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2016 GCEP Report - External Project title: Self-Healing Polymers for High-Energy-Density Lithium Ion Battery Investigators Zhenan Bao, Professor, Chemical Engineering

Yi Cui, Professor, Material Sciences and Engineering

Michael Toney, Professor, SLAC

Zheng Chen, Postdoc Researcher

Sean Andrews, Postdoc Researcher

Jeffrey Lopez, Graduate Researcher

Abstract

In this report, we detail our recent progress toward the goal of enabling high energy density

lithium (Li) ion batteries through the application of self-healing polymers (SHPs). Here we

specifically focus on improving the cycling stability of silicon (Si) negative electrodes through

careful and systematic study of the effects of the SHP on electrode performance. Over the last

year, we have completed the synthesis of a family of SHPs with varied density of crosslinking

junctions in order to better understand how viscoelastic materials properties affect cycling

stability. We found that binders with relaxation times on the order of 0.1 s gave the best cycling

stability with 80% capacity maintained for over 175 cycles using large silicon particles (~0.9

um). We have also carefully investigated the structural change and interfacial stability of Si

particles during charging/discharge cycling. We found that our SHP can restrain the cracking of

the overall electrode layers and stabilize the solid-electrolyte-interface (SEI). Additionally, we

observe Si particles maintain electrochemical connection during the lithiation/delithiation

process, which is in stark contrast to behavior using conventional binders. Therefore, the overall

electrode structure, even with Si micropartilces, can be maintained stably over long-term cycling.

Finally, we discuss ongoing work to synthesize new SHPs to further improve electrode

performance.

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Introduction In light of the ever growing energy demand in our world today, it is critical to develop

sustainable methods of generating and storing this energy in order to prevent catastrophic climate

change. Specifically, transportation contributes 25% of global carbon dioxide emission, which is

one of the leading greenhouse gas products. The development and deployment of electric

vehicles (EVs) would significantly reduce these emissions by shifting transportation energy

generation from petroleum products to renewable sources like wind and solar or other more

efficient grid scale generation. The lithium-ion battery (LIB) is the most promising energy

storage candidate to power these electrical vehicles. Although current lithium ion batteries have

been very successful for portable electronic devices, they are currently too expensive and have

not achieved the performance benchmarks required for ubiquitous use in transportation

applications.

A typical LIB is based on the combination of a carbon anode and a lithium metal oxide or

phosphate cathode (LiCO2, LiMn2O4, and LiFePO4). The relatively low capacities of these

electrodes (370 mAh/g for graphite and 140-170 mAh/g for lithium metal oxides or phosphates)

limit the total specific energy of the battery. To meet the demands of mass market electrical

vehicle applications, much higher specific energy/energy density (3-5x) is needed. Improving the

energy density of LIBs requires exploiting new materials for battery anodes and cathodes.

Silicon (Si) is a promising candidate to replace graphite anodes due to its high theoretical

specific capacity of 3579 mAh/g, which is 10x higher than that of graphite. However, these

materials experience extreme, unavoidable expansion and contraction during the lithiation and

delithiation processes. Since Si is a brittle material, the large anisotropic stresses that build up

cause the active material to crack, and quickly fail. This capacity loss results from two

mechanisms. First, portions of the Si active material can become electrically disconnected from

the rest of the electrode thus preventing them from further charging and discharging.

Additionally, the cracks in the Si provide fresh surfaces for electrolyte decomposition to occur

forming new solid electrolyte interphase (SEI). Tremendous efforts have been made to address

these material challenges by using nanostructured active materials, including nanoparticles,

nanowires, porous structures, nanotubes, hollow particles, yolk-shell particles, thin films and

composite nanostructures. However, these improvements are still not enough to enable their

practical applications in electrical vehicles. Furthermore, the nanostructured materials make it

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more challenging to achieve robust electronic connections between nanoparticles. Even more

important, most nanostructures generally require complex and expensive synthesis and

fabrication processes. Cost and cycling stability thus remain significant barriers for alternative

high energy density LIB materials applied in transportation applications.

