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석사 학위논문

Master's Thesis

혼합 전해질의 시너지 효과를 이용한

리튬 공기전지의 수명 특성향상에 관한 연구

A study on enhanced cycle life of Li-air batteries

by synergistic effect of blended electrolytes

김 병 곤 (金炳坤 Kim, Byung Gon)

EEWS 대학원

Gaduate School of EEWS

KAIST

2013

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by synergistic effect of blended electrolytes

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by synergistic effect of blended electrolytes Advisor : Professor Choi, Jang Wook

By Kim, Byung Gon

Graduate school of EEWS

KAIST

A thesis submitted to the faculty of KAIST in partial fulfillment of the re-

uirements for the degree of Master of Science and Engineering in the Graduate

chool of Energy, Environment, Water and Sustainability (EEWS). The study

as conducted in accordance with Code of Research Ethics1

2012. 12. 20 Approved byProfessor Choi, Jang Wook

1 Declaration of Ethical Conduct in Research: I, as a graduate student of KAIST, hereby declare that I have not

committed any acts that may damage the credibility of my research. These include, but are not limited to: falsi-fication, thesis written by someone else, distortion of research findings or plagiarism. I affirm that my thesiscontains honest conclusions based on my own careful research under the guidance of my thesis advisor.

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김 병 곤

위 논문은 한국과학기술원 석사학위논문으로

학위논문심사위원회에서 심사 통과하였음.

2012 년 12 월 20 일

심사위원장

심사위원

심사위원

최 장 욱 (인)

정 성

윤

(인

)

한 승 민 (인)

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MEEW

20113098

김 병 곤. Kim, Byung Gon. A study on enhanced cycle life of Li-air batteries by

synergistic effect of blended electrolytes. 혼합 전해질의 시너지 효과를 이용한

리튬 공기전지의 수명 특성향상에 관한 연구. Graduate school of EEWS. 2013.

38 p. Advisor Prof. Jang Wook Choi. Text in nglish

ABSTRACT Despite the exceptionally large specific capacities, the use of Li-O2 batteries have

been limited due to their poor cycle lives originating from irreversible reaction processes

during each cycle. Recent investigations have found electrolyte decomposition as one of the

most critical reasons for the capacity decay. Herein, this paper demonstrates that a blended

electrolyte consisting of a carbonate solvent and an ionic liquid improves the cycle lives of

Li-O2 batteries remarkably by engaging a synergistic effect from both components. Both

electrolyte components perform complementary functions to each other, as the ionic liquid

suppresses the decomposition of the carbonate solvent, while the carbonate solvent resolves

the poor ionic conductivity of the ionic liquid. This study confirms the importance and op-

portunities of utilizing electrolytes in Li-O2 batteries.

Keywords: Blended electrolyte; Electrolyte decomposition; Ionic liquid; Propylene car-

bonate; Li-air batteries.

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Table of Contents

Abstract ······················································································ i

Table of Contents ········································································ ii

List of Figures ············································································ iii

Chapter 1 Introduction

1-1.Background ········································································ 1

1-2 Motivation ········································································· 3

Chapter 2. Experimental Procedure

2-1 Preparation of electrolyte ························································ 6

2-2 Li-O2 battery assemblies and Electrochemical tests ·························· 6

2-3 Surface analysis of air electrode ················································ 8

Chapter 3. Results and Discussion

3-1 The effect of ionic liquid on the blended electrolytes ······················· 11

3-2 Electrochemical characteristics and cell performance······················· 13

3-3 Air electrodes surface analysis ················································· 18

Chapter 4. Conclusion

4-1 Conclusion ········································································ 27

Reference ·················································································· 28

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List of Figures

Figure 1. Proposed reaction schematic figure on discharge to explain formation of the

compound: Li propyl dicarbonate, Li formate, Li acetate, Li2CO3, CO2, and H2O.

Figure 2. The Li-O2 battery configuration used for this study.

Figure 3. The oxygen chamber for Li-O2 battery operation.

