffer the large volume changes, and iv) ability to trap polysulphides. Several routes utilising different nano-‐sized and nano-‐structured sulphur based cathodes are possible, and the main research trends are summarised below. Porous carbon-sulphur composites Micro-‐porous carbon proves to be an effective sulphur immobilizer because it has very small pores to confine sulphur and prevent intermediate product poly-‐sulphides from dissolve into the electrolyte [132]. Micro-‐porous carbon-‐sulphur composites with a narrow pore size distribution have shown a large reversible capacity of approximately 650 mAh/g even after 500 cycles [133]. There are, however, some critical factors to consider for improving the performance: pore size and sulphur loading. After optimising the pore sizes and sulphur loading, cathodes with an initial capacity of ca. 1400 mAh/g and after 100 cycles a capacity retention of ca. 840 mAh/g have been demonstrated [134,135]. Bimodal pore structures can also be used to improve the performance. Furthermore, by optimising the initial ratios of carbon/silica/surfactant, cathode materials have been demonstrated having an initial capacity of 995 mAh/g and a capacity of 550 mAh/g after 100 cycles at 1 C rate [132,136]. Sulphur containing nano-tubes/nano-fibres Nano-‐tubes and nano-‐fibres of carbon have been shown to be attractive matrices for sulphur. The main reason is the intimate contact between conductive nano-‐tubes and sulphur enabling fast electron and ion transport in electrodes [137]. In addition, the nano-‐tubes can accommodate the volume expansion of sulphur during cycling. One example is cathodes made by CVD deposition of carbon nano-‐tubes filled with sulphur delivering a capacity of more than 1400 mAh/g [138]. This cathode not only facilitates the transportation of electrons and ions, but also shortens the distance of lithium ions diffusion in electrodes, contributing to fast kinetics. Moreover, polymer materials can be used mainly due to strong physical bonds and chemical interactions among the polymer framework, sulphur, and poly-‐sulphides. Examples of capacities are 837 mAh/g at 0.1 C after 100 cycles and 432 mAh/g at 1 C after 500 cycles [139]. Graphene-sulphur composites Despite the high conductivity, mechanical strength, and flexible structure graphene is not widely investigated as the host for sulphur in the cathode due to its sheet-‐like shape and open structure, causing active materials to readily diffuse out. There are, however, a few attempts to use graphene in Li-‐S cells, [140,141], for example by sandwich sulphur between two graphene layers [142] or by coating sulphur particles with a polymer and then wrapping these coated particles with graphene [143]. The cathodes made have
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displayed reversible capacity of ca. 600 mAh/g and less than 15% degradation after 100 cycles. 3D nano-structured sulphur composites 3D nano-‐architectures provide porosity for increased power by reducing the diffusion lengths and can accommodate volume changes during cycling. Also, porosity offers necessary pathway for electrolyte penetration, resulting in enhanced lithium ion diffusion and fast kinetics. Thus, considerable efforts have been devoted to the development of 3D electrodes in order to achieve high energy density and high rate capability Li-‐S batteries [137,144,145]. 3D multi-‐walled carbon nano-‐tubes have been developed to house sulphur or lithium sulphide and achieved enhanced capacity with 780 mAh/g remaining after 200 cycles at a current density of 0.5 A/g [146]. 3D polymer nano-‐tubes have been investigated as well and compared to carbon nano-‐tubes, polymer nano-‐tubes enable trapping of intermediate poly-‐sulphides effectively, rendering better cell performance [147,148]. Cathodes of a high sulphur loading (70 wt %) have shown a reversible capacity of ca. 500 mAh/g at 1 C rate [140]. Core/yolk-shell structures Core-‐shell structures are attractive addressing the challenges of volume changes during cycling and preventing poly-‐sulphide dissolution. Yolk-‐shell structures have also been investigated, showing promising results [149]. The main difference between the yolk-‐shell structure and core-‐shell structure is that void space exists between the core and the shell in the yolk-‐shell structure. In most core-‐shell structures, sulphur or sulphur-‐based compounds act as the core and high sulphur contents (up to ca. 85%) embedded in the shell have been reported [150,151]. The shell provides protection against poly-‐sulphide dissolution and is usually conductive materials able to facilitate both ion and electron transport. Although most work focuses on sulphur as the core material, some research has been done using sulphur as the shell, however, resulting in poor capacity retention [152]. The coatings can be attributed to lower capacities mainly due to i) insufficient coating with conductive materials, resulting in polysulfide dissolution into the electrolytes, and ii) the volume expansion and constriction during electrochemical processes leading to fracture of the shell structure and leakage and dissolution of polysulphides. There are two general approaches that can be considered to address the aforementioned tricky problems. The first is to modify the sulphur core by using for example smaller sulphur allotropes [153] or ultrafine sulphur [154]. The second approach is to modify the core-‐shell framework by for example making double shells to increase the effectiveness of preventing polysulfide dissolution, making soft shells to accommodate volumetric expansion in discharge, utilising yolk-‐shell structures, or combinations thereof. Li2S cathodes The final discharge product of sulphur electrodes, Li2S, has been investigated as a potential cathode material for high energy Li-‐S cells [e.g. 155]. Moreover, compared to the sulphur cathode, the Li2S cathode material is a pre-‐lithiated material not requiring lithium metal as the anode, mitigating safety concerns caused by lithium metal and its dendrites. Li2S can be paired with other promising anodes, such as silicon [156] and tin [157]. A big challenge for Li2S cathodes are the slow kinetics and low rate capability as a result of the poor electronic and ionic conductivities of Li2S. The high dissolution of poly-‐sulphides is also a concern. Different carbon-‐coated Li2S materials have also been