Our goal is to develop high-energy and long-lifetime lithium ion batteries by making the

electrodes able to repeatedly self-heal during electrochemical cycling. This can be realized by

combining high-capacity active materials with self-healing polymers. Cracks may form in the

self-healing polymer layer due to the huge volume expansion of Si, but, in contrast to traditional

polymer binders, these cracks can be healed automatically, leading to stable electrical

connections among the active particles. With this self-healing design the electrode should have

much improved cycle life. This design is a significant departure from traditional thinking in the

field since instead of trying to avoid cracks during cycling, the electrodes are coated with soft

material that self-heals, which maintains the electrode structure and thus enhances

electrochemical stability. We envision this concept can be widely applied to many types of high-

capacity active materials, including Si, Sn, S, and Ge.

Background

Si-based high-capacity anodes continue to attract growing interest due to their great potential

in next-generation lithium-ion batteries. However, cycling life and areal mass loading remain the

key issues to be addressed. During the past year, most of the developments in this field have still

been focused on making various nano- and/or porous Si structures to mitigate mechanical

failures inside electrodes. However, there has been only incremental improvement on Si

electrode performance. Related to our approach, a few groups continue to develop new polymer

binders to improve the Si cycling stability, but all of the work is based on Si nanoparticles, which

has significant limitations as we mentioned above. Coskun et al have also utilized our

supramolecular approach to develop a host-guest binder that enable electrode level healing of

nano-Si via dynamic crosslinking [1]. Additionally, micron-sized particles have been shown by

Cui et al to have stable cycling after encapsulation in a graphene cage to limit SEI formation on

fractured Si [2].

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Results I. Understanding of Self-healing Polymer Binder

In an effort to improve the performance of our previously reported self-healing polymer

binder [3] and to better understand the requirements of polymer binders in Li-ion battery

applications, we varied the viscoelastic properties of our SHP by fine control of the density of

crosslinking junctions. From this synthesis, we obtain a family of low glass transition

temperature (Tg) supramolecular self-healing polymers. These materials are very similar to

supramolecular hydrogen bonding polymer we used in our initial work and originally developed

by Leibler in 2008 [4]. This material is based on fatty acid derived molecules that are first

functionalized with terminal amines and further reacted with urea to create a large density of

hydrogen bonding sites as the end groups of oligomeric molecules (Figure 1). These

polydisperse oligomers spontaneously associate via their hydrogen bonding end groups to form a

supramolecular polymeric material. This material has been shown to self-heal at room

temperature without external stimulus via these hydrogen-bonding groups.

Figure 1. Synthesis of self-healing polymers. A two-step condensation reaction is used to obtain

randomly branched oligomers with urea functionalized end groups that associate via hydrogen

bonding to form a supramolecular polymer. X is varied between 30% and 70% to obtain a family

of SHPs with different mechanical properties.

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We first measured the viscoelastic properties of this family of materials at room temperature

using frequency sweep and stress relaxation experiments (Figure 2). In each material, the loss

modulus dominates for most frequencies and the stress relaxation is complete on very fast time

scales. This illustrates significant plastic deformation and the strong liquid-like characteristics of

these materials, which contributes to the self-healing capability of our electrodes. As was

expected when comparing the materials, the polymers with a higher density of theoretical

crosslinking junctions are stiffer, with a higher resistance to deformation. The more crosslinked

materials also have higher storage (G’) and loss (G”) moduli. We extracted relaxation times

from both the frequency sweep and stress relaxation data. The relaxation time represents a

characteristic time scale required for the polymer chains to respond to external stress, and was

used to quantify the viscoelasticity of each crosslinking ratio. We see a good agreement between

the frequency sweep and stress relaxation data, and as the percentage of theoretical crosslinking

junctions was increased from 16 wt% to 70 wt% there was an increase in relaxation time over

two orders of magnitude. Specifically, the polymer with 16 wt% trifunctional fatty acids was the

softest with a relaxation time on the order of 10-2 s. The intermediate polymers with 30 wt%, 43

wt%, and 57 wt% trifunctional fatty acids all had relaxation times on the order of 0.1 s, and the

most crosslinked polymer had a relxation time on the order of 1 s.

