Safety Improvement of Lithium Ion Batteries by Fluorine Compounds

愛総研。研究報告 第14号 2012年

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Therma1 stabi1ity， e1ectrochemica1 oxidation stability and charge/discharge characteristics of natura1 graphite powder were investigated by mixing of fiv巴fluoro回carbonateswith 1 mo1/L LiC104-EC/DEC/PC (l:1: 1 vol.). DSC study revealed

that thermal stability ofthe electrolyte solution was improved by mixing offluoro-carbonates by 10.0-33.3 vol.%. Electrochemical oxidation stability was also improved. Oxidation currents for Pt electrode were significantly decreased by mixing of fluoro-carbonates. In the fluoro-carbonate-mixed electro1yte solutions， electrochemica1 reduction of PC decr巴asedwith increasing concentration of fluoro-carbonate and current density. As a result，自rstcoulombic efficiency for natural graphite electrode incr巳ased， that is， irreversible capacity decreas巴din the fluoro-carbonate-mixed solutions. 1. IlI.troduction Lithium ion batteries have a possibility of firing and/or explosion at high temperatures， by short circuit， by overcharging and so on since they employ flammab1e organic solvents. High oxidation stability of 1ithium ion batteries is one of the most important issues for their app1ication to hybrid cars and electric vehicles. In order to improve the oxidation stability of 1ithium ion batteries， new additives or solv巴nts for e1ectro1yt巴solutionshave been investigated [1-44]. Phosphorus compounds were main1y examin巴das flame retardant additives Therma1 stability and e1ectrochemica1 properties were investigated in detai!for various phosphorus compounds. Our recent study revea1ed that fluoro-carbonates and f1uoro-ethers are ab1e to be used as nonflammab1e solvents for 1ithium ion batteries [39ラ40].F1uorine substitution of organic compounds improves th巴町 oxidation stability [39， 40， 44]. However， the fluorine substitution simu1taneously increases reduction potentia1s of organic compounds， i.e. causes e1ectrochemica1 decomposition at higher potentia1s than thos巴 for organic solvents such as EC (ethy1ene carbonate)， PC (propy1ene carbonate)， DEC (diethy1 carbonate) etc. [39ラ 40]. If e1ectrochemica1 reduction of organo向日uorine compounds continues without forming a protective surface fi1m (Solid 十Department of Applied Chemis仕y，Aichi Institute of Techno1ogy 十 ↑ Graduate Schoo1 of Engineering， Aichi Institut巴 of Techno1ogy 十十十Chemica1Division， Daikin Industries， Ltd E1ec廿olyt巴Interphase or Interface: SEI) on carbon anode， irreversib1e capacity high1y increases. However， if decomposed products quick1y form SEI on carbon e1ectrodeヲsuchf1uorine compounds can be used as nonflammab1e solvents for 1ithium ion batteries. Mixing of fluoro-carbonates with 1 mol/L LiC104-EC/DEC significantly improved the oxidation stability without decrease in charge capacities and first cou1ombic efficiencies for natura1 graphite(NG) e1ectrodes [39]. The results show that m叩yfluorine compounds with high oxidation stabi!ity can be used for EC-based solvents such as ECIDEC as nonflammab1e solvents [39]. However， it is difficult to use high crystalline graphite such as natura1 graphite in PC-mixed e1ectrolyte solutions due to the continuous decomposition of PC. EC with a high melting point， 360C shou1d be used for such high crystalline graphite e1ectrode for the quick fonnation of SEI.Therefore if PC with a 10w melting point， -550C can be used for graphite， lithium ion batteries are used in a wide range of temperatur巴.For example， the melting points of 1 mol/L