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utilized directly as the cathodes [e.g. 157]. The main concern is the high sensitivity towards water, putting high restrictions on the production process. Anode: The most common anode used is metallic lithium and has been described previously. In the special environment in the Li-‐S cell the metallic lithium anode needs to be protected to prevent redox shuttle mechanisms, to reduce gassing (swelling), and to increase the safety. In order to protect Li effectively, the material of the protective layer should be: i) insoluble in the liquid electrolyte, ii) chemically stable against poly-‐sulphides and metallic Li, and iii) highly ionic conductive. The protection can mainly be of the characters: physical barrier, using a gel polymer electrolyte or lithium alloys as anode, or the metallic lithium can be pre-‐passivated before cell assembly. The main issue for these protective layers is their ability to not hamper the power capabilities of the anodes. A separator can be utilised, and one example is to use fluorinated polymers forming a LiF passivation layer on the surface of Li anode through a limited reaction between the Li metal and polymer [158]. This approach has proven to improve the cycling efficiency and morphology of Li metal although the concern with the chemical stability of fluorinated polymers against the poly-‐sulphides still remains. Moreover, it is possible to use organosulphur compounds, which can form complex with Li metal and thereby a protective layer [159]. A gel polymer electrolyte able to stick the separator and Li anode together can be used, which helps to improve the morphology of Li plating by forming a layer between the separator and Li anode. The capacity retention have shown to be improved and the on-‐set temperature for thermal runaway increased by at least 50 °C due to the compact deposition of the Li metal [160]. Furthermore, a Li-‐Al alloy by laminating a thin Al foil with a Li anode has been proposed to reduce the redox shuttle mechanism and has been shown to improve the specific capacity and capacity retention [161]. A similar concept has been demonstrated using Pt and a Li-‐S cell with a specific capacity of 750 mAh/g after 90 cycles has been demonstrated [162]. Pre-‐treatment of the Li anode using a reactive chemical to form a stable passivation layer before the Li-‐S cell is assembled has been investigated by using oxidative compounds and inorganic acids to form the insoluble and stable protective layer on the Li metal. A Li-‐S cell with pre-‐treated Li showed higher discharge capacity as compared with the baseline cell [163]. Electrolytes: Considerable efforts to solve poly-‐sulphide dissolution and the shuttle phenomenon have been paid to electrolyte studies. Developing new electrolytes is extremely desirable for realising good interfacial architectures and great properties of Li-‐S batteries. In the following the main research trends concerning electrolytes for Li-‐S cells are summarised. Liquid electrolytes Properties for electrolytes applicable for Li-‐S cells include good polysulfide solubility, chemical stability towards polysulfide species (anions and anionic radicals) and the Li anode, and low viscosity for fast ion and charge transport. Based on the requirements above, conventional carbonate solvents used in Li-‐ion cells are usually not suitable for
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Li-‐S cells due to their chemically reactivity with poly-‐sulphides [164,165]. Therefore, alternative ether solvents and poly(ethylene glycol) are considered [166,167]. It is found that cyclic and linear ethers, including tetrahydrofuran, 1,3-‐dioxolane, 1,2-‐dimethoxyethane, and tetra(ethylene glycol) dimethyl ether, are suitable electrolytes, where cells based on the latter has shown high capacities of over 1200 mAh/g [168-‐170]. Solid electrolytes The shuttle phenomenon is inevitable when liquid electrolytes are employed in Li-‐S cells due to poly-‐sulphide dissolution and reduction on the anode surface. In order to eliminate the existence of poly-‐sulphide ions, solid electrolytes have attracted intense attention as an alternative approach. The requirements of solid electrolytes for Li-‐S cells include good Li-‐ion conductivity, high stability towards the lithium metal anode, and high contact area between electrodes and electrolytes. The main hurdle for the practical application of solid electrolytes is their low ionic conductivity. However, with the emergence of solid media possessing lithium ion conductivity comparable to that of liquid electrolytes [171], all-‐solid state electrolyte cells could become the next-‐generation batteries (as described previously). Since the ion conductivity is challenging for all-‐solid-‐state Li-‐S cells, PEO with lithium salts containing finely dispersed nano-‐sized ZrO2 particles or LiAlO2 filler has been developed [172,173]. A capacity close to theoretical and high Coulombic efficiency are both achieved only at elevated temperatures (90 °C), and a stable anode interface is obtained, likely due to the dispersed ceramic filler as an interfacial stabiliser [172]. Also, various other solid electrolytes, such as Li2SeSiS2 powers, thio-‐LiSICONs (lithium super-‐ionic conductor), and Li2SeP2S5 glass-‐ceramics [e.g. 174,175], have been investigated. Gel polymer electrolytes Theoretically, gel polymer electrolytes (GPE) have advantages of solid electrolytes impermeable to poly-‐sulphides and suppressing dendrite formation and advantages of liquid electrolytes with good conductivity. One example is PEO/LiCF3SO3, EC/DMC and LiPF6, resulting in high interfacial resistance and slow capacity retention [157]. A favourable alternative to have higher capacity retention is to use electro-‐spun nano-‐fibrous membranes based on different polymers [176]. These GPE not only have high interfacial compatibility, wide oxidation stability and high ionic conductivity, but also possess high liquid electrolyte uptake and serve as a good host due to a fully interconnected pore structure of polymer membranes [177]. Another approach to enhance the ionic conductivity and maintain the liquid electrolyte within the GPE is to use functional groups to bond with the liquid electrolyte; for example functionalised poly(methyl-‐methacrylate) (PMMA) containing trimethoxysilane domains blended with PVDF-‐HFP [178]. Li-‐S cells with this GPE have delivered higher ionic conductivity compared to that of GPE without functionalized groups, and displayed little capacity decay. Ionic liquid electrolytes In addition to the three types of electrolytes discussed above, ionic liquid (IL) electrolytes have also attracted much interest. Ionic liquids are defined as liquid comprising entirely ions. They have a variety of merits, including negligible vapour pressure, non-‐flammability, high lithium ion conductivity, wide electrochemical stability, and the ability to inhibit the formation of lithium dendrites [e.g. 179]. IL electrolytes can
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enhance the performance of sulphur cathodes: for example EMITFSI [180] P1A3TFSI [179], have been investigated as the electrolytes for Li-‐S cells. Moreover, IL can suppress the solubility of Li2Sx (2≤x≤8) in the IL, whereas the dissolution and precipitation of Li2Sx take place in the organic electrolyte [181]. However, considering the viscosity and high price of IL, it is not cost-‐effective to totally employ IL as the solvents of electrolytes, and combinations of IL and organic solvents are investigated [165,182]. For example the viscosity and conductivity of electrolytes can be adjusted with different contents of PYR14TFSI, and have shown that the shuttle mechanism is greatly inhibited and a Coulombic efficiency over 98% upon 100 cycles can be obtained when the content of PYR14TFSI is 50 vol.% [165,182]. It should be noted that the SEI exhibits different morphology when ionic liquids are used including more stable properties against the corrosion of poly-‐sulphides [183].
