공학석사 학위논문

Development of electrolytes to

suppress the dissolution of polysulfides

for Li-S batteries

리튬-황 전지의 다황화물 용해 억제를 위한

전해질 개발

2019년 2월

서울대학교 대학원

화학생물공학부

양 홍 선

i

Abstract

Development of electrolytes to

suppress the dissolution of polysulfides

for Li-S batteries

Hongsun Yang

School of Chemical & Biological Engineering

Seoul National University

Lithium-sulfur batteries are one of the most promising next-generation

batteries due to its high theoretical energy density, which is up to 6 times that of

lithium-ion batteries, and the abundance of active material sulfur reducing the cost.

The demand for batteries with high energy density is on the rise and this is

emphasized in the case of electric vehicles that need to be able to travel at least

500km.

However, the commercialization of Li-S batteries is hindered due to several

issues that remain to be solved. The most important problem is the insulating

nature of sulfur because it limits the utilization of the active material. Sulfur

requires a conductive network that can transport electrons instead of it. Therefore,

carbon/sulfur composites are the main active material that is widely researched to

maximize the energy density.

Since sulfur particles relatively far from the carbon network is unable to

contribute to the electrochemical reaction, nano-sized porous carbon became an

ideal material candidate and typical electrolytes used for this system dissolve sulfur

and its reduction product polysulfides to ensure the contact with conductive carbon.

These electrolytes, which usually are ether electrolytes, have high solubilites of

polysulfides and low viscosities to improve electrode reactivity.

ii

Catholyte-like electrolytes suffer from active material loss because of the side

reactions of polysulfide and lithium metal. The consequential shuttle phenomenon

renders the battery dead due to continuous decrease in capacity and efficiency of

recharging.

In this thesis, I will propose a nonpolar electrolyte in order to suppress the

dissolution of polysulfides. Controlling the solubility of polysulfide is key to

separating the issues of increasing the sulfur composition and the shuttle effect

caused by polysulfide dissolution with the use of soluble electrolyte. The

performance of this nonpolar electrolyte will be compared to common electrolytes

and electrochemical studies will analyze the difference of this novel electrolyte.

iii

Keywords : Lithum-sulfur battery, nonpolar electrolyte, sparingly solvating

electrolyte, shuttle phenomenon suppression, difluorobenzene, crown ether

Student Number : 2016-25285

iv

Table of Contents

Abstract ....................................................................................................... i

List of Tables .............................................................................................. v

List of Figures ........................................................................................... vi

Chapter 1. Introduction ............................................................................ 1

1.1. A general introduction of Li-S batteries ........................................... 1

1.2. Electrochemistry ............................................................................... 3

1.3. Sulfur cathode ................................................................................... 7

1.4. Lithium anode ................................................................................. 11

1.5. Electrolyte ....................................................................................... 12

1.6. Objectives of this thesis .................................................................. 15

Chapter 2. Experimental details ............................................................. 16

2.1. Materials preparation ...................................................................... 16

2.2. Characterizations and electrochemical tests ................................... 17

Chapter 3. Development of electrolytes to suppress the dissolution of

polysulfides for Li-S batteries ................................................................. 18

3.1. Results and discussion .................................................................... 18

Chapter 4. Conclusion ............................................................................. 33

References ................................................................................................. 34

국문초록 (Abstract in Korean) .............................................................. 43

v

List of Tables

Table 2.1. Solubility limits of K2Sx determined by UV-Vis spectroscopy at an

elevated temperature 60℃ and ionic conductivities at 45℃ for different

electrolytes. ................................................................................................. 20

vi

List of Figures

Figure 1.1. Electrochemistry of the Li-S battery at different stage: a) open circuit;

b) discharge process; c) charge process. d) Typical cyclic voltammogram (CV) for

an S cathode in a Li-S battery. ......................................................................... 4

Figure 1.2. a) A schematic diagram of the S (yellow) confined in the

interconnected pore structure of mesoporous carbon, CMK-3. b) Cycling stability

comparison of CMK-3/S-PEG (upper points, in black) versus CMK-3/S (lower

points, in red) at 168 mA g-1 at room temperature. ............................................ 9

Figure 1.3. Hierarchical designs of carbon-based sulfur composites: (a)

microporous carbon spheres, (b) spherical ordered mesoporous carbon

nanoparticles, (c) porous hollow carbon, (d) graphene oxide sheets, (e) porous

carbon nanofibers, and (f) hollow carbon nanofibers to encapsulate sulfur. ....... 10

Figure 1.4. a) Cyclic performance and Coulombic efficiency of Li-S batteries at a

current rate of 0.2C. SEM images of b) fresh Li metal and c) Li metal with a SIS

electrolyte after 280 cycles in Li deposition and dissolution experiments. ........ 14

Figure 2.1. Polysulfide solubility measurement done by UV-Vis spectroscopy. (a)

Visual confirmation of polysulfide solubility in various solutions. The UV-Vis

spectra of potassium (poly)sulfide in (b) DFB-CE solution at 45℃ and (c)

TEGDME electrolyte at 45℃. ....................................................................... 19

Figure 2.2. 1st cycle voltage profiles during galvanostatic cycling at C/30 rate

(1C=1675 mAhg s-1) with CMK-3/S cathodes (a) at 30℃ in DFB-CE compared to

DOL-DME and (b) at 45℃ in DFB-CE compared with DOL-DME. Specific

discharge and charge capacities over multiple cycles for S/CMK-3 cathode in DFB-

CE are compared to DOL-DME (c) at 30℃ and (d) 45℃. The DOL-DME

electrolyte contains 0.1M LiNO3 additive. ..................................................... 22

vii

Figure 2.3. 1st cycle voltage profiles of Li-S cells using S/CMK-3 cathodes with

DFB-CE (a) at C/30 rate and different temperatures and (b) at a temperature 45℃

using various cycling rates(1C=1675 mAhg s-1). GITT profiles of (c) DOL-DME

and DFB-CE at 30℃ and (d) DFB-CE at 45℃. .............................................. 24