Figure 4. Schematic diagrams showing the electrolyte of LiTFSI dissolved in PC in a) the

absence and b) presence of the IL(PYR 13TFSI). The TFSI- ion is from both LiTFSI and the

IL.

Figure 5. Room temperature conductivities and viscosities of the blended electrolytes with

various IL/PC ratios. 0.3 M LITFSI was dissolved in all of the electrolytes.

Figure 6. a) CV characterization to test the reversibility of oxygen reduction for various

electrolytes when measured at 40 mV s-1 in the voltage range of −1.3 - 0.1 V vs. Ag/Ag+.

For these tests, 0.05 M TBATFSI was used as a salt in the blended electrolytes. b) Capacity

retentions of various electrolytes when measured in the voltage range of 2.25 - 4.35 V vs.

Li/Li+ at a current density of 200 mA g-1carbon.

Figure 7. Voltage profiles and impedance spectra (insets) of the various electrolytes at a)

the 1st cycle, b) the 10th cycle, and c) the 50th cycle. The color codes used to denote the dif-

ferent electrolytes in b) and c) are the same as in a).

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Figure 8. XPS results of the discharged air electrodes employing the electrolytes with dif-

ferent IL/PC ratios. a) C 1s, b) O 1s, c) F 1s, and d) Li 1s. The color codes used to denote

the different electrolytes in b-d) are the same as in a).

Figure 9. Raman spectra of the discharged air electrodes based on various electrolytes.

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Chapter 1. Introduction

1-1 Background

Lithium-ion batteries (LIBs) have become one of the most prominent secondary bat-

teries due to their superior energy densities compared to those of other counterparts such as nick-

el-metal hydride (Ni-MH) and nickel-cadmium (Ni-Cd) batteries.[1,2]

Nevertheless, future large-

scale applications such as various types of electrical vehicles (EVs) impose more challenging

standards with regards to energy density. However, the energy densities of the current LIBs are

quite limited as the Li storage relies mostly on the intercalation within the channel structures of

active materials.

As a solution to overcome the limited energy density of the conventional LIBs, vari-

ous metal-air batteries including Zn-air and Al-air systems have been investigated for many

years.[3]

These metal-air batteries use gaseous state oxygen as an active material on the cathode

side and thus enable significantly higher energy densities compared to those of other rechargeable

batteries relying on solid state active materials. Despite this conspicuous advantage, these metal-

air batteries have suffered from low operating cell potentials below 2 V and electrode corrosion,

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preventing their wide propagation into practical applications.[4]

More recently, Li-air (or Li-O2) batteries have also emerged because of their unpar-

alleled theoretical energy densities of 11000 Wh kg-1 (based on the mass of a Li electrode)[4,5]

and practical energy densities around 362 Wh kg-1.[6] Ever since the first demonstration by the

Abraham group in 1996,[7]

however, the cycle lives of Li-O2 batteries have not been fully ad-

dressed and thus constitute the most critical issue. In particular, finding reversible reaction path-

ways between the reactants of Li ions and oxygen molecules has been the most challenging

task.[8-11]

In order to resolve the unclear reversibility issue, a number of groups have investigated

various cell components particularly focusing on the cathode catalysts and electrolytes. In the

earlier stage, a variety of cathode catalysts mostly made of metals[12,13]

and metal oxides[14-16]

have been introduced to accelerate the decomposition of the discharge product, lithium peroxide

(Li2O2). However, further studies revealed that the use of electrolytes containing carbonate sol-

vents such as propylene carbonate (PC) indeed produces lithium carbonate (Li2CO3) rather than

Li2O2, thus raising the possibility that the electrolyte plays a key role in determining the cathode

reaction mechanism and thus overall cell performance.

Initially, PC was most widely used because its low volatility makes the electrolyte

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more sustainable upon exposure to air. Nevertheless, PC is vulnerable to the nucleophilic attack

of radicalized superoxide (O2•– ), which is formed from oxygen molecules through electron dona-

tion processes from the cathodes. The superoxide attack to PC generates Li 2CO3 and other reac-

tion products such as C3H6(OCO2Li)2, HCO2Li, and CH3CO2Li during the discharge (Figure

1).[17]

The most critical issue with the formation of these products is that these discharged prod-

ucts are very difficult to be decomposed during the charge in the same cycle, which makes each

cycle irreversible and subsequently results in rapid capacity fading. Also, the decomposition of

the electrolyte during each discharge continuously consumes the electrolyte, eventually leading

to exhaustion of the electrolyte.