2.6 Li-oxygen Lithium air or, more accurately, Li-‐oxygen (Li-‐O2) batteries have been particularly tantalizing because of their very high gravimetric theoretical energy densities (11-‐13 kWh/kg). These numbers are, however, misleading and come from a simple calculation of how lithium metal reacts electrochemically, and the reaction products are not taken into account. Accounting for Li2O2 formation the theoretical energy density drops to about 3500 Wh/kg, and practical values on the system level will be even lower (likely less than 300 Wh/kg). Currently, four types of Li-‐O2 cells are under development and are designated by the type of electrolyte employed: aprotic, aqueous, solid-‐state, and hybrid aqueous/aprotic. For all types of Li-‐O2 cells, an open system is required to obtain oxygen (from the air), as oxygen is the active material of the cathode. Li metal must be used as the electrode to provide the lithium source for all the systems at the current stage. In aprotic Li-‐O2 cells, porous carbons are used as the reservoir for the insoluble discharge products, presumably Li2O2. In most cases, electrocatalysts are essential to promote the oxygen reduction and oxygen evolution reactions during the cell discharge and charge processes. In the following focus is on the aprotic Li-‐O2 cells, since it is the dominant concept for research efforts and presently being the most mature. A typical aprotic Li-‐O2 cell composes a metallic lithium anode, a non-‐aqueous electrolyte, and a porous O2-‐breathing cathode that contains carbon particles and, in most cases, an added electrocatalyst. It should be noted that the oxygen reduction reaction during discharge and oxygen evolution reaction during charge of a Li-‐O2 cell occur at a three-‐phase boundary involving the solid electrode, liquid electrolyte, and oxygen gas, which makes the Li-‐O2 cell more complicated than the conventional Li-‐ion cell (more resembling a fuel cell). During discharge oxygen is reduced at the cathode and combines with lithium ions supplied from the anode to form Li2O2 at a voltage of about 3 V vs. Li/Li+. Practical discharge voltages range from ca. 2.5-‐2.8 V vs. Li/Li+ [184-‐190] and therefore the gravimetric energy advantage compared to Li-‐ion cells arises from the significantly larger gravimetric capacities attainable with the non-‐intercalation O2 electrodes. Energy densities of Li-‐O2 cathodes in the discharged state (considering the weight of carbon, catalyst, the Li2O2 formed) have been obtained in the range of 1800-‐2800 Wh/kg, depending on cathode material used [184-‐188], corresponding to ca. 55-‐85 % of the
29
theoretical upper limit. These energy densities, which conservatively consider the performance when the electrode is in the discharged (heaviest) state, represent roughly a 3-‐5 times improvements compared to Li-‐ion cells with electrode energies of ca. 600 Wh/kg at comparable low power density (50 W/kg) [191]. When comparing the capacity and energy density for Li-‐O2 cells with other battery technologies, the weight and volume of the oxygen or the reaction product (Li2O2) must be included to make the comparison of results fair. There are many challenges to make rechargeable Li-‐O2 cells attainable for practical usage. The round-‐trip efficiency of Li-‐O2 cells with carbon cathodes without catalysts is below 70% [187,192], significantly lower than the round-‐trip efficiency of conventional Li-‐ion cells, often at 85-‐95% [193]. Moreover, the current densities demonstrated of Li-‐O2 cells are in the range of 0.1-‐1 mA/cm2 [186,194], which is about 10 to 100 times lower than that of Li-‐ion cells (ca. 30 mA/cm2) [194]. The main reason for the low rate capability is related to oxygen. There is a practical limit how fast oxygen can be brought into the cell, which in turn diminishes the reaction rates. Most cells produced so far are small laboratory-‐scale cells where limited oxygen flow is not yet an issue, as it will be for large automotive cells. This will result in the need of more cells in parallel in order to achieve the same performance, adding more cost, weight, and volume of the complete battery pack for vehicle installation. Furthermore, the cycle life of Li-‐O2 cells shown to date is up to 100 cycles [195,196], which is significantly lower than that of Li-‐ion cells (up to 5000 cycles) [197]. These technological challenges are strongly related to various scientific challenges of the materials, including the chemical instabilities and the lack of fundamental understanding of the reaction and transport kinetics. Moreover, the safety characteristics of the anode and the sensitivity for contaminations from H2O and air are still to be understood. A major challenge to moving ahead even at the research level is to find a stable electrolyte for the oxygen electrode. The research efforts to tackle these challenges are summarised in the following.
2.6.1 Research trends Electrolyte: The organic electrolytes (both solvents and lithium salts) play the most critical role in an aprotic Li-‐O2 cell and determine whether a truly rechargeable Li-‐O2 cell can be realised. Numerous studies have shown that the stability of the electrolytes during the oxygen reduction and oxygen evolution processes is the key challenge for the aprotic Li-‐O2 cell. With no doubt, searching for fully stable electrolytes in the oxygen-‐rich including super-‐oxide and peroxide electrochemical environment is the research priority at present. Unfortunately, no single electrolyte investigated so far meets these demanding requirements, despite extensive efforts in the past few years. Understanding the reaction mechanisms between the electrolytes and active oxygen reduced species will, no doubt, be the key to develop a stable electrolyte for Li-‐O2 cells [198]. Carbonate-‐based electrolytes have proved to be highly unstable towards the oxygen reduction species. However, there is still a large amount of research work using carbonate-‐based electrolytes to investigate the catalytic activities of the cathode materials [199-‐201], despite the fact that the severe instability of these electrolytes has been reported. Ether-‐based electrolytes have been shown to be relatively stable in the presence of the reduced oxygen species; the most promising solvents are DMSO and TEGDME [202-‐206].
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Their electrochemical behaviour and durability remain to be investigated. The research direction is towards mixed-‐solvent electrolytes [207], as for Li-‐ion cells. Also the lithium salt used has an effect on the stability of the electrolytes in Li-‐O2 cells. For example, lithium hexafluorophosphate (LiPF6), which is used in most commercialised Li-‐ion cells, has been shown to react with Li2O2 [208,209] and the nickel foam current collector could be oxidised at a potential beyond 3.5 V vs. Li/Li+ [210]. Several Li-‐salts (LiBF4, LiPF6, LiClO4, and LiTFSI) have been investigated in terms of anion stability. LiClO4 has shown to be the least reactive towards O2-‐ radicals [211], but it is unstable in an O2 rich environment [212]. Lithium bis(oxalato)borate (LiBOB) has also been investigated as a potential salt [213]. The BOB-‐ anion reacted, however, with the O2-‐ radicals to form LiB3O5. Moreover, the anions react with the current collectors, such as the widely employed Al foils. LiTFSI, LiC(CF3SO2)3, and LiCF3SO3 have all been reported to react with Al [214,215]. Therefore, Al foils should be avoided and replaced by other current collectors. Although limited attention has been paid to Li-‐salts in Li-‐O2 cells, the possible decomposition and accordingly the effect of the anions on the performance of Li-‐O2 batteries should not be neglected. Electrolyte additives can improve the dissolution of Li2O2 and O2 in electrolytes and increase the discharge capacity of the cell. One example is to add tris(pentafluorophenyl)borane to carbonate-‐based electrolytes to increase the dissolution of the discharge product Li2O2 and enable further oxygen reduction reaction to occur at the released active sites [216]. To increase the O2 solubility in PC-‐based electrolytes per-‐fluorotributylamine has been used [217,218]. The idea of storing the discharge product Li2O2 in the electrolyte is promising for improving the specific capacity and avoiding the blockage of channels in the cathode, but the stability of this system needs careful evaluation. Cathode: One of the largest hurdles for the rechargeable Li-‐O2 cells is the large overpotential during discharge and charge (about 1 V), even at very low current density (0.01− 0.05 mA/cm2), which results in low round-‐trip efficiencies and low power capability. This limitation is strongly believed to depend on the nature of the catalytic properties of the cathode, in addition to the stability of the electrolytes. Many catalysts, including metal oxides, non-‐precious, and precious metals on a porous carbon support, have been examined as parts of the cathode material for the oxygen reduction and evolution reactions, showing large differences in discharge capacity among different catalysts. A nearly identical discharge voltage plateau at about 2.6-‐2.7 V is observed for different catalysts, similar to that for the bare carbon without any catalyst loading. This probably implies that either the oxygen reduction reaction kinetics in cathodes is limited by the oxygen mass transport toward the catalysts or carbon itself can provide sufficient electrochemical activity. Understanding the role of carbon in the electrochemical reactions in the Li-‐O2 cell could provide guidance and a baseline for identifying efficient catalysts and to improve the cell performance. Porous carbon is the most commonly used cathode material, mainly due to the sufficient charge transfer for the electrochemical reactions and space for housing the discharge products. Moreover, the low mass of the carbon-‐based electrode results in high specific capacities. Different types of carbon have shown certain catalytic activity towards