Figure 2. a) Frequency sweep at 1% strain of polymers at various crosslinker ratios. Loss

modulus dominates at most frequencies and both moduli increase with crosslinking. b) Stress

relaxation of polymers at various crosslinker ratios at 10% strain. Relaxation time increases with

crosslinking concentration, but all materials fully relax showing large plastic deformation.

a) b)

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Once the materials had been characterized, the electrochemical performance of the Si/SHP

electrodes was tested. High mass loading (0.75-1.1 mg cm-2) of 800 nm Si particles was used in

all cells in an effort to ensure that any positive performance was not just due to low mass

loading, which is not relevant for commercial applications. After loading the Si particles, a

coating of SHP/CB composite was applied to the electrode. After 3 activation cycles at C/20, the

cells were all cycled at C/10 until they reached 80% of their initial capacity. Surprisingly, there

was little difference in the performance of the cells with different amounts of crosslinking until

the amount of trifunctional groups in the polymer was increased to 70 wt%. Until this point,

cells had a specific capacity ~1700 mAh/g after activation (1st C/10 cycle) and reached ~1360

mAh/g (80%) at an average of 133 cycles, 146 cycles, 178 cycles, and 159 cycles, for 16 wt%,

30 wt%, 43 wt%, and 57 wt% trifunctional SHP respectively (Figure 3a). This relatively low

capacity compared to Si nanoparticle electrodes is likely due to poor activation of the large sized

Si particles and the increased Ohmic resistance of the thick Si layer. As we have shown before,

further improvement to the electrochemical performance of these cells can be achieved by using

thinner and more conformal SHP coatings, and by mixing in conductive additives with the Si

particles to improve the conductivity of the Si layer and improve the activation, but such

performance optimization was precluded by the necessity for consistent electrodes for each

polymer binder tested. In contrast to the other electrodes, the cells with the most crosslinked

binder only had a specific capacity of ~1300 mAh/g after activation and reached 80% of their

initial capacity after only 83 cycles. This result illustrates a further reduced activation of the Si

particles. This is due to a reduced ability of the 70 wt% trifunctional SHP to flow over newly

exposed Si surfaces after fracture. Additionally, the application of the composite coating was

noticeably more difficult for the highly crosslinked binder and resulted in generally thicker

binder layers. This increased thickness likely caused fewer of the Si particles to participate in

lithiation. Using slower rates (C/40) allowed the capacity to increase to ~2400 mAh/g for the 70

wt% trifunctional SHP and showed that the low capacity is a result of kinetic issues in the

electrodes (Figure 3c). The performance of the Si/SHP electrodes is limited at high rates due to

the poor ionic conductivity of the binder and the large particle size.

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Figure 3. a) Cycling stability of SHP binder with 800 nm Si particles. b) Charge/discharge

profile of SHP binder showing normal Si lithiation and delithiation profiles. c) Capacity of 70

wt% Triacid Si/SHP electrode at different C-rates.

The increased rate of capacity decay is related to the relaxation time. Between the binders

with 57 wt% and 70 wt% crosslinking there is an increase in the relaxation time of more than an

order of magnitude. As the crosslinking is increased to 70 wt% the bulk material is less able to

flow, and its rheological properties are markedly different from those in the rest of the study with

a much slower stress relaxation and higher shear modulus. This prevents the polymer from

maintaining the electrode’s capacity during cycling in the same way that the other polymers do.