Figure 2.4. Ex-situ XRD characterization of sulfur electrochemistry in DFB-CE

using S/CMK-3 cathodes. (a) The Ex-situ XRD patterns of S/CMK-3 cathodes in

DFB-CE at 45℃ at varied stages and (b) the corresponding discharge/charge

profile. The red pattern indicates the end of discharge and the blue pattern

corresponds to the end of charge. ................................................................... 26

Figure 2.5. TGA curve of CMK-3/S composite active material. ..................... 27

Figure 2.6. Electrochemical performance comparison of different cathodes C/S,

KB/S, CMK-3/S with DFB-CE. The 1st cycle voltage profiles at C/30 rate and (a)

30℃ and (b) 45℃. ....................................................................................... 29

Figure 2.7. Ex-situ SEM images and EDS of (a, d) a pristine CMK-3/S cathode,

(b, e) a retrieved CMK-3/S cathode cycled in DFB-CE for 10 cycles, and (c, f) a

CMK-3/S cathode in DOL-DME after 10 cycles. All cells were cycled at 45℃ and

C/30 rate. ..................................................................................................... 30

Figure 2.8. Linear sweep voltammetry (LSV) scans of DFB-CE with Li metal as

counter and reference electrode and (a) Cu as the working electrode for lower limit

and (b) Al as the working electrode towards higher limit. Both cells were scanned

at 45℃ and with scan rate 1mV s-1. ............................................................... 32

１

Chapter 1. Introduction

1.1. A general introduction of Li-S batteries

The increase of global population and industrialization in developing nations

has brought forth an unprecedented need for more energy. In 2012, 70% of the

world’s electrical energy came from fossil fuels [1] and the following critical

environment issues and demand of new forms of energy have been one of the

forefront of problems mankind needs to address.

In the 1960s, the first lithium-sulfur battery was proposed [2], but the concept

was abandoned because of the lack of tools to understand the system and other

systems were simply better. In the 90s, the lithium ion batteries were

commercialized due to its long lifetime and larger energy densities than most

secondary batteries [3-6]. Li-ion batteries naturally became the focus of numerous

researches which aimed to develop it to be suitable for electric vehicles. Nearly 50

years later since its first appearance, Li-S batteries were addressed again due to

demands of yet higher energy density which Li-ion batteries simply could not reach

[7-10].

Most transition metals used as intercalation materials for the Li-ion batteries

are only able to support the transport of only one electron per site due to the

instability of higher redox states [11]. This limited the theoretical capacity of Li-ion

battery technology and optimal capacity has almost reached the theoretical value.

Compared to Li-ion batteries, Li-S batteries have higher energy densities and

the abundance of sulfur makes it more appealing to lower the cost. Sulfur being a

nontoxic element is also another advantage since the environmental pollution risk

will be lesser. In short, Li-S batteries are expected to be the next-generation energy

２

storage system that could possibly replace conventional Li-ion batteries.

３

1.2. Electrochemistry

A typical Li-S battery is composed of a sulfur cathode, a lithium anode, and an

electrolyte. The open-circuit voltage of the Li-S battery is proportional to the

electrochemical potential difference between the sulfur cathode and the lithium

anode [12]. During battery discharge, sulfur S8 is reduced by reacting with lithium

and forms polysulfide intermediates (Li2Sx, x = 2~8). At the end of discharge,

lithium sulfide (Li2S) is generated and the electrochemical potential of the cathode

increases until the battery reaches the terminal voltage. The two-electron redox

reaction of the Li-S battery endows the electrodes with high theoretical capacities

and high specific energy.

However, there are several problems that hinder the application of Li-S

batteries for practical usage. Firstly, the low electronic and ionic conductivities of

sulfur and lithium sulfide (Li2S) increases the internal resistance of the battery.

High internal resistance results in large polarization and the decrease of energy

efficiency. The insoluble insulation layer of Li2S2 and Li2S, which forms on the

surface of sulfur particles during discharge, impedes sulfur reduction and

contributes to conductivity decrease. Thus, the utilization of insulating sulfur

becomes poor.

The second problem is the polysulfides crippling the battery performance. The

first plateau of the discharge curve corresponds to the transformation of sulfur S8 to

Li2S4. The second plateau is where the reduction of Li2S4 to Li2S2 occurs and then

further reduction results in lithium sulfide (Li2S). Multiple polysulfide species are

in equilibrium because the Gibbs free energy of formation of the polysulfides,

which are formed by the sulfur reduction in the presence of Li ions, are very close.

４

Figure 1.1. Electrochemistry of the Li-S battery at different stage: a) open circuit;

b) discharge process; c) charge process. d) Typical cyclic voltammogram (CV) for

an S cathode in a Li-S battery. Adapted from Reference [13] (Yin, Y.X., et al.,

Lithium-sulfur batteries: electrochemistry, materials, and prospects. Angew Chem Int Ed

Engl, 2013. 52(50): p. 13186-200.)

５

Disproportionation and chain-like polysulfide growth reactions occur and

multiple polysulfide species are simultaneously formed. Disproportionation is a

type of redox reaction in which species are simultaneously reduced and oxidized to

form multiple products.

The lower plateau region of the charge curve accounts for the oxidation of

Li2S to short-chain polysulfides and further oxidation to high-order polysulfides. S8

is the final product of the following second plateau overall reaction. The Li-S

battery cyclic voltammogram (CV) shows two pairs of redox peaks that match with

the plateaus of the discharge-charge profiles.