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oxygen reduction [219-‐221]. Aside from the commercially available carbon black, recent studies have shown that other carbon-‐based materials could also be very successful when used with a stable electrolyte, for example hollow carbon fibres showing an energy density about four times that of a LiCoO2 cathode for Li-‐ion cells (ca. 2500 vs. 600 Wh/kg) [222]. This was due mainly to low carbon packing and highly efficient utilisation of the available carbon mass and void volume for Li2O2 formation. The visualisation of Li2O2 morphologies upon discharge and disappearance upon charge represents a critical step towards understanding of the key processes that limit the rate capability and low round-‐trip efficiencies of Li-‐O2 cells. The morphology of Li2O2 leads to very non-‐uniform surface coverage, which is beneficial to increasing the discharge capacity of the cell due to easier oxygen diffusion at the late stage of the discharge. Therefore, understanding and controlling the nucleation and morphological evolution of Li2O2 particles upon discharge is the key factor to achieving high volumetric energy density of the Li-‐O2 cell [219,223]. Graphene-‐based materials have also been investigated due to their low weight, high conductivity, and catalytically active surface. High electrode capacities have been reported based on graphene (about 15000 mAh/g graphene) [224]. It should be noted, however, that this capacity has been obtained in a primary cell containing porous functionalised graphene sheets. The Li2O2 morphology (e.g. shape and thickness) and structure (e.g. crystallinity and surface vs. bulk composition) are important parameters that can influence the discharge capacity, rate capability, and cyclability of Li-‐O2 cells and is critical for identification of new approaches to reduce the overpotentials during cycling [225]. Moving on to catalysts, various catalysts have been examined, such as metal oxide, precious metals, and non-‐precious metals, and the most studied is MnO2 [220,226,227]. The crystal structure (e.g. α, β, δ, γ, and λ) and the morphology of the MnO2 nano-‐particles can be tailored to achieve different properties and, thus catalytic performance in Li-‐O2 cells: nano-‐sheet δ-‐MnO2 microflowers, α-‐MnO2 nano-‐wires, and α-‐MnO2 nano-‐tubes [228]. In terms of the electrocatalytic activity for these different MnO2 nano-‐particles, the α-‐MnO2 nano-‐tubes exhibit much better performance to catalyze the electrochemical processes in aprotic Li-‐O2 cells. As-‐prepared MnO2/C composites with porous structures and high specific surface area provide more active sites for the oxygen reduction and evolution reactions and, therefore, lead to significant enhancement of the electrochemical performance of Li-‐O2 cells, and hence a lower charge overpotential (3.5 V) could be achieved [220]. The use of carbon supported metal oxides as catalysts for Li-‐O2 cells was initially motivated by studies in aqueous Zn–air or fuel cell systems, where MnO2, Co3O4, LaNiO3-‐x, and Pb2Ru2O7-‐x oxides have shown significant catalytic activities for oxygen [229,230]. Carbon supported precious metal catalysts have been extensively studied in fuel cells, and they have now also been investigated for Li-‐O2 cells. Pt and Pd supported on carbon has shown to be functionable [231,232] and carbon supported Ru could significantly increase the kinetics of Li2O2 decomposition on charge [233]. The possible catalytic activity towards electrolyte decomposition has to be taken into account, also for Pt and Pd [234,235]. One example of a non-‐oxide, non-‐precious catalyst is Fe–N–C which has been demonstrated to have catalytic activity towards the reduction of oxygen in aqueous electrolytes [235,236]. The cell voltage was lower, however, compared to an MnO2-‐based catalyst, but a reduction in the overpotential of ca. 0.6 V was observed, which is promising. Moreover, only O2 was released upon charge; for the MnO2-‐based catalyst
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also CO2 was released, and a suggested reason proposed is an increased interfacial contact with lithium peroxide, which could efficiently lower the activation barrier and reduce the overpotential during charge [237]. Anode: Currently, Li metal is used as anode and will possibly be replaced by another large capacity materials for the sake of safety before deployment, if the metallic lithium anode cannot be stabilised. This has been described previously and will therefore not be further discussed here. From the known anode materials for Li-‐ion cells, Si and its related materials are the most promising alternatives [238]. The difficulty is to buffer the volume change and reduce the pulverisation of Si particles during cycling. Assuming a discharge voltage of 2.4 V, the energy density of the LixSi-‐O2 cell has been estimated to 980 Wh/kg, considerably higher than the 384 Wh/kg offered by conventional 3.6 V Li-‐ion graphite//LiCoO2 cells [238]. The reactions occurring on the interface between the Li electrode and electrolyte in Li-‐O2 cells are complicated due to O2 crossover from the cathode. For this reason, we only discuss the effect of oxygen crossover on the degradation of the Li electrode and the possible solutions to improve the Li electrode performance in Li-‐O2 cells. As for all other anodes having a potential lower than the HOMO level of the electrolyte, the electrolyte will decompose at the anode forming various compounds – the SEI layer – which may block the Li-‐ion diffusion leading to poor cell performance. Controlling the SEI reactions at the lithium electrode through suitable membranes or passivation films is essential for achieving good performance of Li-‐O2 cells. Previous studies have suggested the need for an efficient protective layer for metallic lithium to avoid decomposition of the Li electrode due to contamination by discharge/charge products in the Li-‐O2 cells [239-‐242]. The separators currently used in Li-‐O2 cells cannot prevent O2/H2O diffusion to the metallic Li anode, and oxygen crossover can be a significant problem, leading to fast decay of the Li electrode. It is critical to develop thin, active membranes that can be embedded within passive porous polymeric membranes to effectively eliminate the O2 crossover, and thereby increasing the reversibility of Li-‐O2 cells. To address this issue, studies have been performed on Li-‐ion conducting membranes. One example is to utilise Li+ conducting Si-‐membranes [241]. Compared with the NASICON-‐type lithium ion conducting membranes, the Li+ conductivity of these Si-‐membranes is, however, still too low (3-‐4 times lower). Research efforts towards protective and stabilising membranes are expected to grow.
2.6.2 System implications Packaged Li-‐O2 cell prototypes have not been widely developed and the true gravimetric energy advantage of devices is not known, although it is expected to be significantly lower than the theoretical values. A calculation of the energy densities for a Li-‐O2 cell (Li2O2 as the final reaction product) and a comparison with a graphite//LiCoO2 based Li-‐ion cell has been performed [243]. A two-‐fold excess of lithium beyond the capacity of the lithiated positive electrode was used due to imperfect plating of Li upon cycling. The gravimetric energy density was found to be roughly a factor of two higher than for the Li-‐O2 cell. On a volumetric basis the projected Li-‐O2 cell has no advantage. The American
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battery company Polyplus, however, predicts that it should be possible to construct a rechargeable Li-‐O2 cell with an energy density of 700-‐800 Wh/kg using a protected lithium anode and claims to have achieved this energy density already (for a primary cell) [244,245]. Moreover, the power capabilities are limited and one major limiting factor being the diffusion of the O2 molecules in the electrolyte. The attractive gravimetric energy density will be further reduced in practical Li-‐O2 cells; by the need of the conductive porous matrix into which the Li2O2 must be deposited owing to its very low conductivity, by the electrolyte, by separators, by current collectors and by the packaging required at the system level, and by pumps needed for the oxygen handling and supply. If air is the source for oxygen, the air should preferably be free of water, particles, CO2, and impurities, which will require further cleaning steps and adding to the system complexity. Furthermore, if oxygen is used directly, an oxygen tank, preferably compressed oxygen, will add to the system weight. Which air or O2 quality is needed? How large an oxygen tank is needed for the driving range aimed at? Another challenge for vehicle applications is the large voltage hysteresis and polarisation of about 1 V between the charge and the discharge. From a material perspective the electrolyte decomposition and the poor cyclability and catalyst degradation are research challenges where the electrolyte seems to be the key.
