Mixed solvent electrolytes containing f luorinated carboxylic acid

esters to improve the thermal stability of lithium metal anode cells

Kazuya Sato, Ikiko Yamazaki, Shigeto Okada, Jun-ichi Yamaki*

Institute of Advanced Material Study, Kyushu University, 6-1 Kasuga Koen, Kasuga-shi, 816-8580 Fukuoka, Japan

Abstract

A safety aspect is one of the most important problems to utilize lithium metal anode battery which can show significant

higher capacity than conventional lithium ion battery. The thermal stabilities of electrolytes containing fluorinated carboxylic

acid esters were assessed at the coexistence of lithium metal using differential scanning calorimeter. Among the fluorinated

carboxylic esters used here, CHF2COOCH3 (Methyl difluoro acetate; MFA) exhibited the highest onset temperature and

smallest amount of exothermic heat at the coexistence of lithium. A similar trend was observed for apparent 1-M solution of

LiPF6 with fluorinated carboxylic acid esters and ethylene carbonate (EC) + dimethyl carbonate (DMC) (1:1 in vol.). Precise

thermal study was undertaken for 1 M LiPF6/MFA and 1 M LiPF6/EC +DMC or propylene carbonate (PC), changing the

mixing ratio at the coexistence of lithium metal. Surviving lithium metal content was estimated by endothermic vs. exothermic

heat ratio at lithium melting and freezing from 300 jC. With the increase of volume ratio of MFA from 0% to 100%, the amount

of surviving lithium metal increased from 0% to 95%. D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Fluorinated carboxylic acid esters; Thermal stability of lithium metal anode cells; Surviving lithium metal

1. Introduction

Lithium ion batteries are widely used for power

sources of various portable electronic devices because

of their high voltage and high energy density. Lithium

ion batteries are using graphite as anode material. If

lithium metal that has electrochemical equivalence

about 10 times as high as graphite is used as anode,

the higher energy density of the cell is gained. In the

past, much effort has been paid to develop lithium

metal rechargeable batteries with lithium metal as

anode material [1–5]. Nevertheless, the commercial

success of these batteries is not realized even up to

now. The main problems inherent in the practical

applications of lithium metal rechargeable batteries

have been indicated as poor cycle life and safety

concern due to the reaction of lithium metal and

electrolyte components [6–10]. The thermal stability

of 0.2 M fluorinated esters had been studied [11]. The

exothermic reaction temperatures of the fluorinated

esters with lithium metal were higher than that of the

corresponding esters. Non-fluorinated esters may react

with lithium metal at low temperature because of their

enol form, and fluorinated esters may help to produce a

thick and effective solid electrolyte interphase layer to

prevent farther reaction of electrolyte. Unfortunately,

many fluorinated carboxylic acid esters did not dis-

solve 0.2 M LiPF6, and they used saturated solutions

for the experiments. This paper focuses on reaction of

lithium metal and electrolyte components. The thermal

0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0167 -2738 (02 )00088 -7

* Corresponding author. Fax: +81-92-583-7790.

E-mail address: yamaki@cm.kyushu-u.ac.jp (J. Yamaki).

www.elsevier.com/locate/ssi

Solid State Ionics 148 (2002) 463–466

stability of 1-M LiPF6-mixed solvent electrolytes with

fluorinated esters was studied to compare the stability

of the same concentration electrolytes. A precise

thermal study on the mixing ratio of a fluorinated ester

was also studied.

2. Experimental

An electrolyte solution used in the present study was

prepared by a reagent grade fluorinated carboxylic acid

esters [CHF2COOCH3(MFA), CHF2COOCH2CH3,

CF3CF2COOCH3, CF3CF2COOCH2CH3, F(CF3)2CCOOCH3, F(CF2)3COOCH3, F(CF2)3COOCH2CH3,

F(CF2)4COOCH2CH3 and F(CF2)7COOCH2CH3] pur-

chased from Daikin Industries.