1-2 Motivation

In order to overcome the irreversibility associated with the use of carbonate solvents,

diverse electrolyte systems have been interrogated.[18-21]

For instance, Laoire et al.[18]

investigated

ether-based electrolytes such as tetraethylene glycol dimethylether (TEGDME), but capacity re-

tentions were limited for only a small number of cycles. Among various electrolytes, ionic liq-

uids (ILs) could be one of the useful options as ILs hold a number of advantages including neg-

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ligible vapor pressure, low flammability, and high thermal stability.[22-25]

Moreover, for the

cases of Li-O2 batteries, the IL’s capability of interacting with the O2•– radical and thus mitigat-

ing its attack should be very attractive for robust cycling. [25,26] Although Kuboki et al.[27] investi-

gated 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (EMITFSI) as an IL for

Li-O2 batteries, the high viscosity and low oxygen solubility of the IL hindered the cell from op-

erating in a decent manner.[28,29]

Cecchetto et al.[30]

also introduced a similar blended electrolyte

mainly to improve the ionic conductivity, but its cycling performance was not evaluated exclu-

sively.

This paper demonstrates that once IL is blended with one of the most commonly

used solvents, PC, in an optimal condition, the IL’s protection capabili ty from O2•–

radical attack

is effective and makes the cycle life far superior to those of the cases where either liquid electro-

lytes or ILs are solely used. Throughout a series of analyses, it was found that the use of the

blended electrolyte alleviates the decomposition of PC and thus diminishes the formation of irre-

versible reaction products, such as Li2CO3, while engaging the sufficient ionic conductivity of

PC and thus enabling substantially enhanced cycling performance.

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Figure 1. Proposed reaction schematic figure on discharge to explain formation of the com-

pound: Li propyl dicarbonate, Li formate, Li acetate, Li2CO3, CO2, and H2O.[17]

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Chapter 2. Experimental Procedure

2-1 Preparation of electrolyte

Propylene carbonate (PC, PANAX ETEC Co. Ltd, Korea) and N -propyl- N -methyl pyr-

rolidinium bis(trifluoromethanesulfonyl)imide (PYR 13TFSI, C-TRI, Korea) were used as received.

The blended electrolytes were prepared by mixing PC and the IL at weight ratios of 5:5 and 3:7, re-

spectively. The concentrations of lithium salt (lithium bis(trifluoromethane-sulfonyl)imide or LiTFSI,

Aldrich, USA) for all of the electrolytes were 0.3 M. Conductivity measurements of the electrolytes

were carried out using a conductivity meter (LF340, WTW, Germany). Viscosities of the electrolytes

were measured using a vibrational viscometer (SV-10, AND, Japan).

2-2 Li-O2 battery assemblies and electrochemical tests

In order to investigate the stability of oxygen radical anions, cyclic voltammetry (CV)

tests were performed using a three electrode beaker cell. A glassy carbon (GC, Pine Instruments Inc,

USA) disk with a 5 mm diameter, Ag/AgCl, and a Pt wire were used as working, reference, and coun-

ter electrodes, respectively. 0.05 M tetrabutylammonium bis(trimethane-sulfonyl)imide (TBATFSI)

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salt was dissolved in each case of PC and the IL. The scan rate was 40 mV s-1

in the voltage range of

−1.3 - 0.1 V vs. Ag/Ag+ at room temperature. The porous air electrodes were prepared by casting a

slurry of Super-P (TIMCAL, Switzerland), α-manganese oxide (α-MnO2) nanowires, and

poly(vinylidene fluoride) (PVDF, Aldrich, USA) binder onto carbon paper (TGPH-090, NARA cell-

tech, Korea). The weight ratio of the components was super-P : α-MnO2 : PVDF = 24 : 42 : 34. For