The polysulfides (Li2Sx, x = 4~8) dissolve into the electrolyte and may react

with it to cause capacity loss. Dissolved polysulfides are capable of migrating to

the Li metal anode by a phenomenon called the shuttle effect and form an

electrochemically inactive layer of Li2S2 and Li2S. The shuttle phenomenon

irreversibly harms the battery performance and prevents the battery from achieving

the full capacity of the sulfur cathode. As a result, the full recharge of the Li-S

battery becomes impossible [14-20].

The third problem is the significant change in volume of sulfur in the cathode

during cycling [21]. S8 has higher density than the final product of discharge, Li2S

and thus sulfur expands as it accepts lithium. Li2S is pulverized and it loses

electrical contact with the conductive carbon substrate. If the fraction of carbon-

contacting sulfur decreases, the capacity drops significantly. The volume change of

sulfur is a safety threat, so it must be contained within the carbon structure.

The lithium metal anode also has several problems to address. Since the Fermi

energy of lithium is higher than the unoccupied molecular orbital of common liquid

electrolytes, they cause side reactions and the electrolyte is reduced on the Li anode

６

surface and a solid electrolyte interphase (SEI layer) is formed. This SEI layer

causes capacity loss and lowers deposition efficiency of lithium during the

charging process [22-24]. Uneven deposition of lithium can cause the growth of

dendrites which may result in safety problems due to short-circuiting [25].

The electrolyte directly influences battery performance. The compatibility of

the electrolyte in choice with the Li-S battery electrochemistry is important. Ether

electrolytes, which were reported to be highly active in facilitating the

electrochemical reactions between lithium and sulfur, have also exhibited high

solubilities of polysulfides which led to strong shuttle effects [26]. Ionic liquid

electrolytes [27] and solid state electrolytes [28] are capable of forestalling the

shuttle effect, but are limited with slow Li+ ion diffusion due to high viscosity or

the solid state nature of the electrolyte [29].

７

1.3. Sulfur cathode

The sulfur cathode endows the Li-S battery with high energy density, but has

several problems as well. Sulfur is unsuitable for direct use as an active material

because of its poor electrochemical properties [30]. To overcome this weakness,

sulfur is usually combined with a carbon substrate with high electronic

conductivity to improve electrical contact. Porous substrates are effective at

adsorbing polysulfides and they can restrain sulfur from dissolving into the

electrolyte. Also, the elastic properties of these substrates can deal with the volume

variation of sulfur.

Two common methods to obtain carbon-sulfur composites are surface coating

and sulfur loading. Sulfur coating is an effective method for improving the

electrochemical performances of sulfur cathodes. Carbon is the ideal coating

material because of its high electrical conductivity, elastic nature, and dense

structure. Graphene is usually utilized since the thermal heat treatment needed is

relatively low in temperature and the high conductivity, large surface area, and 2D

morphology are strong advantages. Graphene easily wraps around sulfur particles

and forms a conducting network. Which provides excellent electrical contact and

flexibility. However, if the graphene composition is too high, then the sulfur

content will not be enough and the desired energy density will not be reached.

Conductive polymers, such as polypyrrole and polythiophene, are other

materials used for sulfur coating [31-34] . The polymer-sulfur composite is a core-

shell structure in which the polymer shell transfers electrons through its conductive

polymer chain. The elastic polymer framework accommodates the sulfur volume

variation but the inner part of the sulfur particle is insufficiently utilized.

８

Surface coating sulfur particles go against the utilization of sulfur and perfect

coating has been proven difficult. The dissolution of polysulfides is inevitable and

the cycle performance of sulfur is not guaranteed for this method.

Loading sulfur within porous carbon substrates is an attractive method that

has already been applied to other metal based secondary batteries. Ordered

mesoporous carbon CMK-3 loaded with sulfur was the first case to prove that

porous substrates were effective at improving the cycle performance of Li-S battery.

Porous substrates may vary in chemical composition and structure [35-43], but they

have a common goal of retaining sulfur. The pores are classified into macropores

(pore dimeter > 50 nm), mesopores (2 ~ 50 nm), and micropores (< 2 nm). The

main issue of incorporating sulfur into porous carbon is maintaining a high level of

sulfur composition without compromising the carbon structure.

Macropores are unsuitable for Li-S batteries since their large size makes it

difficult to enclose sulfur and suppress polysulfide dissolution. Most researches

focus on mesoporous substrates because it exhibits high sulfur loading and

relatively effective constrainment of polysulfides via strong interactions between

the carbon substrate and sulfur. Sulfur stored in mesoporous channels exist in small

sized particles and thus the electrochemical activity is enhanced. The downside are

the dissolution and shuttle effect of polysulfides remaining unsolved. Microporous

carbon [44-47] has an average pore diameter of 1 nm, which can greatly improve

the electroactivity and cycle performance of the Li-S battery. However, the

problems of the electrochemistry of elemental sulfur still remains an issue.

９

Figure 1.2. a) A schematic diagram of the S (yellow) confined in the

interconnected pore structure of mesoporous carbon, CMK-3. b) Cycling stability

comparison of CMK-3/S-PEG (upper points, in black) versus CMK-3/S (lower

points, in red) at 168 mA g-1 at room temperature. Adapted from Reference [13]

(Yin, Y.X., et al., Lithium-sulfur batteries: electrochemistry, materials, and prospects.

Angew Chem Int Ed Engl, 2013. 52(50): p. 13186-200. Copyright 2009, Nature Publishing

Group.)