LiCI04-EC/DEC (1:1 vol.)and ECIDEC/PC (1:1:1 vol.)are

-10C and -310Cヲrespectively[40]. It was found in a pr巴VlOUS study that mixing of cyclic and 1inear f1uoro-carbonates with 1 mol/L LiC104-EC/DEC/PC high1y increased not only oxidation stabi1ity of e1ecむolyte solutions but a1so frrst cou1ombic efficiencies for natura1 graphit巴 巴l巴ctrodes [39]. This resu1t indicates that e1ectrochemically reducedf1uoro-carbonates quick1y fonn SEI on graphite e1ectrode. It is therefore an additional advantage for organo-f1uorine compounds that the fluoro-carbonates enable the use of PC四containingsolvents for graphite巴1ectrode 53

54 愛知工業大学総合技術研究所研究報告，第 14号， 2012年 ln the present study， therrnal and electrochemical oxidation stability of fluoro-carbonate-mixed electrolyte solutions was investigated by differential scanning calorimetry (DSC)and oxidation Cl町ent measurements， and charge/discharge characteristics of natural graphite electrode were evaluated using the same electrolyt巴solutionsas functions of mixing ratio of fluoro-carbonates and current density. 2. Experimantal 2.1 Fluorine compounds Five fluoro-carbonates (purity: 99.9%ヲ H20:く10ppm)， synthesiz巴din Daikin lndustries， Ltd.， were used in the present study (Fig. 1). Viscosities and specific conductivities of fluorine compounds， A and C were 4.79 and 0.80 cP at 200Cヲ and 5.5xl0-3 _{and 8.9xlO-}4 _{S/cm at room temperature，} r巴spectively.Those for EC江)ECwere 0.50 cP at 200C and 2.8xl0-3 _{S/cm at room temperature} 2.2 Thermα1 stα，'bility by DSC meαsurements Therrnal stability of fluoro-carbonate-mixed elec仕olyte solutions was examined by differential scanning calorimetry (DSC-60， Shimadzu). DSC measurement was carried out using a mixture of 0.90， 0.78 or 0.67 mollL LiCI04-EC/DEC/PC (Aヲ

B，C，DヲorE) (1:1:1:0.33， 0.83 or 1.5 vol，.10.0ラ21.7or 33.3 vol.%， respectively) and lithiated or delithiated graphite (NG15 μm) between room temperature and 3000C at a temperature increasing rate of 50C/min. Fully lithiated and dellithiated graphite samples w巴re el巴ctrochemically prepared after 3 cycles. Electrolyte solution (3μL) and lithiated or delithiated graphit巴(0.8-1.0mg) wer巴 sealedin an airtight Al cell to examine the therrnal stability 2.3 Electrochemical oxidation stability by oxidαtion cuγrent measurements Oxidation currents for 0目90，。目78 or 0.67 moνL LiCIOrECIDECIPC (1・1:1vol.) and 0.90，0.78 or 0.67 mol/L LiCIOrEC/DEC/PC/(A， BヲC，D or E) (1: 1: 1 :0.33， 0.83 or1.5

volラ 1. 0.0，21.7or 33.3 vol.%， respectively) were measured by linear sweep of potential at 0.1 mV/s between 4 and 10 V vs LilLi+ using Pt wire electrode (diameter: 0.3 m m， geometrical surface area: 0.22 cm2_{) (Hokuto Denko， HZ-5000). Counter} and reference electrodes was w巴relithium foil.

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2-trif1uoroethyl)carbonate Fig.:1.Fluorine compounds used in the present study 2.4Charge/discharge chαrαcteristics for NG 15 f1.m electrode in jluorine compound-containing electrolyte solutions Natural graphite (purity: >99.95%) with average pa抗icle size of 15μm (abbreviated to NG 15μm) was used as an electrode material. The d002 value obtain巴d by X-ray diffractometry (XRD-61 00ラShimadzu)was 0.3355 nm. Surface area and meso-pore volume obtained by BET surface area measurement (Tristar 3000， Shimadzu) were 6.9 m2_/gヲ and