XPS analyses of the air electrodes, the air electrodes were prepared by the same procedure but with-

out the α-MnO2 catalyst. For this case, the weight ratio was Super-P : PVDF = 66 : 34. After the cast-

ing, the air cathodes were dried at 70 ºC for 12 h. For electrochemical characterization, Swagelok

type cells were assembled in a glove box. A Swagelok type cell was composed of a Li metal anode

(thickness = 600 μm, Honjo, Japan), a glass fiber membrane (GF/D, Whatman, USA) impregnated

with the electrolyte, and the air electrode (Figure 2). Galvanostatic charge and discharge tests were

conducted using a WBCS 3000 battery cycler (WonAtech, Korea) at a current density of 200 mA g car-

bon-1

in the voltage range of 2.25 - 4.35 V vs. Li/Li+ at room temperature. Electrochemical impedance

spectroscopy (EIS) tests were conducted using a VMP3 (Biologic, France) tester after charge of dif-

ferent cycles with an AC amplitude of 10 mV and a frequency range of 1 MHz to 0.1 Hz. All of the

electrochemical tests were carried out at 1 atm of pure O2 (99.999 %) in a pressure balanced oxygen

chamber (Figure 3).

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2-3 Surface analysis of air electrode

For the analyses of the discharged air electrodes, the discharged batteries were disas-

sembled in a glove box and the air electrodes were rinsed with dimethoxyethane (DME) and dried in

an Ar filled glove box. The chemical compositions of the air electrodes were analyzed by high resolu-

tion dispersive raman microscope (Horiba Jobin Yvon, France) and XPS (XPS, Thermo VG scientific,

England) with a Mg K α line as X-ray source.

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Figure 2. The Li-O2 battery configuration used for this study.

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Figure 3. The oxygen chamber for Li-O2 battery operation.

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Chapter 3. Result and Discussion

3-1 The effect of ionic liquid on the blended electrolytes

The vulnerable nature of PC and the IL’s effective role can be understood by Pear son’s

hard soft acid base (HSAB) theory.[31]

Between Li+ and PC, Li

+ is classified as a hard Lewis acid,

whereas PC is classified as a soft base. Hence, although Li+ ions are usually solvated by PC mole-

cules, the interaction between these two components is relatively weak, leaving the strong acidic

character of Li+

in the Li+

– PC complexes. The remaining acidity of Li+

renders the interaction of the

Li+ – PC complexes with soft base O2

•– ineffective,

[32] giving superoxide the chance of attacking PC

(Figure 4a). The attack of O2•–

at the methylene group of PC decomposes PC[17]

and leads to the for-

mation of the aforementioned irreversible products and subsequent capacity fading during cycling. By

contrast, the introduction of N -propyl- N -methyl pyrrolidinium bis(trifluoromethanesulfonyl)imide

(PYR 13TFSI), the IL of interest in the present study, endows the electrolyte with the capability of sta-

bilizing O2•–

.[25] As schematically illustrated in Figure 4b, the underlying mechanism for the O 2•–

sta-

bilization is that the bulky cation PYR 13

+

of the IL has soft acidity and thus efficiently neutralizes soft

base O2•–

. In the pure PC case, Li+ ions are expected to be too acidic to interact efficiently with O2•–

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even after the solvation with PC particularly compared to the PYR 13 cations of the IL because the ra-

dius of the IL cation is larger and its charge density is lower.[25,32] The weak interaction of PC with Li+

ions can also be understood by the low donor number of PC (=15.1). [33] In general, the donor number

indicates the degree of nucleophilic character (electron donating, Lewis basicity). The donor number

of PC is indeed smaller than those of other solvents considered for Li-O2 cells such as dimethylfor-

mamide (=26.8),[33,34]

again suggesting the weaker interaction of PC with Li+ ions. Overall, the allevi-

ation of the superoxide attack to PC is the very key motivation of using the IL to improve the cycle

lives of Li-O2 batteries.