Mixing a fluorinated carboxylic acid ester (15 ml)

with 2 M LiPF6/EC +DMC) (1:1) (15 ml), ca.1 M

electrolyte was prepared. Fluorinated carboxylic acid

ester (15 ml) was completely dissolved in 2 M LiPF6solution with the exception of F(CF2)7COOCH2CH3.

The thermal stability of those electrolytes was

assessed at the coexistence of lithium metal by differ-

ential scanning calorimeter (DSC) (Rigaku Thermo

plus) using stainless steel sealed pan. The electrolyte

volume for DSC is 5 Al, lithium weight is 1.3 mg,

heating rate is 5 jC/min and reference is a-Al2O3.

The temperature range is from room temperature to

400 jC. Conductivity of electrolyte is measured by

LCR METER (Hewlett Packard 4284A) using T-

shaped glass cell. The electrolyte volume is 0.5 ml

and used frequency is 100 kHz. Cycling efficiency of

lithium metal was estimated using a coin cell. Lith-

ium metal was deposited on stainless steel electrode

at the current density of 0.2 mA/cm2 for 20 min,

followed by dissolving the deposited lithium metal up

to 1 V. Cycling efficiency of lithium metal was

estimated by the ratio of the amount of dissolving

and depositing electricity. Preparation of the mixed

solvent, assembly of the electrochemical cell, and

sealing stainless steel pan were done in a dry box

filled with argon.

3. Results and discussion

Table 1 summarizes onset temperature and exo-

thermic energy of fluorinated carboxylic acid esters

with 2 M LiPF6-EC/DMC (1:1) at the coexistence of

lithium metal. The onset temperature became higher

by addition of MFA. MFA, CF3CF2COOCH2CH3, and

F(CF2)3COOCH3 decreased the exothermic energy.

These results indicate that MFA is the most effective

additive in this study.

Fig. 1 shows DSC profiles of the mixture of 1 M

LiPF6/MFA and 1 M LiPF6/EC +DMC, changing the

mixing ratio at the coexistence of lithium metal. The

heat flow is based on the total weight of electrolyte

and lithium metal. A very large exothermic peak of

EC +DMC electrolyte (a) from 180 jC (mp. of lithium)

decreased with the addition of MFA. Fig. 2 shows

percentage of survived lithium metal and exothermic

energy of the experiments shown in Fig. 1. The

survived lithium metal percentage was estimated by

endothermic heat of lithium melting and exothermic

heat of lithium freezing after increasing the temper-

ature to 300 jC. The results indicate that exothermic

energy was reduced and percentage of survived lith-

ium metal increased with the increase in the amount of

1 M LiPF6/MFA. It was also found that the large

amount of MFA is needed to improve the thermal

stability. When the electrolyte is pure MFA solution,

the survived lithium is 95%. However, when the

mixing ration of MFA becomes 60%, the survived

lithium goes down to 18%.

Fig. 3 shows DSC profiles of the mixture of 1 M

MFA and 1 M LiPF6/PC, changing the mixing ratio at

the coexistence of lithium metal. The PC electrolyte (a)

did not exhibit a large exothermic peak after melting

lithium at 180 jC. By the addition of MFA, a new

Table 1

Onset temperature of ca. 1 M LiPF6 mixed solvent electrolytes

(EC +DMC+ fluorinated 1:1:2 in volume), and exothermic energy

of the electrolytes from 180 jC to 220 jC

Fluorinated solvent

added to EC+DMC

Onset

temperature

(jC)

Exothermic

energy (J/g)

no 180 2900

CHF2COOCH3 210 700

CHF2COOCH2CH3 175 larger than 3500

CF3CF2COOCH3 168 2100

CF3CF2COOCH2CH3 172 larger than 3500

F(CF3)2CCOOCH3 173 3400

F(CF2)3COOCH3 167 1900

F(CF2)3COOCH2CH3 169 3000

F(CF2)4COOCH2CH3 168 3000

K. Sato et al. / Solid State Ionics 148 (2002) 463–466464

exothermic reaction was observed after lithium melt-

ing, though the total exothermic energy decreased with

the increase of MFA amount. Fig. 4 shows percentage

of surviving lithium metal and exothermic energy of

PC-mixed electrolytes shown in Fig. 3. A similar

tendency as the EC +DMC +MFA was observed.