0.026 cm3_{/g. Peak intensity ratios of D-band to G-band}

(R=ID/IG) obtained by Raman spectroscopy(NRS-IOOO， Jasco) with Nd:YV04 laser (532 nm) was 0.25 Three-elec仕odecell with natural graphit巴 as a working electrode and lithium foil as counter and reference elec仕odes was used for galvanostatic charge/discharge巴xperiments Natural graphite elec仕odewas prepared as follows. Natural graphite powder was dispersed in N-methyl同2-pyηolidon巴 (NMP) containing 12 wt% poly(vinylidene fluoride) (PVdF)

Safety Improvement of Lithium Ion Batteries by Fluor泊eCompounds and the slurry was pasted on a copper current collector. The electrode was dried at 1200C under vacuum for half a day. After dryingラtheelectrode contained 80 wt% graphite and 20 wt% PV dF. Elec甘olytesolutions w巴reprepared by mixing the fluoro-carbonate with 1 mol/L LiCI04-ECIDEC/PC (1:1:1vol.)目 The fluoro-carbonates are miscible with 1 mollL LiCI04-ECIDEC and EC/DECIPC in whole range of composition at room temperature. The 0.90， 0.78 or 0.67 mol江 LiCIOcEC/DECIPC/(AラB，C， D， or E) (1町1:1 :0.33，0.83 or1.5

voラ.l 10.0，21.7 or 33.3 vol.%， respectively) was used for galvanostatic charge/disch紅g巴experiments.Preparation of 1

mol/L LiCIOcEC/DECIPC/(A， s， C， D， or E) (1:1:1:1.5 vol吋

33.3 vol.%)can be made at room temperature by dissolving LiCI04 in 0.67 mollL LiCI04-ECIDECIPC/(A， BヲCラD，or E)

(1:1:1:1.5 vo，.l33.3 vol.%)， r巴spectively.However， the 0.90，

0.78 or 0.67 mollL LiCI04-ECIDEC/PC/(A， B， C， D， or E) (1:1:1:0.33， 0.83 or1.5 vo，.l 10.0， 21.7 or 33.3 vol.%， respectively) was used for charge/discharge cyclings to simpli今 the exp巴riments. Galvanostatic charge/discharge cyclings were performed using N G 15μm at current densities of 60ラ 150and 300 rr凶Jgbe制leen0 and 3 V relative to Li/Lt refl巴r巴nceelectrode at 250C (Hokuto Denko， H_J1001 SM8A) 3. Results and discussion 3.1 Thermal st，α'biliか

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fluorine compound-mixed electrolyte solutions Thermal stability of fluoro-carbonat巴-mixed electrolyte solutions was evaluated by differential sca皿 ingcalorime町 (DSC) under various conditions as a function of mixing ratio of fluoro回carbonatewith 1 mol/L LiCI04-ECIDECIPC (1:1:1 vol.).

Fig.2 shows DSC curves for the mixtures ofO.67， 0.78 or 0.90 mol江LiCI04-EC/DECIPC(1:1:1 vol.)and d巴lithiatedgraphite.

Exothermic reactions started above 260oC. Exothermic peaks were found at 280oC， 2890C and >300oC， increasing with decreasing concentration ofLiCI04 from 0.90 to 0.67 moνL， which suggests that the exothermic reactions are caused by decomposition of not only organic solvents and SEI but also LiCI04・Mixtures of fluoro-carbonate-mixed solution (33.3 vol.%)and delithiated graphite showed the similar exothermic curves to those in Fig. 2 though exothermic reactions started at slightly lower temperatures between 2500C and 2600C (Fig. 3). No exothermic peaks were found below 2500C in both Figs. 2 and 3. Reactions of lithiated graphite with electrolyte solution and SEI are verγimportant for the safety oflithium ion batteries. Fig. 4 shows DSC curves for lithiated graphite with SEI obtained after 3 cycles. DSC measurements were mad巴