While the use of ILs is advantageous in many aspects of Li-O2 battery operations, the

exclusive use of ILs would suffer from low ionic conductivities originating from their high viscosities.

As a solution, in the current investigation, a blend of electrolytes by mixing PYR 13TFSI with PC in

different ratios is introduced. For the blended electrolytes, the ionic conductivity and viscosity were

examined prior to battery performance tests. In these characterizations, instead of testing the bare

mixtures of PC and the IL, 0.3 M lithium bis(trifluoromethane-sulfonyl)imide (LiTFSI) salt was dis-

solved in each solution because it is known that the presence of excessive lithium salt increases the

viscosity and consequently decreases the ionic conductivity due to increased ionic interaction within

the solution.[35]

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As shown in Figure 5, the ionic conductivity is the highest when the mass ratio of IL:PC

is 1:1, indicating that the ionic conductivity increases in the range of small IL content, but decreases

once the IL content passes a certain point. This trend of showing the maximum value in the ionic

conductivity can be interpreted in a way that in the range of small IL content, the increased content of

the IL increases the number of ions involving in the ionic transport. However, after the maximum

point, the further increase in the IL content rather increases the viscosity, which leads to decrease in

the ionic conductivity. This ionic conductivity and viscosity trend is consistent with the previous re-

ports by the Zaghib[35]

and Balducci[36]

groups. It should also be stated that methyl pyrrolidinium cati-

on series have planar arrangements and thus facilitate fast ionic transport.[37]

Beside these features,

pyrrolidinium cation series are known to interact with Li metal in a quite reversible manner, which

must be beneficial to robust battery cycling.[38]

3-2 Electrochemical characteristics and cell performance

The stability of the O2•–

radical was first characterized for various blended electrolytes.

In particular, the reversibility of the O2/O2•–

redox couple was investigated by cyclic voltammetry

(CV) measurements. For these characterizations, 0.05 M tetrabutylammonium

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bis(trimethanesulfonyl)imide (TBATFSI) salt was dissolved in each blended solvent, and the actual

scanning was performed while pure O2 gas was bubbled in. The voltage was first cathodically swept

from open circuit voltage (OCV) to −1.3 V vs. Ag/AgCl, and then reversed to 0.1 V at 40 mV s -1. As

displayed in Figure 6a, the PC curve shows a cathodic peak around −1.07 V, which corresponds to the

formation of superoxide (O2•–

).

However, the scan in the opposite direction shows an almost negligible peak, implying

that the oxidation of O2•–

back to O2 is considerably impaired. The irreversible nature of the redox

couple throughout the cycle must be associated with the superoxide attack to PC and the subsequent

decomposition of PC.[39]

In other words, the generated superoxide is consumed for the reaction with

PC and therefore loses the capability of converting back to O2. By contrast, the blended solvents

(PC:IL = 5:5 or 3:7) show improved reversibility between the cathodic (−1.00 V at PC:IL = 5:5,

−1.04 V at PC:IL = 3:7) and anodic (−0.87 V at IL:PC = 5:5, −0.90 V at PC:IL = 3:7) peaks, suggest-

ing that the addition of the IL improves the reversibility of O2/O2•–

redox reaction. Indeed, the en-

hanced reversibility was indicated by the increased ratio of the integrated areas between the anodic

and cathodic peaks (│Qa/Qc│) with the IL content: │Qa/Qc│= 0.0655 at pure PC, │Qa/Qc│= 0.4404

at PC:IL = 5:5, │Qa/Qc│= 0.7879 at PC:IL =3:7, and │Qa/Qc│=0.9694 at pure IL.