2.7 Organic concepts As an alternative to inorganic electrode materials electroactive organics or polymers with reversible redox reactions are promising candidates as electrode materials for the new generation of “green batteries”. This is mainly ascribed to higher theoretical capacity, safety, sustainability, environmental friendliness, and potential low cost [246,247]. Many organic alternatives indeed have several distinct merits over inorganic electrode materials in reaction kinetics, structure diversity, flexibility, and processability. The large-‐scale use of transition-‐metal based electrode materials is somewhat of an unsustainable route towards the devolvement of batteries, mainly because of resource limitations, environmental pollution, and large energy consumption in both synthesis and recycling [246]. Ideally, organic electrode materials could be extracted directly or synthesised from biomass [248-‐251]. For a long time, organic electrode materials have received much less attention compared to inorganic electrode materials mainly due to their relatively poor electrochemical performance and the great success of inorganic electrode materials in both research and application. During the past decades, the research on organic electrode materials has increased and a lot of different organic structures and redox mechanisms have been investigated. Nowadays, the comprehensive electrochemical performance of some organic cathode materials, including energy density, power capability, and cycling stability, are comparable or even superior to the conventional inorganic cathodes [247]. One of the main drawbacks is, however, the currently attainable volumetric energy density. A battery pack with the energy needed will likely be larger than the available space in the vehicle. Being a rather new research field there are some interesting concepts, which could reach the vehicle industry even if the time horizon likely is far beyond 2025. Therefore, only a short review of the technology and few potential routes is given here.
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2.7.1 Basics For inorganic electrode materials, the redox reactions are related to the valence change of the transition-‐metal or elemental substance, while for organic electrode materials, the redox reactions are based on the charge state change of an electroactive organic group. Generally, organic electrode materials can be divided into three types according to their different redox reactions: i) n-‐type organics, the reaction is between the neutral state and the negatively charged state, ii) p-‐type organics, the reaction is between the neutral state and the positively charged state, and iii) bipolar organics, for which the neutral state can be either reduced to a negatively charged state or oxidised to a positively charged state. There must of course be a potential difference between anode and cathode large enough for practical use and the redox state of the active material in both the cathode and anode must hence be considered. To be of interest in vehicle applications the cell voltage should preferably be higher than 2.5 V and the cell should not only show high energy density, but also high rate capability. One strength of organic electrode materials is their inherent fast reaction kinetics, compared to the slow Li-‐ion diffusion kinetics in the bulk inorganic particles. Conducting polymers, nitroxyl radical polymers, and conjugated carbonyl compounds are all promising candidates for high power electrodes [252-‐256]. As a first example benzoquinone (redox potential of 2.8 V) has a theoretical energy density of about 1400 Wh/kg [257] much higher than that of both commercial LiCoO2 (ca. 550 Wh/kg) and potential Li-‐Mn rich NMC (ca. 1000 Wh/kg) [258]. Nitroxyl radical polymers can retain 97% of the theoretical capacity even at a charge rate of 1200 C and discharge rate of 60 C [259]. The electronic conductivity of organic electrode materials is usually low, or non-‐existing, except for conducting polymers. This is a serious hinder for the full utilisation of the high rate performance of the active materials. Just like the approaches for the LiFePO4 and S cathode, adding conductive carbon could significantly improve the electron transport in the electrode and ensure a high utilization ratio of the active material. One approach to overcome this drawback is to mix the active materials with excellent conductive carbon, e.g. graphene [260]. From a durability perspective dissolution of active materials in the electrolyte is a major issue for organic electrode materials.
2.8 Asymmetrical super capacitors Super capacitors have been plentifully investigated for hybrid electric vehicles, especially for heavy-‐duty applications. The high power density and long durability are the main advantages. The drawbacks are mainly the low energy density and thereby the high cost of a pack. Besides the ‘traditional’ capacitance, a capacitor can be made of pseudo-‐capacitance character; asymmetrical super capacitors. The energy is achieved by redox reactions, electrosorbtion on the surface of the electrode by specifically absorbed ions, and result in a reversible faradic charge-‐transfer of the electrode. Depending on the materials chosen for electrodes and electrolyte, different kinds of high-‐energy density capacitors can be tailored to suit a variety of applications and needs. There are two fundamental ways to increase the energy density of a capacitor: by increasing the cell voltage or the capacitance. One way to increase the cell voltage is by changing the type of electrolyte. Another way is to utilise the advantages of
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asymmetrical capacitors employing both faradic and non-‐faradic processes to increase the capacitance. Coupling a redox material with a high capacitance material results in both higher operational cell voltage and higher cell capacitance. One attractive type of asymmetrical super capacitors is made by using activated carbon as one electrode and an insertion electrode of a Li-‐ion cell as the other, so called Li-‐ion capacitors. The high operational voltage enables Li-‐ion capacitors of both high power and high energy density: ca. 5 kW/kg and 20-‐30 Wh/kg is currently possible. Some companies have already commercialised Li-‐ion capacitors, e.g. JM Energy, FDK, ATC. Another commercial example of this type of capacitors is developed by Fuji Heavy Industry, using a pre-‐lithiated carbon anode together with an activated carbon cathode, resulting in a cell of 3.8 V and an energy density of more than 15 Wh/kg [261,262]. The drawback is a limiting charging rate and the low-‐temperature performance due to the graphite based insertion anode. Moreover, the process of pre-‐lithiation of the anode may lead to poor cost-‐effectiveness or instability of the quality in mass production. Therefore, research is ongoing to find alternative solutions mainly for the anode.