using only lithiated graphite without electrolyte solution. Lithiat巴dgraphit巴obtainedin 0.78 mol/L LiCI04-EC/DEC/PC

gave a weak巴xoth巴rmicpea1王at1280C and another strong one

at 1640C while only one exothermic peak was observed at

1620C for lithiated graphite prepared in 0.67 mol_/L

LiCI04-EC/DECIPC. These exothermic peaks would be due to the reaction of deintercalat巴dlithium with SEI because lithiated

graphite is decomposed by temp巴ratureincrease to~200oC

[45-50].It was reported that LiC6 decomposes toLiC12 by temperature increase to 1200C and Li-intercalat_巴d graphite completely decomposes at around 2000C [50]. The main reactions of d巴intercalatedLi with main compon巴ntsof SEI were also report巴d:2Li+ (CHzOCOzLi)2→ 2Li2C03十C2H4ラ 2Li+ 2CH30Li→2Li20 + CH3CH3 [50]. Strong exothermic peaks shifted to higher temperatures than 162-1640C or almost disappeared for lithiated graphite samples (Lio.94-0.9SC6) prepared in 0.78 mol/L LiCI04-ECIDECIPC/(AヲB，C and D)

(Fig. 4(a))， and for thos巳(Lio.92-0.9SC6)prepared in 0.67 mol/L LiCI04占C/DECIPC/(A，B and C) (Fig. 4(b))， though exothermic peaks were observed at 130 and 1370C for the electrolyte solutions containing fluoro-carbonate B. Thus no significant difference was found between the reactions of deintercalated Li with surface films (Solid Electrolyt巴 Interphase: SEI) prepared in the electrolyte solutions with and without fluorine compounds probably because main reactive species with Li in surface film (Solid Elec仕olyteInterphase: SEI) are lithium alkyl carbonates and lithium alkoxides such as ROCOzLi and ROLi， respectively [50]. Fig. 5 shows DSC curves obtained for mixtures of elec柱。lyte solution and lithiated graphit巴.Mixtures of 0.90， 0.78 or 0.67 mollL LiCI04-ECIDECIPC and lithiated graphite yielded three exothermic peaks at 148-1530C (medium peaks)， 194-2030C

(weak peaks) and 284-2880C (s_仕ongpeaks). The medium and

weak peaks at 14ふ1530Cand 194-2030C， respectively， would

be due to the reactions of deintercalated Li with SEI and elec仕olytesolutions. The medium peaks at 148-1530C may

arise from the decomposition of LiC6， and weak peaks at 194-2030C are probably due to the decomposition ofhigh stage lithiated graphite as mentioned above [50]. The strong peaks at 284-2880C would be due to the decomposition of electrolyte solutions目 Reactions of deintercalatedLi with SEI and elec仕olyte solutions changed depending on the used fluoro-carbonates and their concentrations as shown in Fig. 5 With increase in the concentrations of fluoro-carbonatesヲ exothermic peaks at around 1500C and 2000C disappeared or 55

愛知工業大学総合技術研究所研究報告，第 14号， 2012年 15 20 10 マ国自陸自¥偉岳ロ冨曲国 of shift巴dto higher temperatures. When fluoro-carbonates were

mixed with 1 mol/L LiCI04-EC/DEC/PC by 10.0 vol.% (Fig.

5(a))， fluoro-carbonate C gave the highest effect. The exothermic peak at 1480C disappeared by mlxmg fluoro-carbonate C. A strong peak shifted to higher temperatur巳 in the solution with fluoro四carbonateD though the solution containing fluoro-carbonate B gave an exothermic peak at 1240C._京市enthe amount of mixed fluoro-carbonate increased 56 5 300 mol/L 0.90 mol/L to 21.7 vol.%， improvement of thermal stability was clearly seen as shown in Fig. 5(b). The exothermic peaks due to the deintercalated Li were suppressed for the electrolyte solutions containing fluoro-carbonates B