More specifically, for each of the other binders with lower crosslinking tested in this work the

smaller change in relaxation time and other viscoelastic properties only slightly affected cycling

a)

c) b)

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performance. The viscoelasticity of our SHP allows the binder to accommodate the stress that

builds up at the Si particles expand. The low Deborah numbers (De = τchar/τobs) of these SHPs

also allow it to flow and cover any new Si surface that is exposed as the particles crack. Even the

slowest relaxation time measured for the 70 wt% trifunctional SHP was ~10 s, which is 2-3

orders of magnitude faster than the charging or discharging time of a battery. We suspect that

this flow property is the main mechanism of improving cycling stability in our self-healing

binder and maintains the electrical connectivity of these Si pieces. While the mechanism and

performance of the materials with lower crosslinking are similar, the three intermediately

crosslinked polymers (30 wt%, 43 wt%, 57 wt%) show a slight improvement in cycling stability

over the 16 wt% trifunctional material. This suggests that there is a balance between stress

dissipation and the maintenance of the mechanical integrity of the electrode. The materials with

the slightly slower relaxation time could provide an improvement in the mechanical stability of

the electrode during the volume changes that occur with the Li alloying and dealloying processes

without sacrificing the flow that allows for long cycling life in the first place. After the fracture

of the large Si particles has been stabilized, the electrodes with different binders all are able to

maintain similar capacity decay, except for the binder with the highest amount of crosslinking,

due to their identical backbone and end group structure. This similar cycling stability over a

wide range of polymer viscosities is an exciting property of our SHP for industry applications,

and will make the polymer useful in a variety of processing situations without concern for the

final device performance.

In conclusion, this study has shown that the viscoelastic properties of our supramolecular

SHP binder can be varied over a wide range with little effect on the binder’s performance in Si

negative electrodes. This is because the relaxation times measured at all crosslinking ratios,

while varied, were still very small when compared to battery cycling times. This ability to vary

the viscosity of our polymer without affecting cycling stability makes our binder attractive for

industry where it can be modified to meet various processing demands. It was found that a

relaxation time on the order of 0.1 s gives the best cycling performance due to a balance of

viscous flow and improved mechanical integrity. Large Si particle electrodes could be cycled for

over 175 cycles while maintaining above 80% capacity for three of the five binders tested. We

also found that, at a high amount of crosslinking, the benefits of the SHP are lost when

processing becomes more difficult.

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II. Investigation of Structure and Solid-Electrolyte-Interface (SEI) Stability

Our SHP coated Si electrodes show significantly higher cycling stability than conventional Si

electrodes. The cycling performance of Si electrodes made from different polymer binders (SHP,

CMC and PVDF) are compared using electrochemical and ex-situ SEM and XPS. The Si/SHP

electrodes showed much better capacity retention than Si/CMC and Si/PVDF electrodes, which

is consistent with our previous work. This result confirms a much better stability of Si electrode

structure provided by the SHP. We examined the structure and morphology of electrodes before

and after cycling test. Electrode scanning electron microscopic (SEM) images show that the

surface of Si/SHP electrodes maintained high integrity while both Si/CMC and Si/PVDF

electrodes exhibited clear cracks. The effect of SHP on the formation of solid-electrolyte-

interface (SEI) is investigated by X-ray photoelectron spectroscopy (XPS), which showed that

the SEI can strongly attach on the surface of Si particles and maintain a uniform coating even

during cycling, unlike Si/CMC electrodes. A possible mechanism is also provided. For Si/SHP

electrodes (Figure 4a), a robust SHP layer is coated on Si particle surface, which not only

maintains the mechanical integrity of electrode during drastic structure change, but also protects

the Si surface from uncontrolled growth of SEI layer. For Si/CMC and Si/PVDF electrodes

(Figure 4b), the Si particles often lose electrochemical connections after cycling due to their

instable coatings. And moreover, the particle surface is exposed due to loss of polymer coating

and/or uncontrollable growth of local SEI growth.

Figure 4. The schematic showing the mechanism of SHP and CMC binder on the stabilizing of

electrode structure and their SEI formation behavior. (a) Si/SHP and (b) Si/CMC.