2.8.1 Research trends In most cases, the faradic electrodes lead to an increase in the energy density at the expense of cyclability (for balanced positive and negative electrode capacities). This is certainly the main drawback of asymmetrical capacitors and it is important to avoid transforming a good super capacitor into a mediocre battery [261]. The main activities on asymmetric super capacitors arise, however, from the Li-‐ion batteries research field. At first, a nano-‐structured anode of Li4Ti5O12 was combined with an activated carbon cathode, resulting in a 2.8 V cell exceeding 10 Wh/kg [263]. The Li4Ti5O12 anode ensured high power capacity, as well as long-‐life cyclability thanks to low volume change during cycling. Following this pioneering work, many studies have been conducted on various combinations of a lithium-‐insertion electrode with a capacitive carbon electrode. An advantage for these types of materials is the very high rate capabilities. Laboratory cells have shown C-‐rates of 100-‐300 [264,265]. At these high C-‐rates, the reversible capacity is about 95% of the capacity obtained at 1C rate [265]. Generally metal oxides have been the mostly employed active electrode materials for the super capacitors applications due to their physico-‐chemical properties. Various metal oxides, such as RuO2, MnO2, V2O5, Fe3O4 and α-‐Co(OH)2 have been studied. MnO2 is one of the most studied materials as a low-‐cost alternative to RuO2 (see below). Its pseudo-‐capacitance arises from the MnIII/MnIV oxidation state change at the surface of MnO2 particles [266]. Nano-‐powders and nano-‐structures of MnO2 can further improve the capacitance [267]. Amorphous nano-‐structured MnO2 electrode has shown impressive stability results: up to 1200 cycles at 250 F/g of capacitance [268]. Furthermore, 2D nano-‐sheets of MnO2 have been prepared by the exfoliation-‐reassembling method, to achieve specific capacitance values of about 140-‐160 F/g and a cycling stability of 93-‐99% up to 1000 cycles [269]. Moreover, different morphologies of amorphous RuO2 are promising electrode materials with excellent electrochemical capacitance behaviour: nano-‐needles [270], nano-‐porous film [271] and nano-‐particles [272]. Depending on the synthesis method large variations
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of capacitances can be achieved. For example nano-‐tubular RuO2 has shown high capacitance of about 1300 F/g [273], with another method only 390 F/g [274], or with yet another method as low as 50 F/g [271]. Recently, binary oxides of nickel and cobalt (Ni-‐Co) have been investigated. Nano-‐sheets with mesoporous structure were studied showing a capacitance up to 1846 F/g and excellent rate capability [275]. Based on this material, asymmetrical super capacitors were made by using the Ni-‐Co oxide as the cathode and three kinds of activated carbons respectively as the anode. The operating voltage range was 0–1.6 V with a capacitance of 202 F/g and a maximum energy density of 71.7 Wh/kg in combination with a maximum power density of 16 kW/kg (at an energy density of 41.6 Wh/kg). Composite cathodes of Ni3S2 nano-‐particles and 3D graphene have also been investigated. Due to a synergistic effect, the capacitance and the diffusion coefficient of electrolyte ions of the activated composite electrode are ca. 4-‐6 times higher than electrodes made of only Ni3S2 [276]. The composite cathode showed a specific capacitance of 3296 F/g. In full cell experiments, a composite anode of Fe3O4 nano-‐particles and chemically reduced graphene oxide was used, and the asymmetric super capacitor was operated reversibly between 0 and 1.6 V and a specific capacitance of 233 F/g was obtained, which delivers a maximum energy density of 82.5 Wh/kg at a power density of 930 W/kg [276]. Thus, for both the Ni3S2, Fe3O4, and Ni-‐Co oxides the cell voltages are too low to be attractive for vehicle applications. Therefore, other types of electrodes are needed to increase the cell voltage. Moving from aqueous to organic electrolytes is crucial to enable an increased cell voltage from 0.9 V to 2.5–2.7 V. As the energy density is proportional to the voltage squared, numerous research efforts have been directed at the design of highly conducting, stable electrolytes with a wider voltage window. Today, the state of the art is the use of organic electrolytes based on acetonitrile or propylene carbonate, the latter becoming more popular, because of the high flash point and lower toxicity compared to acetonitrile. Ionic liquids have been shown to enable a cell voltage of about 4V and consequently the energy density will increase by about 80 % compared to a 3V cell if the same capacitance can be achieved. The power capability of the cell depends on the ionic conductivity of the electrolyte, which affects the cell resistance. Ionic liquids often have low conductivities at low temperatures, why elevated temperatures often are needed. For vehicle applications, and in the temperature range –30 to +60 °C, where batteries and super capacitors are mainly used, ionic liquids still fail to satisfy the requirements because of their low ionic conductivity. However, the choice of a huge variety of combinations of anions and cations offers the potential for designing an ionic liquid electrolyte with an ionic conductivity of 40 mS/cm and a voltage window of >4 V at room temperature [277]. A careful choice of both the anion and the cation allows the design of high-‐voltage super capacitors, and 3 V, 1000 F commercial cells are available [278]. The ionic conductivity of these liquids at room temperature is, however, low (few mS/cm) and they are therefore mainly used at higher temperatures. One example is CDC with an EMITFSI ionic liquid electrolyte, which has shown a capacitance of 160 F/g and ca. 90 F/cm3 at 60 °C [279]. Moreover, hybrid activated carbon/conducting polymer devices also show an improved performance with cell voltages higher than 3 V [280-‐282]. Supported by the efforts of the Li-‐ion community to design safer systems using
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ionic liquids, the research on ionic liquids for super capacitors is expected to have an important role in the improvement of capacitor performance.
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3 Emerging battery technologies – Vehicle implications Based on the research trends and the general research field overview the vehicle implications of the emerging technologies can be treated in some detail with respect to future possibilities and limitations. As previously indicated the energy density is often available from researchers and companies developing alternative battery solutions. From a vehicle perspective, the power capabilities are, however, equally interesting, or even more interesting. These data are often not available. The main reason is that the electrode and cell designs highly affect the power capabilities; the materials as such are not the main source of power as it is for the energy. Therefore, only plausible indications on how different technologies will affect the battery pack power when its ready for vehicle installation will be given. Moreover, likely cost trends for the different technologies will be summarised based on their basic layouts and materials used; exact costs can only be given when a mass-‐production is in place. The emerging battery technologies have been assigned levels of readiness in terms of time needed to reach vehicle integration. A list of proposed actions for the research and development of materials, cells, and battery packs for the different emerging technologies, all aiming towards vehicle applications and integration, is provided.
3.1.1 Benchmark and measures In order to set the emerging battery technologies in relation to the state of the art Li-‐ion batteries, the following cell data (Table 2) and battery pack constraints (Table 3) are used. These cells are available on the market today and used in xEV applications. Table 2. Data for state of the art cells. Brand Chemistry Capacity
(Ah) Voltage (V)
Weight (kg)
C-rate
Energy (Wh/kg)
Energy (Wh/L)
HEV A123 C//LFP 4.4 3.3 0.2 5C-‐30C
71 161
EV LG Chem
C//LMO/NMC 36 3.7 0.9 C/3-‐2C
157 275
Table 3. Battery pack constraints for technology evaluations. xEV type Nom.
Voltage (V) Current (A) Power (kW) Total
energy (kWh)
Passenger car
HEV 300 200 60 1.5
EV 300 200 60 25 Heavy-duty bus
HEV 600 250 150 5
EV 600 250 150 100* *Assuming 100 km driving range and an energy consumption of 1.0 kWh/km
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Using these cells to build the battery packs according to the specifications set in Table 3, the corresponding battery pack constraints achieved are given in Table 4. Moreover, the cost for the cells and the corresponding battery packs are summarised. The data for cell cost are based on Total Battery Consulting; $1200/kWh for HEV cells and $220/kWh for EV cells [1]. Table 4. Battery pack performance and cost of cells and battery packs. All packs are designed to meet the power requirements of Table 3. xEV type No of cells Total Cell
weight (kg) Total energy (kWh)
Total cell cost ($)
Passenger car
HEV 180 36 2.5 3000
EV 240 216 36 7500 Heavy-duty bus
HEV 360 72 5.1 6150
EV 640 576 90* 19800 *10% lower than needed.
3.2 Voltage The different emerging battery technologies exhibit different cell voltages, both nominal and range. Furthermore, some are attributed with voltage hysteresis, which will add constraints on the electrical system, and be a source of losses. In Table 5 the voltage ranges are given. Please note that some emerging battery technologies consist of a wide range of possible material combinations, consequently resulting in wide variations of the voltage. Therefore, some rough estimates have been made in the Table 5. Table 5. Voltage ranges of the emerging battery technologies and corresponding number of cells for a 300 V battery pack. Nominal
voltage (V) Voltage range
(V) No. of cells for ca. 300 V pack
Next gen Li-ion 4.6 2.5-‐5 65 Solid state Li 4.6* 2.5-‐5 65 Na-ion 3.7 2-‐4.4 80 Mg 2 0-‐2 150 Li-S 2 1.7-‐2.8 150 Li-O2 2.5 2.5-‐2.8 cha,
3.5-‐4.5 dch 120
Organic 2.8 ca. 2-‐3 105 Asym. S.C. 3.8 0-‐3.8 80 *Assuming NMC cathode The large variation of the nominal voltage, as well as the voltage profiles, will affect the battery pack cost due to the number of cells needed to sustain the vehicle system voltage level.