，

C and D. In case of the solution with fluoro悶carbonateAラtheexoth巴rmlc peak shifted to higher temperature， 1670C. However， veηr broad exothermic peak was observed in the electrolyte solution containing fluoro-carbonateE.Fig. 5(c) indicates DSC curves obtained when fluoro-carbonates were mixed with 1 mol/L

250 Temperature / oc

Fig. 2 DSC cmves for 0.67， 0.78 and 0.90 LiCI04-ECIDEC/PC (1: 1: 1 vol.) and delithiated graphite

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fluoro-carbonate by 33.3 vol.%ラ thereaction of deint紅 白lated SEI and electrolyte pronouncedly suppressed for the solutions containing fluoro-carbonates A， B， C and D. Even in case of the solution with fluoro】carbonateE， the exothermic peak shifted to 1910C目 Exothermic peaks due to the d巴composition of巴lectrol)ぺc solutions were observed between 278 and 2960C for the more was solution with 1 n I L -h t l l 250 Temperature / oc Fig. 3 DSC curves for 0.67 mol/L LiCI04明EC厄EC/PC(l:1:1

vol.) or 0.67 m01生 LiCI04-EC/DECIPC/(A，B， C， D or E)

(1:1:1:1目5vol 3，. 3.3 vol.%) and d巴lithiatedgraphite E

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ECIDECIPC， ECIDEC/PC/ん 300 200 150 8 100 solutions with fluoro-carbonates A， C， D andE.It was above 3000C in the fluoro-carbonate B-mixed electrolyte solutions Thus fiv巴 fluoro-cabonates used in the study improve the The amounts of suppress deintercalated lithium with SEI and electrolyte solution are 10 vol.% for fluoro-carbonate C， 21.7 vol.% for fluoro-carbonat巴S B and D， and 33.3 vol.% for fluoro【carbonatesA andE.It was reported that reduction products in the SEI contain lithium alkyl carbonates (ROC02Li) and lithium alkoxides (ROLi) and Li2C03 is fonned by the reaction of deintercalat巴dLi with (ROC02Li)2 in SEI [50， 51]司 electron-withdrawing ability r巴duce oxygen atoms in carbonate type fluorine compounds， which would suppress the r巴actionof Li with carbonates yielding lithium al匂1carbonates and lithium alkoxides. Fig. 4 clearly shows that the r巴actionsof deintercalated Li with EC， DEC and PC and SEI are significantly reduced by coexistence of fluorine (a) EジDEC/PC/C

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57 Safety Improvement of Lithium Ion Batteries by Fluorine Compounds

ECIDECIPC/(A， B， C， D or E) (1:1:1:1.5 vo 3，.l 3.3 vol.%) and Fig. 4. DSC c町vesfor lithiat巳dgraphite (Lio._92-0 98C6) with SEI film fluorine EC/DEC/PC/A E/DEC/PC/C

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EC/DECIPC， EC/DECIPC乃込 ECIDEC/PC/(;λヲ Elect，γochemicαi 3.2 Electrochemical oxidation stability of electrolyte solutions was evaluated by oxidation current measurements as shown in Fig. 6. Small oxidation currents were observed from 5.8 V， increasing after 6.0 V vs. Li/Lt in the EC江)EC/PCsolvents. Verγsmall currents below 5.8 V were not detected probably because Pt wire electrode had a small surface area. Oxidation ECIDECIPC此， EIDEC/PC/Cヲ ECIDEC/PC/E currents were significantly reduc巴d with of mixed fluoro】carbonates. In th巴 electrolyte solutions containing fluoro-carbonates AラBand C by 21.7 and 33.3 vol.%， oxidation currents were much lower than those in EC/DEC/PC and the solutions containing fluoro-carbonates D and E. Reduction of oxidation currents may be caused due to decrease in surface area of Pt elec仕odeby adsorption of stable fluorine compounds at high potentials.Ithas been found that mixing by 21.7 vol.% is巴noughfor fluoro-carbonates A， B and