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III. X-ray Investigations of Self-Healing Silicon Anodes

In order to further understand the effects the SHP binder has on the SiMPs, we monitored

particle behavior at various points during the electrochemical process using operando and in-situ

synchrotron-based x-ray techniques. Specifically, transmission x-ray microscopy (TXM) and

small angle x-ray scattering (SAXS) were used to track particle changes as a result of the

lithiation and delithiation process, on the single particle and ensemble levels, respectively.

Surprisingly, particles that were active in the 1st cycle remained active in the second cycle, which

is a departure from what has been observed in other high expansion anodes when PVDF is used

as the binder. In work done by Nelson-Weker et al., micron-sized Ge particles in PVDF that

were active in the first cycle were found to become inactive in the second. It was postulated that

the physical expansion and contraction that occurs during lithiation and delithiation severs the

electrical connection between the particle and the rest of the circuit. The Si-SHP system breaks

from this trend, wherein the electrical connections are not lost during the large structural changes

that occur during cycling. Therefore, through a combination of some viscosity and strong

attraction to the hydrophilic Si surface, the SHP is able to maintain the spatial relation between

the Si particle and an electrical connection (carbon black) despite the large changes in volume.

The special confluence of properties and interactions that exist in the Si-SHP system were

also observed on an ensemble level using SAXS. Briefly, SAXS measurements can provide

particle size, shape, and porosity information of an aggregate sample. Here, we used SAXS to

track Si particle size as a function of cycle. These experiments did not require the same low

loadings as previously mentioned for imaging, therefore it was possible to map cycling

performance to the measured signal. Qualitatively, this indicates that Si particles are cracking

into smaller particles after expansion and contraction. These differences in particle sizes for the

different binders can be explained in combination with what was observed in the TXM images.

Within the PVDF electrodes, the electrical connections to many of the Si particles break during

the first cycle, leading to electrically isolated particles that are unable to participate in the

lithiation process. These particles avoid continual fracture, thus remaining large. With use of the

SHP binder, however, these connections are not broken, leading to continual expansion and

fracture of all Si particles. By the fifth cycle, average particle size is nearly to the point where

expansion does not lead to further fracture (~150 nm). Therefore, the SHP binder does not

prevent cracking and fracture from occurring, but rather accommodates it.

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Progress In the past year, self-healing polymers with different mechanical properties were designed

and synthesized successfully. We identified the ideal range of relaxation times (on the order of

0.1 s) for maximum cycling stability. At the same time, detailed studies of the electrode

morphology and SEI composition were carried out. With careful investigation of electrode

structure and SEI formation, it was found that a robust SHP layer is coated on Si particle surface

in Si/SHP electrodes, which not only maintains the mechanical integrity of electrode during

drastic structure change, but also protects the Si surface from uncontrolled growth of SEI layer.

For Si/CMC and Si/PVDF electrodes, their Si particles often lost electrochemical connections

after cycling due to their instable coatings. Additionally, we directly observed that Si particles

are able to maintain electrical connectivity during cycling when SHP binder is used, unlike in for

electrodes using conventional non-adaptable binders. This strategy will enable next-generation

lithium ion battery with about 30% of higher energy density, corresponding to about 10% of

potential greenhouse reduction in the transportation sector (responsible for for ~30% of global

greenhouse).

Future Plans 1) Using what we have learned from the study of our modified self-healing binder

systems, we have designed a new generation of self-healing polymers specifically for

battery applications. Electrochemical characterization is ongoing, and we expect these

new tailored polymers to provide further enhancements to the cycling stability of

silicon electrodes and potentially other active materials.

2) Soft, self-healing elastomeric materials may provide further performance

enhancements base on our self-healing electrode concepts. We plan to synthesize a

covalently crosslinked elastomer that is capable of undergoing large, highly reversible

deformation to accommodate the expansion of the Si particles and provide a restoring

force to help maintain electrical connectivity of the electrode.

3) New processing strategies that allow us to use a slurry coating method will be

explored. We plan to use different solvents with newly designed self-healing

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polymers to create stable slurry that is more compatible with current industrial

process.