The effect of the blended electrolytes on the cycling performance of Li-O2 batteries was

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investigated. Again, the cycling performance is the most challenging aspect in Li-O2 battery opera-

tions, and the key purpose of employing the IL is to improve this property. he air electrode is pre-

pared in the same way (choice of catalyst and mass ratio of cell components) as the Bruce group’s [17]

to focus mainly on the IL effect. As described in the Experimental section, Swagelok cells containing

the various electrolytes were galvanostatically tested and their cycling performances are displayed in

Figure 6b. The gravimetric capacities in this figure were calculated based on the mass of carbon in the

air electrodes. As shown in Figure 6b, the cell with pure PC continuously loses its capacity and, as a

result, retains only 25 % of the initial capacity (880 mAh g-1

) after 70 cycles. The capacity fading is

attributed to the electrolyte depletion and the accumulation of the irreversible products generated

from the electrolyte decomposition.[40]

By contrast, the cell with the 50 % IL retains 94.6 % of the

initial capacity (750 mAh g-1

) after the same number of cycles, verifying that the addition of optimal

amount of the IL improves the cycling performance remarkably. To the best of our survey, this capac-

ity retention is superior to any of the reported based on carbonate electrolytes[16]

and is also compara-

ble to the best ones showing stable retentions for 100 cycles using dimethylsulfoxide (DMSO)[41] and

TEGDME-based electrolytes.[42] The cell with the 70 % IL retains 42.3 % of the initial capacity after

the same number of cycles. Also, the initial capacity (683 mAh g

-1

) of this cell is obviously lower

than those of the other two cells, indicating that the increased IL content increases the resistance in

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the cell, which makes the initial capacity smaller. The cell with the pure IL also shows a low initial

capacity (533 mAh g-1) and poor cycling performance (7.69 % retention after 70 cycles), due to a

combined effect of the higher viscosity of the IL as well as the smaller diffusivity and solubility of O2

in the IL.[28,29] In particular, the inferior capacity retentions of the cells with the larger portions of the

IL can be understood such that as the cycling progresses, the smaller diffusivity and solubility of O 2

in the IL make the supply of oxygen less sufficient, eventually leading to the point where the oxygen

supply becomes a limiting factor for the capacity. Also, it is noteworthy that the degree of the reversi-

bility is different between Figure 6a and Figure 7a-c due to the different main reactions under the dis-

tinctive cell configurations. The data in Figure 6a is associated mainly with O2 ⇋ O2•–

in a cell closed

configuration, whereas the data in Figure 7a-c are associated mainly with 2Li+ + O2 ⇋ Li2O2 in an

open cell configuration where the cathode is exposed to oxygen. Still, the effect of the IL on the en-

hanced reversibility is valid for both cases.

In order to further understand the distinctive cycling performance among the cells based

on the different electrolytes, the galvanostatic charge/discharge profiles are comparatively presented

(Figure 7). In particular, overpotentials (or halves of charge/discharge gaps) indicate the resistances at

the air electrode/electrolyte interfaces and thus give a clue on the degree of electrolyte decomposition.

Along this line, Figure 7a-c shows a consistent picture with the cycling data and validates our original

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electrolyte design of engaging the IL. In the case of pure PC, the gap between charge and discharge

profiles becomes larger more significantly over cycling compared with that of the mixture containing

the 50 % IL. In the case of the mixture containing the 70 % IL, the overpotential becomes larger than

the 50 % case because of the increased IL content. [28,29] In addition, the averaged charge-discharge

potential values are indicative of dominant reaction products especially in the first cycle (Figure 7a)

for the cases of the different electrolyte combinations. The pure IL cell shows an average value of

~2.9 V, whereas the pure PC cell shows an average value of ~3.4 V. Although the average value of the

pure PC cell is a bit different from the equilibrium potential of Li2CO3 (~3.86 V)[43]

perhaps due to

formation of other lithium alkylcarbonates, the average value of the pure IL cell matches well with

the equilibrium potential of Li2O2 (~2.96 V),[44]

confirming the IL effect toward Li2O2 formation dur-

ing the discharge. Electrochemical impedance spectroscopy (EIS) data also show a consistent trend

between the samples (Figure 7a-c, inset). Unlike the galvanostatic data, the EIS data show the de-

crease in the impedance during the first ten cycles for all of the four samples, which is seemingly as-

cribed to some electro-activation processes such as electrolyte wetting [45] and resultant uniform oxy-

gen supply. However, the trend of impedance increase after the 10th cycle is consistent with the

aforementioned electrochemical data: In the cycle range of 10-50 cycles, the impedances of the cells

based on PC and the IL increase much more significantly than those of the cell based on the 50 % IL.