However, adding MFA to 1 M LiPF6 / PC was more

effective than adding to 1 M LiPF6 /EC + DMC, when

the content of surviving lithium metal was compared.

If we compare the 60% mixing of MFA, the surviving

lithium metal in PC-mixed electrolyte is 56%, which is

higher than 18% for EC +DMC-mixed electrolyte.

The onset temperature of 1 M LiPF6 / PC (193 jC) ishigher than that of 1 M LiPF6 / EC +DMC (180 jC) atthe coexistence of lithium metal, which means the

reactivity of 1 M LiPF6 / PC with lithium metal is weak

compared with that of 1 M LiPF6/EC +DMC.

Fig. 2. Dependence of MFA content on thermal stability of ca. 1 M

LiPF6 /EC+DMC+MFA with lithium metal. o: The total exother-

mic energy from 140 to 300 jC..: The amount of surviving lithium

metal after heating.

Fig. 3. DSC profiles of the mixture of 1 M LiPF6/PC and 1 M

LiPF6 /MFA with Li metal. 1 M LiPF6 / PC:1 M LiPF6 /MFA (vol.

ratio) are (a) 1:0, (b) 4:1, (c) 1:1, (d) 3:7 and (e) 0:1.

Fig. 4. Dependence of MFA content on thermal stability of ca. 1 M

LiPF6 / PC +MFA with lithium metal. o: The total exothermic

energy from 140 to 300 jC. .: The amount of surviving lithium

metal after heating.

Fig. 1. DSC profiles of the mixture of 1MLiPF6/EC+DMC and 1M

LiPF6 / MFAwith Li metal. 1 M LiPF6 / EC+DMC:1M LiPF6 /MFA

(vol. ratio) are (a) 1:0, (b) 4:1, (c) 1:1, (d) 3:7 and (e) 0:1.

K. Sato et al. / Solid State Ionics 148 (2002) 463–466 465

The cycling efficiencies of 1 M LiPF6/EC +DMC,

1 M LiPF6/MFA-mixed EC +DMC (50 wt.%), 1 M

LiPF6/PC, 1 M LiPF6/MFA-mixed PC (50 wt.%), and

1 M LiPF6/MFA are 70%, 75%, 71%, 71%, and 85%,

respectively. The result indicates that single solvent

electrolyte of MFA shows the higher cycling effi-

ciency than that of LiPF6/EC +DMC, and the addition

of MFA to EC+DMC can improve cycling efficiency.

However, the addition to PC cannot improve cycling

efficiency.

Conductivities of 1MLiPF6EC +DMC, 1MLiPF6/

MFA-mixed EC+DMC, 1 M LiPF6/PC, 1 M LiPF6/

MFA-mixed PC, and 1M LiPF6/MFA are 12, 14, 7, 13,

and 12 mS/cm, respectively. The conductivity in-

creased by the addition of MFA to EC+DMC (50

wt.%) or PC (50 wt.%). The reason of this result is that

viscosity of MFA is lower than that of EC+DMC or

PC.

4. Conclusions

From the DSC study of 1 M LiPF6/EC +DMC

electrolyte mixed with various kind of fluorinated

carboxylic acid esters, MFAwas found to be the most

stable co-solvent with lithium metal. In order to obtain

precise information on MFA, MFA was mixed with

EC+DMC or PC, and the reactivity of the ca. 1 M

LiPF6 solution with lithium metal was studied. The

amount of surviving lithium metal after heating up to

300 jC at the heating rate of 5 jC/min was estimated

by DSC. The MFA single solvent electrolyte showed

95% of surviving lithium metal, which means only a

very small amount of lithium metal reacts with the

electrolyte. If we compare the 60% mixing of MFA,

the surviving lithium metal in PC-mixed electrolyte is

56%, which is higher than 18% for EC +DMC-mixed

electrolyte.

Acknowledgements

The authors wish to thank Daikin Industries, and

NEC for the solvent supply and financial support.

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