１０

Figure 1.3. Hierarchical designs of carbon-based sulfur composites: (a)

microporous carbon spheres, (b) spherical ordered mesoporous carbon

nanoparticles, (c) porous hollow carbon, (d) graphene oxide sheets, (e) porous

carbon nanofibers, and (f) hollow carbon nanofibers to encapsulate sulfur. Adapted

from Reference [10] (Manthiram, A., Y. Fu, and Y.-S. Su, Challenges and Prospects of

Lithium–Sulfur Batteries. Accounts of Chemical Research, 2013. 46(5): p. 1125-1134)

１１

1.4. Lithium anode

The lithium anode has been studied for a very a long time since the 70s [48-

51]. The problems caused by the lithium metal are mainly due to the side reactions

it causes with the electrolyte solution and further involving polysulfides. The Li

anode could be isolated from the electrolyte solution and polysulfides by

introducing a thin and stable passivation layer on the surface. This passivation layer

is designed to protect the Li anode and permeable for Li+ ions.

The metallic Li anode is prone to dendrite formation, low deposition

efficiency, and deposition of Li2S2 and Li2S. Finding a suitable surface

modification to protect the Li anode while maintaining high energy capacity and

sufficient rate capability will be a daunting task.

The safety risk of metallic Li anodes is another problem that must be solved

for its practical application to Li-S batteries. If a short circuit occurs within the cell,

heat dissipation will lead to a thermal runaway and dangerous explosions.

１２

1.5. Electrolyte

The electrolyte is a medium in which the Li+ ions transfer between the

electrodes. The electrolyte interacts closely with each electrode and thus the

electrochemical behavior of Li-S batteries is heavily dependent on the electrolyte.

Evaluating suitable electrolytes from candidates like liquid electrolytes, RTIL

electrolytes, and solid-state electrolytes for Li-S batteries is an important matter.

The commercial success of lithium ion batteries brought liquid electrolytes to

the main focus, but commonly used electrolytes suffer from polysulfide dissolution

and this problem warrants for more research to investigate the electrolyte

thoroughly.

The dynamic equilibrium, redox chemistry, and kinetics of sulfur and

polysulfides in nonaqueous liquid solutions are connected to the solvent

complexation [52-54]. Carbonate and ether are liquid organic electrolytes mainly

utilized for Li-S batteries. Early researches sought solvents with high solubilities of

polysulfides. Polysulfides are soluble for highly basic solvents and therefore, most

ether are able to dissolve polysulfides very well. DOL, DME and TEGDME [55]

are good polysulfide solvents.

It was found that LiTFSI salt in DOL and DME was a very good match for Li-

S batteries. DOL-DME is highly soluble of polysulfides and enables more

complete reduction of soluble polysulfides. Electrolytes with high solubility of

polysulfides have the advantage of high sulfur utilization but the polysulfides cause

Li corrosion and shuttle phenomenon and in the long term of cycling the cathode

system is greatly harmed.

Ether based electrolytes are mostly used and studied due to the improvement

１３

of electrochemical activity of sulfur that follows. However, ether suffers from poor

electrochemical stability and high volatility. Carbonate solvents are seldom used in

Li-S batteries because of the side reactions with dissolved polysulfides during the

first discharge process which kills the Li-S battery.

LiClO4, LiPF6, LiTFS, and LiTFSI are commonly used salts for Li-S batteries.

Amongst them, LiTFS and LiTFSI are the main focus due to their greater thermal

and hydrolytic stability. There is no clear difference of electrochemical behavior

for each salt, although DOL-DME electrolytes with high concentrations of LiTFSI

salt showed slow lithium metal corrosion and suppressed shuttle phenomenon.

DOL-DME with ultrahigh salt concentration (7M LiTFSI) was developed to

restrain the polysulfide dissolution and stabilize the Li anode [56]. This electrolyte

coupled with CMK-3/S cathode exhibited an initial specific discharge capacity of

1041mAhg-1 with high Coulombic efficiency of 93.7% in the first cycle which

stayed near 100% afterwards. Ultrahigh concentrations of LiTFSI close to

saturation cannot easily dissolve polysulfides and thus the shuttle effect is

suppressed. The metallic Li anode, which is accompanied with SIS electrolyte,

shows less roughness and less damage done to it compared with a pristine anode.

Additives passivate the surface of the Li anode and protect it from side

reactions that may occur with the electrolyte or polysulfides. LiNO3 is an oxidizing

additive that stabilizes the metallic Li anode by forming a protective layer on the Li

anode surface. While LiNO3 is reduced into insoluble LixNOy species, the sulfides

are oxidized to LixSOy species. The Li anode is passivated and thus the side

reaction with polysulfides is prevented [57].

１４

Figure 1.4. a) Cyclic performance and Coulombic efficiency of Li-S batteries at a

current rate of 0.2C. SEM images of b) fresh Li metal and c) Li metal with a SIS

electrolyte after 280 cycles in Li deposition and dissolution experiments. Adapted

from Reference [13] (Yin, Y.X., et al., Lithium-sulfur batteries: electrochemistry,

materials, and prospects. Angew Chem Int Ed Engl, 2013. 52(50): p. 13186-200.)

１５

1.6. Objectives of this thesis

In this thesis, I attempted to introduce a new perspective of electrolytes

applied to Li-S battery systems. A nonpolar solvent 1,4-difluorobenzene was

prepared in order to minimize the dissolution of polysulfide species. LiTFSI salt

was solvated with the help of chelating 15-crown-5. Since equimolar (0.6M) salt

and crown ether were dissolved, all crown ether molecules are effectively bound

and as a result, the solubility of polysulfides in DFB-CE was reduced to very low

levels.