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3.3 Energy and Power density from cell to pack/system The different material combinations will give rise to different energy densities, both gravimetric and volumetric. The electrode and cell designs are, however, the main driver for energy or power optimised cells. Depending on cell format (cylindrical, prismatic, or pouch) the same materials can give rise to large variations in these performance parameters. The energy and power densities will therefore also vary widely among the different cell suppliers. In Table 6, new and ‘old’ Li-‐based technologies are summarised and the corresponding energy densities compared using only the material properties as input. The cells are intended to perform the same amount of work and it is the total energy that is referred to, not the available. Table 6. Gravimetric and volumetric energy densities for different Li-‐based technologies. Only cell materials taken into account in the cell data. The calculations are based on [283]. Wh/kg
(cell) Wh/L (cell)
Wh/kg (pack†)
Wh/L (pack†)
Wh/kg (system†)
Wh/L (system†)
NMC//C 250 520 150 230 150 230 High-‐energy NMC//C
285 575 170 250 170 250
High-‐voltage Li-‐Ni-‐rich NMC//C
265 550 160 240 160 240
High-‐energy NMC//SiC
420 850 240 325 240 325
High-‐energy NMC//Li(m)*
540 1050 290 375 290 375
Li-‐S 550 620 300 260 300 260 Li-‐O2** 770 820 380 320 280*** 240*** USABC† 350 750 235 500 †A battery pack of a total energy content of 40 kWh is assumed. *50% excess Li; **assuming Li2O2 as final product; ***25kg and 30L has been added for the air-‐handling system; †USABC targets [284] For the corresponding pack data, the same power capabilities are assumed and in all calculations, a 40 kWh (able to deliver 80 kW) battery pack has been assumed [283]. The same amount of pack components (housing, electronics, fuses etc.) beside the cells is assumed. As the Li-‐O2 concept requires air or oxygen handling components extra weight and volume has been included at the system level. A simple estimate gives the following: one cycle of 40 kWh will require about 300 mole of O2. To be able to deliver 80 kW, about 40 g air/s is required. An air-‐handling system (compressor, water and CO2 removal units, etc.) from a corresponding fuel cell system has been used: an air compressor of 15 kg and 12 L and an air-‐cleaning system of about 10 kg and 17 L. An alternative would be to use compressed clean oxygen. In such a case, the oxygen (tank not included) would
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require about 2500 L at 3 bar, 75 L at 100 bar, or 10 L at 700 bar, even further reducing the volumetric energy density of the Li-‐O2 technology. Of course also the weight and volume of the tank and supporting equipment must be included, again reducing the energy density. For Li-‐S batteries, the vehicle installation pack would be of the same weight as an advanced Li-‐ion pack, but about 40% larger in size. The energy density of the non-‐Li based emerging battery technologies offer other promising prospects: the Na-‐ion cells currently offer about 125 Wh/kg and 340 Wh/L [110]. Using the same calculation basis as above, the corresponding battery pack would be about 60% heavier, but ca. 10% smaller than the improved Li-‐ion cells (Table 6). Today it is too early to compare the Mg battery technology with the present Li-‐ion technology in terms of energy density. Only smaller research cells have been made and most data are valid for just the cathode material as such. Yet, energy densities above 300 Wh/g do seem possible at cell level. This will, however, require further research and cell optimisation efforts. Organic battery technologies are a wide range of concepts, some not at all of interest for vehicle applications. Some concepts have, however, high theoretical energy densities of for example 1400 Wh/g at the cell level. The volumetric energy densities have to be further investigated, but are at a first instance questionable. The question is how or if high gravimetric and low volumetric density cells can be used for vehicles and how a battery pack can look like. Asymmetrical super capacitors with high energy densities up to the range 70-‐80 Wh/kg at cell level have been shown. Compared to Li-‐ion batteries these energy levels are low, but compared to conventional super capacitors these numbers are about one order of magnitude higher. Having an increased energy density the power capabilities are not affected, resulting in an attractive candidate for high power demanding applications. The targets for battery packs for battery electric vehicles set by USABC are 350 Wh/kg and 750 Wh/L at cell level, and 235 Wh/kg and 500 Wh/L on system level [284]. Just considering the gravimetrical energy density the organic concepts may seem as the most attractive alternative, but when both the volumetric and the gravimetric densities are considered, the main attractive technologies are Li-‐based concepts: Li-‐ion battery technology with Si-‐based anodes, Li-‐batteries with metallic Li anode, Li-‐S, and Li-‐O2 technologies. Overall, high-‐voltage cathodes in combination with a metallic lithium anode are the more attractive concepts. The power capabilities of the cells are fundamental for the system design, and for the fast-‐charging possibilities. The C-‐rate is the mainly used reference and the power capabilities are related to the capacity of the cells. Moreover, if the cell is optimised towards energy or power will highly affect the power capabilities. The fast charging capabilities will follow the same trends. USABC power goals for different types of vehicles give needs at pack level of about 2-‐3 kW/kg for an HEV, about 1 kW/kg for a PHEV, and about 0.2 kW/kg for an EV [284]. The
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power densities of the reference cells used in this study (see Table 2) are in line with the goals of USABC; ca. 2 kW/kg for the power optimised cell and ca. 0.3 kW/kg for the energy optimised cell. The power capabilities are mainly determined by the electrode design. Smaller particles with high surface area and thinner electrodes are routes for increase the power capabilities, and at the same time the energy density will be sacrificed. Therefore, it is difficult to put numbers on the power densities and C-‐rates. In general terms, goal must be to achieve the same or improved power capabilities as of the present Li-‐ion cells. Today, indications for the Na-‐ion technology is comparable power capabilities as the Li-‐ion technology. For the other emerging technologies, lower capabilities are to be expected if the energy density advantages should be able to be utilised. The Mg-‐cells would need a C-‐rate of ca. C/5 to be attractive from an energy point of view. Even lower C-‐rates are needed for the Li-‐S and Li-‐O2 technologies, which will have lower or much lower power capabilities and C-‐rates of ca. C/20 are needed. As a consequence, fast-‐charging is an issue/challenge for these technologies.