C to reduce oxidation currents. However， mixing by 33.3 vol.% is necessary for fluoro-carbonates D and E. The results show that mixing of fluoro-carbonates well improves electrochemical oxidation stability of electrolyte solutions. in the mcrease amounts 20 、_B U ， ， F L u i 園 、 15 10 0 20 15 10 5 5 同 l岡 田 ヘ 広 田 ¥ 陰 。 田 富 由 国 3.3 Chαγge/dischαrge characteristics 01 N G 15μ'Inin fluorine compound-mixed electrolyte solutions 0 20 15 Charge/discharge characteristics of natural graphit巴 electrode were investigated in low potential region because fluorine compounds generally show high oxidation stability but they are electrochemically r巴ducedat higher potentials than EC， DECヲ PC etc.Itwas already reported that elec仕ochemical reduction of fluoro-carbonates start between 1.9 and 2.7 V vs Li/Li+ [39ヲ40]，which are higher potentials than those of EC (1.4V)， DEC (1.3 V) and PC (1.0-1.6 V) [52， 53]. As stated in the Introduction， EC】basedsolvents should be used for high crystalline graphite such as natural graphite to reduce irreversible capacity by the quick formation of SEI on the electrode. Many fluorine-containing compounds can be used for EC/DEC solvents because EC easily forms SEI on natural graphite electrodes [39]. Several exampl巴sare given in Table 1， which indicates that the first coulombic effici閉 じiesobtained in 10 0 100

(a)Li094-0.9SC6 prepared in 0.78 mol/L LiCI04-ECIDEC/PC (1:1・1vol.) or EC/DEC/PC/(A， B， C， D or E) (1:1:1:0.83 vo，.l 21.7 vol.%).

(b) Lio.92-0.9SC6 prepared in 0.67 mol/L LiCI04-EC/DECIPC (1:1:1 vol.) or EC/DEC/PC/(A， B， C， D or E) (1:1:1:1.5 volラ 33.3 vol.%).

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ECIDEC/PC/己診 5 or 300 Fig.5.DSC curves for mixtures of fluoro-carbonate containing electrolyte solution and lithiated graphite (Lio.92-0.9SC6). (a) 0.90 mol/L LiCI04-EC/DEC/PC (1:1:1 vol.) EC/DEC/PC/(A， B， C， D or E) (1:1:1:0.33 vo，.l10.0 vol.%) and Lio.96_0.98C6・

(b) 0.78 mol/L LiCI04-ECIDEC/PC (1:1:1 vol.) EC/DECIPC/(A， B， CラD or E) (1:1:1:0.83 vo，.l21.7vol.%) and Lio.96-0.97C6・

(c) 0.67 mol/L LiCI04-EC/DECIPC (1:1:1 vol.) or

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(al) and (a2): 0.90 mol/L LiCI04-EC/DEC/PC (1:1:1 voI.)and ECIDEC/PC/(A， B， C， D or E) (1:1:1 :0.33 voラ.I10.0voI.%).

(bl) and (b2): 0.78 mol/L LiCI04-ECIDEC/PC (1:1:1 voI.)and ECIDEC/PC/(AラBラCラDorE) (1 :1:1 :0.83 voラ.I2l.7voI.%).

(cl) and (c2): 0.67 mol/L LiCI04-ECIDEC/PC (1:1:1 voI.)and ECIDEC/PC/(A， B， CヲDor E) (1:1:1:1.5vo，.I33.3 voI.%).