The DFB-CE electrolyte showed distinctive electrochemistry from typical catholyte

systems. Coulombic efficiency remained close to 100% for long term cycling and the cycle

performance was improved compared to DOL-DME electrolyte. However, the limited

solubility of the DFB-CE electrolyte resulted in relatively higher polarization and further

optimization of the Li-S system coupled with DFB-CE was needed.

Along with maximizing the sulfur composition, controlling the solubility of

polysulfides is another important issue that must be solved in order to commercialize Li-S

batteries. Nonpolar electrolytes were designed for a similar purpose of highly concentrated

electrolytes, but are vastly different in terms of composition and solubility. Nonpolar

solvents may become the key to separating the issues of increasing the sulfur composition

and the shuttle effect caused by polysulfide dissolution with the use of soluble electrolyte.

The performance of the DFB-CE electrolyte will be compared to common electrolytes and

electrochemical studies will analyze the difference of this novel electrolyte.

１６

Chapter 2. Experimental details

2.1. Materials preparation

The C/S cathode slurry consisted of 50wt% sulfur, 30wt% conductive carbon(Super P),

and 20wt% PVDF(Polyvinylidene fluroride) binder in NMP(N-Methyl-2-pyrrolidone). The

KB/S and CMK-3/S carbon-sulfur composites were prepared by melt-diffusion of sulfur

into each conductive carbon at 155℃ for 12 hours and 200℃ for 2 hours to remove excess

sulfur. The mass ratio of the carbon-sulfur composite was 40wt% carbon and 60wt% sulfur.

The slurry contained 80wt% carbon-sulfur composite, 10wt% conductive carbon(Super P),

and 10wt% PVDF(Polyvinylidene fluroride) binder in NMP(N-Methyl-2-pyrrolidone). All

slurries were mixed with a Thinky Mixer and cast onto an aluminum current collector with

a doctor blade, then dried at 60℃ overnight.

The DOL-DME electrolyte was a solution of 1M LiTFSI (lithium

bis(trifluoromethanesulfone) imide) salt solvated into a mixture of DOL(1,3-dioxolane) and

DME(1,2-dimethoxyethane) at a volume ratio of 1:1 with 0.1M LiNO3 salt additive. The

TEGDME electrolyte consisted of 1M LiTFSI salt and the TEGDME solvate. The DFB-CE

electrolyte was prepared by dissolving 0.6M LiTFSI salt into DFB(1,4-difluorobenzene)

with the help of 0.6M 15-crown-5 cosolvent.

Sulfur, PVDF, NMP, DME, TEGDME, DFB, LiTFSI, potassium (poly)sulfide were

purchased from Sigma Aldrich. DOL, LiNO3, 15-crown-5 were purchased from Alfa Aesar.

Ordered mesoporous CMK-3 was purchased from ACS Material. The LiTFSI salt was dried

at 80℃ overnight and all solvents were dried with activated molecule sieves for 24 hours.

１７

2.1. Characterizations and electrochemical tests

2032 type coin cells were assembled in an argon gas filled glovebox with a single

sheet of PE (polyethylene) separator, Li metal as the counter electrode, and 20μl of

electrolyte. Galvanostatic experiments were done with a WBCS-3000 cycler from

WonATech(Korea) with a voltage window of 1.5V to 3.0V vs. Li/Li+. GITT experiments

were performed with C/10 pulses(10 minutes) and OCV periods(2 hours). LSV tests were

carried out at a sweep rate of 1mV s-1 with a Cu working electrode for the lower limit and

the Al working electrode for the higher limit. The C-rates used for this study were based in

the theoretical specific capacity of sulfur, 1672mAh g-1.

The sulfur composition of the carbon-sulfur composite was evaluated by a TA SDT

Q600 instrument from Artisan Technology group under a nitrogen atmosphere. The

solubility of polysulfide was measured by a Optizen 2120 UV/Vis spectrophotometer from

Mecasys(Korea). Potassium (poly)sulfide was finely grinded to be used as solutes for the

samples and solutions were prepared at 60℃. The saturated solution was diluted with the

same respective solvate at a specific ratio. The morphology of cathodes was characterized

by SEM imaging and EDS analysis done with a JSM-7800F Schottky field emission

scanning electron microscope(FESEM). X-ray diffraction was performed by a D2 model X-

ray diffractometer from Bruker using a monochromatic Cu Kα (1.5405 Å) radiation.

Cathodes recovered from disassembled coin cells were thoroughly washed with DME then

dried for 12 hours.

１８

Chapter 3. Development of electrolytes to suppress

the dissolution of polysulfides for Li-S batteries

3.1. Results and discussion

LiTFSI salt and potassium polysulfide(K2Sx) did not dissolve in pure DFB. This is due

to DFB molecules being nonpolar whereas the solute molecules are polar. This difference

resulted in DFB showing very low solubilities for each solute. With the help of 15-crown-5,

which acts as a cosolvent, the dissolution of LiTFSI into DFB is enabled. Crown ethers

such as 12-crown-4 and 15-crown-5 can coordinate with Li+ ions and form a complex by

chelating them [58]. 15-crown-5 can also dissolve K2Sx by forming a complex with a

molecular ratio of 2:1 which is shown in Figure 2.1.(a) [59]. The DFB-CE electrolyte

consists of equimolar LiTFSI salt and 15-crown-5 to ensure that no excess crown ether

solvates unwanted polysulfide species. Free excessive 15-crown-5 molecules in the

electrolyte were able to solvate polysulfide.