Publications and Patents 1. Lopez, J.; Chen, Z.; Wang, C.; Andrews, S. C.; Cui, Y.; Bao, Z. The Effects of Cross-Linking in a

Supramolecular Binder on Cycle Life in Silicon Microparticle Anodes. ACS Appl. Mater. Interfaces 2016, 8, 2318–2324. DOI: 10.1021/acsami.5b11363

2. Y. Sun, J. Lopez, H.-W. Lee, N. Liu, G. Zheng, C.-L. Wu, J. Sun, W. Liu, J. W. Chung, Z. Bao, and Y. Cui, "A Stretchable Graphitic Carbon/Si Anode Enabled by Conformal Coating of a Self-Healing Elastic Polymer", Advanced Materials 2016, 28, 2455-2461. DOI:10.1002/adma.201504723

3. Andrews, S. C.; Chen, Z.; Lopez, J.; Weker-Nelson, J; Cui, Y.; Toney, M; Bao, Z. “Understanding the Role of Self-Healing Polymers in Long Cycle Life Silicon-Based Lithium Ion Battery Electrodes”, In Preparation, 2016

Presentations

1. Lopez, J. Effects of crosslinking in a supramolecular binder on cycling stability of silicon microparticle anodes. Oral presentation at the ACS 251st National Meeting, PMSE Division, March 16, 2016

2. Zheng Chen, Yi Cui, Zhenan Bao, Self-healing Polymer for High-Performance Si Anode in Li-ion Batteries, Oral presentation at the AICHE meeting, Nov 10, 2015.

3. Sean C. Andrews, Zheng Chen, Jeff Lopez, Michael Toney, Zhenan Bao, Operando X-Ray Imaging of Self-Healing Silicon Anodes in Lithium Ion Batteries, Beyond Lithium VIII, June 3, 2015

4. Sean C. Andrews, Zheng Chen, Operando X-Ray Investigations of Self-Healing Silicon Anodes in Lithium-Ion Batteries, Materials Research Society Meeting, April 4, 2016

References

1. Kwon, T.-W.; Jeong, Y. K.; Deniz, E.; AlQaradawi, S. Y.; Choi, J. W.; Coskun, A. Dynamic Cross-Linking

of Polymeric Binders Based on Host-Guest Interactions for Silicon Anodes in Lithium Ion Batteries. ACS

Nano 2015, 9 (11), 11317–11324.

2. Li, Y.; Yan, K.; Lee, H.-W.; Lu, Z.; Liu, N.; Cui, Y. Growth of Conformal Graphene Cages on

Micrometre-Sized Silicon Particles as Stable Battery Anodes. Nat. Energy 2016, 1 (2), 15029.

3. Wang, C.; Wu, H.; Chen, Z.; McDowell, M. T.; Cui, Y.; Bao, Z., Self-healing chemistry enables the stable

operation of silicon microparticle anodes for high-energy lithium-ion batteries. Nature Chemistry 2013, 5.

4. Cordier, P., Tournilhac, F., Soulié-Ziakovic, C. & Leibler, L. Self-healing and thermoreversible rubber

from supramolecular assembly. Nature 451, 977–980 (2008).

5. (a) Chan, C. K.; Ruffo, R.; Hong, S. S.; Cui, Y., Surface chemistry and morphology of the solid electrolyte

interphase on silicon nanowire lithium-ion battery anodes. Journal of Power Sources 2009, 189 (2), 1132-

1140; (b) Schroder, K. W.; Celio, H.; Webb, L. J.; Stevenson, K. J., Examining Solid Electrolyte Interphase

Formation on Crystalline Silicon Electrodes: Influence of Electrochemical Preparation and Ambient

Exposure Conditions. The Journal of Physical Chemistry C 2012, 116 (37), 19737-19747.

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Contacts Zhenan Bao: zbao@stanford.edu Yi Cui: yicui@stanford.edu Michael Toney: mftoney@slac.stanford.edu Zheng Chen: zchen8@stanford.edu Sean Andrews: sean147@stanford.edu Jeffrey Lopez: jlopez12@stanford.edu