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Once again, the severely increased impedances of the cells based on PC and the IL are associated with

the formation of the irreversible discharge products[17] and the inferior ionic diffusivity/oxygen solu-

bility,[28,29] respectively.

3-3 Air electrodes surface analysis

In order to identify the reaction products on the air electrodes during discharge, XPS

analyses were conducted. For these analyses, the α-MnO2 catalyst was not impregnated in the air elec-

trodes because the O 1s signals from the catalyst could bury the signals from reaction products in the

same regions. From the XPS results, the following points are noteworthy:

1) The C 1s spectra (Figure 8a) clearly show the effect of the IL addition. The peak intensity of

Li2CO3 at 289.7 eV[46]

continuously decreases as the IL content increases. In the case of the pure

IL, the Li2CO3 peak disappears, verifying that the formation of Li 2CO3 is exclusively from PC. In

addition, the peak at 293 eV corresponding to CF3 is intensified with the IL content, as CF3 origi-

nates from TFSI- in both the IL and the lithium salt.[47]

2) The O 1s spectra (Figure 8b) indicate that the peak at 531.4 eV corresponding to Li 2CO3[40] be-

comes intensified with the PC/IL ratio. On the contrary, the peak position is shifted toward 532.7

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eV corresponding to TFSI-,[47]

as the IL/PC ratio increases, reflecting more dominant character of

the IL. Although Li2O2 formation is expected to be detected at 531 eV [48] for the cells containing

the IL, such peaks were not observed perhaps because the amounts of the generated Li 2O2 are

much smaller compared to those of TFSI- originating from the IL with much larger amounts, re-

sulting in the burial of the Li2O2 peaks by the TFSI- peaks. Instead, the presence of Li2O2 at the

end of discharge for the cases of the IL-based electrolytes was confirmed by raman spectroscopy

characterization as discussed below.

3) The F 1s spectra (Figure 8c) indicate that, consistent with the C 1s data, the peaks at 688.9 eV cor-

responding to CF3 of TFSI-[47]

is enhanced with the IL/PC ratio. In contrast, the peaks at 684.9 eV

corresponding to lithium fluoride (LiF) show the opposite trend that the peak intensity decreases

with the IL/PC ratio, which is commensurate with a previous report that the LiTFSI salt is decom-

posed more significantly in carbonate-based electrolytes and forms LiF.[49]

From this trend of the

salt stability, it can be anticipated that the addition of the IL is also beneficial for the stability of

LiTFSI. In addition, the Li 1s spectra (Figure 8d) also support the aforementioned XPS data: the

enhanced Li2CO3 and LiF peaks[50] with a decreased IL/PC ratio.

Raman spectroscopy data (Figure 9) also verify the aforementioned trend of the dis-

charge products and the positive effect of the IL on the formation of more reversible products. While

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the peak at 1080 cm-1[17]

corresponding to Li2CO3 is clearly visible for the cell based on PC, and the

peak completely disappears in the case of the cell based on the IL. The cell based on the 50 % IL also

shows the peak at this position, which is reflective of the presence of PC. By contrast, the peaks at

790 cm-1[48,51] corresponding to Li2O2 are conspicuously noticeable for the cells based on the pure IL

and 50 % IL, whereas the same peak is negligible for the cell based on PC.

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Figure 4. Schematic diagrams showing the electrolyte of LiTFSI dissolved in PC in a) the

absence and b) presence of the IL(PYR 13TFSI). The TFSI- ion is from both LiTFSI and the

IL.

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Figure 5. Room temperature conductivities and viscosities of the blended electrolytes with

various IL/PC ratios. 0.3 M LITFSI was dissolved in all of the electrolytes.

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Figure 6. a) CV characterization to test the reversibility of oxygen reduction for various

electrolytes when measured at 40 mV s-1 in the voltage range of −1.3 - 0.1 V vs. Ag/Ag+.