3.4 Cost trends The next generation Li-‐ion battery cells are attributed with higher energy and power densities per cost due to cheap raw materials. The main trend towards lower cost for the current Li-‐ion technology is, however, driven by improved production processes, higher production volumes, standardised cell formats and balance of plant components. Hence cost reduction in general cannot be expected to follow any linear trend starting from materials cost. Both Total Battery Consultant and Avicenne have analysed the cost trends of improved Li-‐ion cells and battery packs, summarised in Table 7 [1,285]. It should be noted that the trends presented are related to cell and production optimisations of the present cell technologies and materials. Cost is related to the size and capacity of the batteries and also the production capabilities. The reference studies conclude a cell cost of 5 €/[kWh*Ah] for pouch cells and 50 €/[kWh*Ah] for 18650 cells for 2016. Table 7. Cost trends (€/kWh) for energy optimised Li-‐ion battery cells materials, cells, and packs for EV applications [1,285]. 2010 2015 2017 2020 Cost
reduction 2010-2020
Cell material 180 140 115 115 36% Cell 315 235 195 175 45% Battery pack w/o cells
210 135 60 55 74%
Total battery pack cost
525 370 255 230 56%
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As can be noted, the largest cost reduction potential is related to the battery pack components and assembly. This is related to mass-‐production and modularisation in the assembly. For the cost estimations and comparisons in the present study the following cell cost split will be used based on Avicenne’s data [285]:
-‐ 65% of the cell cost refers to material cost -‐ 40% of the material cost refers to the cathode, 12% to the anode, and 10% to the
electrolyte. Moreover, 75% of the cost of a battery pack is assumed be related to the cells. The estimated cost for the emerging battery technologies are based on assessments of a more complete field overview and are summarised in Table 7 including the main cost drivers. The cost estimates are related to the improvements of the present Li-‐ion technology and refers to the 2025 time line. Table 7. Cost trend estimates (cost/storage capacity) for emerging battery technologies, compared to the improved Li-‐ion technology (-‐ refers to relative cost reduction, + refers to relative cost increase). Technology Cost – cell Cost – pack* Cost driver Solid Li-‐metal
-‐ 8% -‐ 6% Anode cost 1/3 of Li-‐ion, no Cu used
Na-‐ion -‐ 13% -‐ 10% 20% lower cell material cost Mg ± 10% + 75% Low cell voltage Li-‐S -‐ 40% > 100% Low-‐cost raw materials. High pack cost
due to low cell voltage and poor rate capabilities
Li-‐O2 ± 0% + 250% Low electrode cost, high electrolyte cost, low cell voltage and poor rate capabilities, extra components for air/oxygen handling not included.
Organic -‐ 50% -‐ 35% Low cell voltage Asymmetric super capacitors**
± 0% ± 0% High rate capabilities, low energy density
*The same cost for electronics, control, and management are assumed for all technologies. **HEV application only. The main drivers for cost improvements are thus related to the cell capacity and rate capabilities. Cells of the same energy density and cell cost, but having a cell voltage 2 V lower will result in an increased pack cost of about 75%. The same is true if twice the number of cell strings is needed to fulfil the power requirements. Therefore, all emerging battery technologies having poor rate capabilities must exhibit an extremely low cell cost to compensate for this. Not only low cost of raw materials, but also low production costs have to be considered. Moreover, cells of low cell voltage must also exhibit much improved rate capabilities.
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3.5 Pro’s and con’s Based on the review of the research trends given in Chapter 2, the following pro’s and con’s and special needs of R&D efforts are concluded (all in relation to improved present Li-‐ion technology): Next generation Li-ion: + High cell voltage ! stable electrolyte needed. + Less amount of Co used ! reduced cost. Drawback: less interesting for recycling? + Minor system changes -‐ Small steps in energy density at cell level Solid-state Li: + High gravimetric energy density + Very high volumetric energy density + Cost reduction mainly due to no Cu needed for current collector. Drawback: less interesting for recycling? -‐ Increased operational temperature needed (ca. 60-‐90 °C) (could though be an advantage depending on the overall vehicle design) -‐ Safety issues related to metallic Li Na-ion: + Abundant and less expensive raw materials. Drawback: less interesting for recycling? + Same basic production tools and schemes as for Li-‐ion + Aluminium current collectors for both electrodes (cost and weight reduction). Drawback: less interesting for recycling? -‐ Slightly heavier and larger cells compared to Li-‐ion of the same capacity Mg: + High energy density due to two electron redox reaction -‐ Low cell voltage -‐ Costly electrolyte? Li-S: + High energy density at cell and pack level + Cost reduction: cheap cathode material -‐ Low cell voltage -‐ Poor rate capability -‐ Production process -‐ Safety issues related to metallic Li Li-O2: + High energy density at cell level -‐ Low cell voltage and large voltage hysteresis -‐ Poor rate capability -‐ Sensitive towards impurities, water, CO2, etc. due to open system -‐ Safety issues related to metallic Li -‐ Low energy density at pack/system level -‐ Complex vehicle integration
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Organic: + High gravimetric energy density + High rate capabilities -‐ Low volumetric energy density -‐ Low cell voltage -‐ System design constraints unknown Asymmetric super capacitors: + High rate capabilities + High cell voltage + Concepts in production -‐ Low energy density compared to battery technologies For all emerging battery technologies, the durability is an issue to be address and is impossible to predict based on cells at the laboratory scale.
3.6 Conclusions and proposed actions The challenges facing all emerging battery technologies beyond the current Li-‐ion technology are numerous. Issues remain to be solved on the cathode, the anode, and the electrolyte – i.e. on all parts. Besides the improvements of the Li-‐ion technology, there are attractive solutions regarding solid-‐state Li technologies and for Na and Mg based technologies. The drivers are mainly energy density, availability of raw materials, cost, and utilisation of more than one electron per transition metal. In addition to these technical challenges, there is continuing uncertainty about the ability of Li-‐O2 and Li-‐S batteries to meet the volume, power, and cost goals of automotive batteries. The different emerging battery technologies are at various development stages for being of interest in vehicle applications by 2025. The conclusions from this study are based on careful assessments on a broad field overview and can be summarised accordingly:
o Most suitable technologies for EV applications for 2025 are: next generation Li-‐ion and Li-‐metal polymer.
o Most suitable technology for HEV applications for 2025 is asymmetric super capacitors having increased cell voltages
This conclusion takes the following advantages in consideration: EV:
o High gravimetric and volumetric energy densities o High cell voltage, resulting in fewer cells needed and higher energy density o High rate capabilities o Communalities with today’s cell production o Pack complexity – no extra components needed. Heating system though needed
for Li-‐metal polymer. o Cost reduction potentials, both of the cell materials and the pack design.
HEV:
o High cell voltage, resulting in fewer cells needed and higher energy density
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o High rate capabilities o Communalities with today’s cell production o Pack complexity – no extra components needed.
The main cost reduction potential is, however, related to the pack design and components included in the pack except the cells. Preferred post-‐2025 technologies are Na-‐ion, Li-‐S, and Mg, mainly due to cost reduction potentials of the cells (Na-‐ion and Li-‐S) and two-‐electron redox reactions (Mg), and in additional all these technologies are more long-‐term sustainable. From this study the following recommendations and actions are proposed for further research and development of these technologies for vehicle applications:
o Continue to support efforts to stabilise the metallic lithium surface during cycling. Such efforts have benefits for Li-‐metal, Li-‐S and Li-‐O2 technologies.
o Advance the development of stable electrolytes for Mg-‐based technologies, as this is the main drawback of an attractive solution utilising two electrons in the redox reaction, and thereby double the capacity.
o Expand and evaluate options for new Li-‐S concepts that enable improved rate capability and cyclablity.
o Fundamental investigations of the reaction mechanisms and dynamics on the air/oxygen cathode.
o For all emerging battery technologies improved electrolytes are the key in order to increase the stability towards new cathode materials, to form stable electrode interfaces, to be stabilized against reaction products in both Li-‐O2 and Li-‐S cells, and last but not least to improve safety.
o Development of solid-‐state high-‐voltage concepts with increased loading of active materials, utilizing advanced solid electrolytes with higher conductivity, reducing the amount of electrolyte for increased energy density of the entire battery
o Continue the investigations of the stability of interphases between all components and materials of the cell, and mastering the interphase between the electrodes and the electrolyte through the use of new solvents, salts, additives, binders, etc.
o At a later stage – efforts on scaling up selected emerging battery technologies towards larger cells suitable for vehicle applications.
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