X: ECIDECIPC， A: EC/DECIPC/九ヲB:EC/DECIPC/瓦 C:EC/DECIPC/C， D: EC/DEC/PC/;)， E: ECIDEC/PC/紅

Table 1 First coulombic effici巴nciesfor natural graphit巴巴lec仕odesin 0.67 moνL LiCI04-EC/DEC (1:1voI.)and EC/DEC/(A， B or

C)(1・1:1vo，I.33.3 voI.%)at 60 mA/g. First coulombic efficiency / % ECIDECffi Natural ECIDEC/C EC/DEC/A EC/DEC 72.3 74.4 76.3 75.1 80.3 80.3 77.9 78.0 84.0 84.4 75.9 77.7 86.2 82.0 82.7 85.6 graphite NG5μm NGlOμm NGl5μm NG25μm NG40μm 87.5 In ref.39ラdataare not given as Tabl巴. 88.4 87.1 85.7

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3

v

o

l

.

ろ

も

150

m

A

g

-

l

o

200 400 600 800 1000 0 200 400 600 800 1000 0

Capadty /

mAhg

・1

1

0

.

0

vol%

300

mA

g

-

1

2

1

.7

v

o

l.%

300

mA

g

胴1

3

3

.

3

v

o

l.%

300

臨

Ag

・1 300 600 900 1200 59

Fig. 7. First charge/discharge curves ofNG15μm electrod巳in0.90ラ0.78and 0.67 mol/L LiCI0cEC/DEC/PC (1:1:1 vol.)and 0.90，

0.78 and 0.67 mol/L LiCI04-ECIDECIPC/(AラBラCラD or E， 1: 1: 1:0.33ラ0.83or1.5 volラ10.0，21.7or 33.3 vol.%，respectively) as

functions of concentration off1uoro-carbonate and current density.

回 出 _ _EC/DECIPC -EC/DEC/PC/九 一 一 一 一 一ECIDECIPC/忍

山 岡 田 間 四 国ECIDECIPC/C ぃEC/DEC/PC/ひ 一広一拍パーECIDECIPC/玄

Table 2 First coulombic efficiencies (%) for NG15μm electrode in 0.90，。司78or 0.67 mol/L LiCI04-ECIDECIPC (1:1: 1 vol.)and

0.90，0.78 or 0.67 mol/L LiCI04-ECIDECIPC/(A， B， C， D or E) (1:1:1 :0.33ラ0.83or1.5 vo，.l10.0，21.7 or 33.3 vol.%，respectively).

Concentration of f1uoro同carbonate/ vol.% 10.0 21.7 33.3 Solvent Current density / mA/g 60 150 300 60 150 300 60 150 300 ECIDECIPC 57.4 50.7 50.7 56.9 43.3 45.5 49.6 37.9 23.5 EC/DECIPC/ A 55.8 55.1 56.1 59.1 53.3 38.7 63.2 54.6 37.9 ECIDECIPCIB 79.8 73.3 57.9 84.1 76.1 70.3 84.1 80.0 73.7 EC/DECIPC/C 47.3 39.3 46.4 51.9 57.6 52.9 65.9 59.6 55.2 EC/DEC/PCID 45.5 35.5 48.9 47.6 50.0 54.5 70.9 57.2 55.1 EC/DEC/PCIE 42.2 38.4 49.0 53.0 58.2 54.7 75.0 58.6 53.2

Fig. 8. Charge capacities ofNG15μm electrode in 0.90， 0.78 and 0目67mol/L LiCI04-ECIDECIPC (1:1:1 vol.) and 0.90，0.78 and

0.67 mol/L LiCI04-ECIDECIPC/(A， B， C， D or E， 1:1:1:0.33ヲ0.83or1.5 volラ 1. 0.0ラ21.7or 33.3 vol.%， respectively) as functions of

concentr低ionoff1uoro凶carbonateand current density

Q E C厄ECIPC

，

蓄

量

EC:DEC/PC/

ふ欝

ECIDECIPCIB

，

傘

ECIDECIPC/C

，

ECIDECIPCH，j EC/DEC/PC尽 f1uoro-carbonate-mixed elec仕01戸e solutions are n巴arly the