For these tests, 0.05 M TBATFSI was used as a salt in the blended electrolytes. b) Capacity

retentions of various electrolytes when measured in the voltage range of 2.25 - 4.35 V vs.

Li/Li+ at a current density of 200 mA g-1carbon.

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Figure 7. Voltage profiles and impedance spectra (insets) of the various electrolytes at a) the

1st

cycle, b) the 10th

cycle, and c) the 50th

cycle. The color codes used to denote the different

electrolytes in b) and c) are the same as in a).

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Figure 8. XPS results of the discharged air electrodes employing the electrolytes with differ-

ent IL/PC ratios. a) C 1s, b) O 1s, c) F 1s, and d) Li 1s. The color codes used to denote the

different electrolytes in b-d) are the same as in a).

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Figure 9. Raman spectra of the discharged air electrodes based on various electrolytes.

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Chapter 4. Conclusion

4.1 Conclusion

This paper presents the substantially improved cycle life of Li-O2 batteries by engaging

blended electrolytes. The key feature of using the blended electrolyte in which the IL and PC are

mixed at an optimal ratio is that these two components compensate the drawbacks of the other com-

ponent and thus bring synergistic effects to the cell performance. While the IL minimizes the decom-

position of PC and thus generates more reversible reaction products, PC makes up for the low ionic

conductivity of the IL. Based on these synergistic effects, the optimized blended electrolyte allows

the cell to demonstrate far superior cycling performance compared to the cases where either the car-

bonate-based solvent or the IL is solely used. Detailed electrochemical and surface analyses give a

consistent picture of the diminished irreversible reaction products upon the introduction of the IL.

This study suggests that the generation of reversible reaction products is very critical for the cycle life

of Li-O2 batteries, and the blended electrolytes might be one of the most feasible solutions.

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Summary in orean

혼합 전해질의 시너지 효 를 이용한

리튬 기전지의 수명특성 향상에 관한 연구

전 지구적인 환경 문제를 해결하고 다가 오는 전기자동차 시대를 열기 위해서는 높은

에너지 밀도를 가진 차세대 전지 시스템이 필요하다. 이러한 이유로 최근 리튬 공기전지가

새롭게 부각되고 있다. 리튬 공기전지는 리튬 이온전지 대비 높은 이론 에너지밀도를 가지며,

가솔린과 비슷한 에너지 밀도를 보인다. 그러나 상당히 큰 에너지밀도를 보임에도 불구하고,

사이클이 진행되는 동안에 발생하는 비가역 반응으로 인한 짧은 수명특성이 지금까지의

사용에 있어서 큰 걸림돌로 작용되어왔다.

수명특성을 줄이는 가장 근원적인 문제 중 하나가 산소라디칼 공격에 의한 카보네이트

계열의 전해질 분해라는 것이 최근 리튬 공기전지의 연구결과들을 통해 밝혀졌다. 이러한

문제를 해결하기 위해서, 본 논문에서는 리튬 공기전지의 전해질로써 카보네이트 용매와

이온성 액체로 이루어진 혼합 전해질을 사용하였다. 이러한 혼합전해질을 구성하는 각 성분은

서로에 대해서 상보적인 역할을 하며, 시너지 효과를 통해 리튬 공기전지가 가지고 있는

수명특성 문제를 해결하였다. 이온성 액체는 산소 라디칼 공격으로 인한 카보네이트 용매의

분해를 억제시키고, 카보네이트 용매는 이온성 액체가 가진 낮은 이온 전도도 문제를

해결함으로써, 리튬 공기전지의 장기 수명특성을 확보하였다. 본 연구는 이러한 혼합전해질을

사용함으로써 리튬 공기전지의 수명특성을 향상시켰을 뿐만 아니라, 나아가 리튬 공기전지의

새로운 연구방향을 제시하는 데에 기여했다고 생각된다.

핵심어: 혼합 전해질; 전해질 분해; 이온성 액체; 프로필렌 카보네이트; 리튬 공기전지.

2013m020113098_s1ver2

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