The TEGDME electrolyte was capable of solvating large amounts of additional salt or

K2Sx. The supposedly K2Sx-saturated DFB-CE solution remained mainly colorless while

the TEGDME solution turned deep green (Figure 2.1a). With this simple dissolution test I

confirmed that compared to the TEGDME electrolyte, the solubility of polysulfide within

DFB-CE was successfully suppressed. The idea of different polarities of solvent and solute

matched the result in DFB being a nonsolvent. These different polarities match the result in

DFB possibly being a nonsolvent of polysulfides.

The solubilities of potassium polysulfide in DFB-CE and TEGDME electrolyte were

evaluated by UV-Vis spectroscopy (Figure 2.1b, c). The solubility in pure DFB was too low

to be detected by the UV-Vis spectrometer. The DFB-CE electrolyte was able to solvate up

to 0.073mg/ml of K2Sx which is too small an amount to consider the electrolyte dissolving

１９

Figure 2.1. Polysulfide solubility measurement done by UV-Vis spectroscopy. (a) Visual

confirmation of polysulfide solubility in various solutions. The UV-Vis spectra of

potassium (poly)sulfide in (b) DFB-CE solution at 45℃ and (c) TEGDME electrolyte at

45℃.

２０

Electrolyte Solubility(mg/ml) Ionic conductivity(mS/cm)

DFB-CE 0.073 2.6

TEGDME 267 1.8

DOL-DME - 11.4

Table 2.1. Solubility limits of K2Sx determined by UV-Vis spectroscopy at an elevated

temperature 60℃ and ionic conductivities at 45℃ for different electrolytes.

２１

the sulfur species from the cathode in a 2032 type coin cell system. The TEGDME solution

could take in up to 267mg/ml at 45℃ and is likely to dissolve more amounts at higher

temperature ranges.

At 45℃, the ionic conductivity of DFB-CE rised to 2.8mS cm-1 from 2.6 mS cm-1.

The ionic conductivity of the DFB-CE wasn’t too poor as to be lower than the TEGDME

electrolyte, although it was still lower than DOL-DME.

At each temperature 30℃ and 45℃, the DFB-CE electrolyte was compared to DOL-

DME. Regardless of the temperature, DOL-DME electrolyte paired with CMK-3/S showed

faster kinetics and greater discharge capacity of 1502mAh g-1. Increasing the temperature

slightly increased the capacity but did not significantly change the behavior of the DOL-

DME catholyte. The discharge curve exhibited two main plateaus at 2.4V and 2.1V.

During discharge, at the first plateau at 2.4V, sulfur is reduced to long polysulfide

chain molecules (Li2Sx, 4 ≤ x). These long chain polysulfides are further reduced to short

chained polysulfides during the second plateau, and then finally into Li2S. DOL-DME is

low in viscosity and capable of providing a more complete reduction of polysulfides. When

the cell was charged, insoluble L2S2 and L2S are oxidized to soluble long chain polysulfide

species and the polysulfides were sequentially further oxidized to solid sulfur.

The discharge capacity of the cell with DFB-CE at 30℃ was only about 811mAh g-1

and the performance suffered greatly from large overpotential. This crippling weakness of

DFB-CE was simply gone when the temperature was raised to 45℃. The first plateau at

2.2V remained the same, and the overpotential decreased enough to actually rival the DOL-

DME electrolyte. The first discharge capacity was 1165mAh g-1 which still was lower than

DOL-DME, but the consequences of lack of activation cycles was much less. Since the

solubility of polysulfides in DFB-CE was very low even at 45℃, the enhanced performance

of the cell cannot be due to DFB-CE suddenly behaving as a catholyte and following a

pathway of a fast-kinetic facile two-phase reaction. The charge process was vastly different

from the discharge process. Only one flat and long plateau was observed, which can only

２２

Figure 2.2. 1st cycle voltage profiles during galvanostatic cycling at C/30 rate (1C=1675

mAhg s-1) with CMK-3/S cathodes (a) at 30℃ in DFB-CE compared to DOL-DME and (b)

at 45℃ in DFB-CE compared with DOL-DME. Specific discharge and charge capacities

over multiple cycles for S/CMK-3 cathode in DFB-CE are compared to DOL-DME (c) at

30℃ and (d) 45℃. The DOL-DME electrolyte contains 0.1M LiNO3 additive.

２３

correspond to the complete conversion of Li2S to S8. This is also unlike catholyte systems

which typically show two charge plateaus that are less defined and more sloped than the

discharge plateaus.

Mesoporous carbons, such as CMK-3, are unable to prevent sulfur and its reduction

products from dissolving into catholytes. Figure 2.2c shows that DOL-DME suffered from

irreversible capacity fading coupled with active material loss. The dissolved polysulfide

species caused the shuttle effect and the battery performance was continuously deteriorated.

After around 15 cycles the Coulombic efficiency of the DOL-DME cell continued to

decline. Cell failure was further accelerated at 45℃ due to increasing polysulfide solubility.

The Coulombic efficiency fell below 100% much quicker and the slope of discharge

capacity was steeper. DOL-DME, as a catholyte for Li-S batteries, has many advantages

such as low viscosity, high ionic conductivity, and facile kinetics. However, like any other

catholyte, DOL-DME brings with it the same problems. Poor cycle performance, severe

capacity loss and shuttle phenomena were all confirmed in this study.

The DFB-CE electrolyte showed better cycle performance. During cycling, the

Coulombic efficiency was always close to 100% after a few cycles. This is further proof

that the DFB-CE electrolyte prevented the dissolution of polysulfide species and as a result

the shuttle effect did not occur in this system.

The effects of slowing C-rates and increasing the temperature are shown in figure 2.3a,

b. The polarization decreases dramatically as the C-rate decreases and the temperature

increases. Figure 2.3 shows that the electrochemistry of DFB-CE is significantly different

from DOL-DME. The first plateau is located at not 2.4V but 2.2V. The reduction reaction

assigned to 2.4V involves sulfur(S8) converted into long-chain polysulfide species.