same as or slightly higher than those obtained in EC厄EC without f1uorine compounds.It means that the f1uorine compounds can be used in ECIDEC solvents. If decomposed products off1uorine compounds quickly form SEI on graphite electrodeラPC-containingsolvents with low melting points can

be also used. Fig. 7 shows白rst charge/discharge curves obtained in EC厄ECIPC(1:1:1vol.) andEC/DEC/PC/(A， B， C， D or E) (1:1・1:0.33， 0.83 or1.5 vol，.10.0， 21.7 or 33.3 vol.%， respectively) as functions of concentration of f1uoro-carbonate and cu汀entdensity. The potential plateaus at 0.8 V vs Li/Li+ in Fi.g. 7 indicates th巴 reduction decomposition of PC. In EC江)ECIPCsolvent without f1uorine compound， the potential plat巴aulengthened with decreasing concentration of LiCI04 企om0.90 to 0.67 mol/L and increasing current density from 60 to 300 mA/g. Particularly long potential plateaus were observed in 0.67 mol/L LiCI04-ECIDEC/PC. According to this change in potential plateau， first columbic efficiency in ECIDECIPC solvent decreased with decreasing concentration of LiCI04 and mcr巴asingcurrent density as giveninTable 2. On the other

Safety Improvement of Lithium Ion Batteries by Fluorine Compounds

hand， in the fluoro-carbonate-mixed el巴ctrolytesolutions， the

potential plateau was shortened with incr巴asingconcentration

offluoro-carbonate from 10.0 to 33.3 vol.%and current density from 60 to 300mAlg.The difference in ECIDEC/PC with and without fluorine compound was clearly observed when fluoro-carbonate was mixed by 33.3 vol.%，where the electrode potentials were quicldy lowered. As shown in Table 2， first coulombic efficiency in EC/DEC/PC/(A， B， C， D or E) solvent mcr巴asedラ that is， irreversible capacity decreased with increasing concentration of fluoro-carbonate and current density. Table 2 indicates that fluoro-carbonate B is the b巴st among five fluoro-carbonates examined， giving much higher fluoro-carbonate containing electrolyte solutions. F1uoro-carbonate B is the best compound among five fluoro-carbonates， giving much higher first cou1ombic ef白ciencies，i.e. 10wer irreversib1e capacities than others in PC-containing solven.t Much higher first coulombic effici巴ncies than those in EC/DECIPC solvent wer巴 also observed for other fluoro-carbonate-mix巳dsolutions by 33目3 vol.%. References

first coulombic efficiencies， i.e. lower irreversible capacities [lJX. Wang， E. Yasukawa， S. Kasuya， J. Electrochem. Soc.， than others. For other fluoro-carbonates except B， much higher 148， A1058-A1065 (2001)

日rstcoulombic efficiencies than those in EC/DECIPC solvent [2JX. Wang， E. Yasukawa， S. Kasuya， J.Electrochem. Soc.，

W巴realso obtained by mixing of fluoro-carbonates by 33.3 148， A1066-A1071 (2001)

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fluoro-carbonates needed to suppress the reaction of Li with electrolyte solution and SEI are 10 vol.%for fluoro-carbonate C， 21.7 vol.%for fluoro-carbonates B and D， and 33.3 vol.% for fluoro-carbonates A andE.Electrochemical oxidation stability was also significantly improved by mixing of fluoro-carbonates. To decrease the oxidation current， mixing by 21.7 vol.%is necessaη

，

for fluoro-carbonates A， B and C， and mixing by 33.3 vol.%for fluoro-carbonates D andE.In the fluoro-carbonate-mixed elec位olyte solutions， reduction decomposition of PC decreased with increasing concentration offluoro-carbonate from 10.0 to 33.3 vol.%and current density from 60 to 300mAlg.As a result，自rstcoulombic efficiency increasedヲthatis， irreversible capacity decr巴asedin the

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