The specific reaction assigned to 2.2V is unclear, but there are two possibilities. Firstly,

the reaction may be completely different with different products. Secondly, it could simply

be the difference in solvation energy that made all the difference in voltage. It is at the dip

that is decided whether the capacity be cut short or not due to the large overpotential. If the

２４

Figure 2.3. 1st cycle voltage profiles of Li-S cells using S/CMK-3 cathodes with DFB-CE

(a) at C/30 rate and different temperatures and (b) at a temperature 45℃ using various

cycling rates(1C=1675 mAhg s-1). GITT profiles of (c) DOL-DME and DFB-CE at 30℃

and (d) DFB-CE at 45℃.

２５

temperature is high enough and C-rate slow enough, sulfur is better utilized and the

performance is greatly enhanced. The effects of both variables are quite similar which

makes it difficult to discern each impact on the electrochemistry of DFB-CE system.

After the first plateau follows a voltage dip that is deeper and wider than the slight

dips that appear in catholyte cell voltage profiles. The voltage recovers to 2.15V which is

close but not quite as high as the first plateau voltage 2.2V. The second plateau occurs here

with following further reduction reactions.

In Figure 2.3c, d, the OCV of DFB-CE at 30℃ and 45℃ are compared to DOL-DME

at 30℃. The OCV at the first plateau of DFB-CE did not deviate from the profile voltage.

This almost nonexistent overpotential means that the reaction occurring at the first plateau

has fast kinetics despite sulfur and its reduction products being unable to dissolve in the

electrolyte. However, the overpotential at the start of the dip region is much larger than that

of DOL-DME and thus becomes the determining stage of whether the battery be cut short

or not. The OCV difference between the two plateaus than the difference of the DOL-DME

electrolyte, although the transition of between the plateaus seem to show different

behaviors. The two different OCVs of each plateau means that two different two-phase

reactions occur during the discharge process. The specific scheme of electrochemical

reactions of low solubility electrolytes is still unclear and a complex problem that needs to

be addressed in the future.

CMK-3/S cathodes were cycled in DFB-CE and retrieved and washed thoroughly for

ex-situ X-ray diffraction (XRD) analysis. The XRD peaks of sulfur are absent for the

pristine cathode due to sulfur being distributed inside the framework of mesoporous carbon

CMK-3. There is no significant fraction on the external surface of CMK-3/S composite,

which was indirectly proved by the TGA analysis (Figure 2.5) of the active material CMK-

3/S composite. The TGA curve shows a constant weight loss with no other rapid change of

the slope. Polysulfide species are amorphous discharge products and thus are undetected by

XRD. Only the XRD peaks of crystalline species, Li2S was observed as the formation and

２６

Figure 2.4. Ex-situ XRD characterization of sulfur electrochemistry in DFB-CE using

S/CMK-3 cathodes. (a) The Ex-situ XRD patterns of S/CMK-3 cathodes in DFB-CE at

45℃ at varied stages and (b) the corresponding discharge/charge profile. The red pattern

indicates the end of discharge and the blue pattern corresponds to the end of charge.

２７

Figure 2.5. TGA curve of CMK-3/S composite active material.

２８

consumption occurred during cycling. Li2S was formed after the dip and it is likely that the

two phase reaction of the second plateau formed the crystalline product Li2S after a series

of disproportionation reactions of polysulfides. Halfway through charging the cell, the Li2S

peak is seen to have decreased and then at the near end the peak has disappeared. No XRD

peak of elemental sulfur has been observed and it is likely that no significant change

occurred for the cathode morphology or the CMK-3/S cathode surface when only one cycle

was done.

The DFB-CE electrolyte exhibited Coulombic efficiency close to 100% and prevented

the shuttle effect, yet capacity fade occurred. This problem may be due to two factors. The

transformation of cathode morphology accumulated over several cycles and side reactions

with the lithium metal anode.

The galvanostatic voltage profile of CMK-3/S was compared to a simple C/S mixed

cathode and KB/S melt-infused cathode. CMK-3 was chosen as the porous carbon

framework for this DFB-CE electrolyte system. Sulfur simply mixed with conductive

carbon (Super-P) failed to extract further capacity after the fall in the dip region due to large

overpotential (Figure 2.6). Melt-infused KB/S was a better performing cathode than C/S,

but the overpotential was still too large and the discharge capacity reached only 556 mAh g-

1 at 45℃. Without a sophisticated conductive carbon framework, the DFB-CE cell did not

perform as well, and this highlighted the insulating nature of sulfur in a particularly

insoluble electrolyte. The sulfur particles of simple-mixed C/S are naturally larger than

KB/S and CMK-3/S and thus the overpotential is greater. Increasing the temperature was

not enough to change this weakness of C/S.

Ex-situ SEM and EDS studies (Figure 2.7) revealed the morphology change of the

CMK-3/S cathode in DFB-CE during several cycles at 45℃ and C/30 rate. There was no

sign of bulk sulfur on the surface of the pristine cathode since all sulfur was melt diffused

into the pores of CMK-3. The SEM images of the cathode after cycling demonstrated

change in surface morphology. The cathode cycled in DFB-CE started to show a few

２９

Figure 2.6. Electrochemical performance comparison of different cathodes C/S, KB/S,

CMK-3/S with DFB-CE. The 1st cycle voltage profiles at C/30 rate and (a) 30℃ and (b)

45℃.

３０

Figure 2.7. Ex-situ SEM images and EDS of (a, d) a pristine CMK-3/S cathode, (b, e) a

retrieved CMK-3/S cathode cycled in DFB-CE for 10 cycles, and (c, f) a CMK-3/S cathode

in DOL-DME after 10 cycles. All cells were cycled at 45℃ and C/30 rate.

３１

number of sites of bulk sulfur bloated out of its initial place which is evident in the sulfur

elemental map. Bulk sulfur without the aid of highly ordered carbon suffered from high

overpotential (Figure 2.6) and in this case was essentially ‘dead’ sulfur because it could not

be reached by electrons from the conductive carbon. In short, the cathode morphology

change resulted in capacity fade and decreasing sulfur utilization. Evidence of the volume

change of sulfur being a significant problem to DFB-CE is given by comparing the cathode

surface cycled in DOL-DME. Larger quantities of sulfur were dissolved into the DOL-

DME catholyte and the EDS image (Figure 2.7f) shows sulfur covering the whole surface.

The volume change of sulfur may not be as impactful in catholyte systems because sulfur

species simply dissolving into the electrolyte mitigates the harm that may be done by sulfur

bloating.

The electrochemical stability of the DFB-CE electrolyte was studied using linear

sweep voltammetry (LSV) at 45℃ at a scan rate 1mV s-1. The results show that DFB-CE

was oxidized around 4.3V (vs Li/Li+), which is higher than the voltage window used for

this study. However, the current density started to increase below 1.8V and drastically

increased below 0.3V. This unwanted current density can be attributed to electrolyte

decomposition and side reaction with Li metal. Although the instability of the electrolyte

against Li metal starts to becomes a severe problem when high concentrations of

polysulfide appear and join in the side reactions, this hindrance may become a problem for

the DFB-CE electrolyte for long-term cycling.

３２

Figure 2.8. Linear sweep voltammetry (LSV) scans of DFB-CE with Li metal as counter

and reference electrode and (a) Cu as the working electrode for lower limit and (b) Al as the

working electrode towards higher limit. Both cells were scanned at 45℃ and with scan rate

1mV s-1.

３３

Chapter 4. Conclusions

The simple concept of mismatched polarities applied to suppressing polysulfide

dissolution was demonstrated with DFB as the solvent with LiTFSI salt chelated with equal

moles of 15-crown-5. The polysulfide shuttle phenomenon was also successfully

suppressed as a direct result of near lacking solubility. Despite the low solubility of sulfur

species, the DFB-CE electrolyte applied Li-S battery showed good capacity and excellent

Coulombic efficiency.

Polarization was significant because the facile dissolution pathway was unavailable at

30℃ but by increasing the temperature to 45℃, the polarization was minimized and sulfur

utilization was enabled to higher levels. The electrochemical discharge voltage profile was

different from catholytes and more close to solid-state electrolytes.

Applying the DFB-CE electrolyte with better sulfur-binding porous carbon/sulfur

composites and protected Li-metal will be the next task to improve the cycle performance. I

expect the nonpolar electrolyte concept to be another possible strategy that can be applied

for Li-S battery systems to free it from problems oriented from the dissolution of sulfur

species.

３４

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４３

국문 초록

리튬-황 전지의 다황화물 용해 억제를 위한

전해질 개발

리튬-황 배터리는 리튬이온배터리의 6배에 달하는 매우 높은 이론

에너지 밀도를 가진 차세대 배터리 시스템이다. 충전 한 번에 500km

이상을 달릴 수 있어야하는 전기차는 무게 당 부피당 대용량 배터리를

요구하게 되는데 이를 리튬-황 배터리가 충족시킬 수 있다.

하지만 리튬-황 배터리의 상용화를 막는 난관은 단일 방법으로

해결하기 어려운 복합적인 문제이며 이를 모두 개선할 수 있는 전지

성능 극대화 연구 개발이 필요하다. 황은 지구상에 존재하는 원소 중에

이차전지 양극 소재로 쓸 수 있는 이론적인 에너지 용량이 가장 높은

물질이지만, 전극 소재로 작용하기 위해서는 전자의 이동을 필요로 하는

산화 환원 반응이 진행되어야 하고 황은 부도체이기 때문에 전자의 이동

경로를 만들어 줘야한다.

황 입자의 형태를 유지하며 탄소와 복합체를 만들면 황 입자의

표면은 도전재인 탄소와의 거리가 가까워 반응속도가 빠를 수 있지만,

황 입자 내부로 가면서 반응 속도가 크게 떨어지게 된다. 따라서

이상적인 전극 소재는 수 nm 크기의 pore가 균일한 porous한 탄소에

sulfur를 주입하는 것이 유효한 전략이 될 것이다.

전해질은 황 입자 내부도 반응에 참여시키기 위해 용매에 용해되어

확산하면서 탄소 표면에서 반응이 진행되는 것을 쓰게 되면 셔틀 현상이

일어나 음극이 소모되는 문제를 겪게 된다.

전극 반응성을 높이기 위해서 전해액의 점도를 낮추고 용해도를

높이는 방향으로 연구가 이뤄졌으나 황과 다황화물이 용해 되면서

점도가 높아지고 반응 속도가 떨어지는 문제가 있었다.

이 논문에서 나는 비극성 용매를 활용하여 다황화물의 용해를

억제하고자 하였다. 다황화물의 용해도 조절은 셔틀 현상을 제어하기

위한 필수적인 단계이며 그 다음 단계인 황 조성 극대화를 노려보고자

４４

한다. 이 연구에서는 비극성 전해질이 보통의 전해질과 비교되며 그

가능성을 증명하고자 한다.

４５

주요어 : 리튬-황 전지, 비극성 전해질, 낮은 용해도의 전해질, 셔틀 현상

억제, 다이플루오로벤젠, 크라운 에터

학 번 : 2016-25285