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EC + DMC + EMC electrolyte for lithium ion batteries
RARE METALS Vol. 25 , spec. issue , oct 2006, p .94
Thermal stability of LiPFJEC + DMC + EMC electrolyte for lithium ion batteries WANG Qingsong‘), SUN Jinhua” , and CHEN Chunhuu” State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, China 2) Department of Materials Science & Engineering, University of Science and Technology of China, Hefei 230026, 1)
China (Received 2006-08-20)
Abstract : The thermal stability of lithium-ion battery electrolyte could substantially affect the safety of lithium-ion battery. In order to disclose the thermal stability of 1 .O mol L- LiPF6/ethylene carbonate (EC) + dimethyl carbonate (DMC) + ethylmethyl carbonate (EMC) electrolyte, a micro calorimeter C80 micro calorimeter was used in this paper. The electrolyte samples were heated in argon atmosphere, and the heat flow and pressure performances were detected. It is found that LiPF6 influences the thermal behavior remarkably, with more heat generation and lower onset temperature. LiPF6/EC shows an exothermic peak at 212 “c with a heat of reaction - 355.4 J * g - . DMC based LiPF6 solution shows two endothermic peak temperatures at 68.5 and 187 9: in argon filled vessel at elevated temperature. EMC based LiPF6 solution shows two endothermic peak temperatures at 191 and 258 “c in argon filled vessel. 1.O mol*L-’ LiPF6/EC + DMC + EMC electrolyte shows an endothermic and exothermic process one after the other at elevated temperature. By comparing with the thermal behavior of single solvent based LiPF6 solution, it can be speculated that LiPF6 may react with EC, DMC and EMC separately in 1 .O m0l.L-I LiPFs/EC + DMC + EMC electrolyte, but the exothermic peak is lower than that of 1 - 0 mol * L- LiPF6/EC solution. Furthermore, The 1 . 0 mol ’ LLiPFs/EC + DMC + EMC electrolyte decomposition reaction order was calculated based on the pressure data, its value is n = 1.83, and the pressure rate constants k, = 6.49 x lO-’kPa. -0.83*min-’. Key words: lithium ion battery ; electrolyte; thermal stability; C80 micro calorimeter
[ This study was jimncially supported by ‘‘ 100 Talents Project” of Chinese Academy of Sciences , Nature Science Funds of Anhui Province ( No .050450403) , and Youth Funds of USTC are also appreciated. ]
1.
Introduction
Lithium ion batteries are currently widely used in many portable electronic devices and in recent years the high performance lithium ion batteries are used for electric vehicle (EV) and other large-sized equipment[ 1-21. The extensive applications of the lithium ion batteries generate increasing safety concerns and many studies were concern on the safety of lithium ion batteries[ 3-81 . It is thought that safety is related mainly to the thermal reactivity of the mateCorresponding author: SUN Jinhua
rials in the battery. As the electrolyte links to anode and cathode, its thermal stability is a key element for the safety of lithium ion batteries [ 9111. Therefore, knowledge of the thermal behavior of the electrolyte is essential for designing safer and higher performance batteries. 1.0 mol*L-’ LiPF6/ethylene carbonate (EC) + dimethyl carbonate (DMC) + ethylmethyl carbonate (EMC) electrolyte is widely used in the commercial lithium ion battery, and in order to disclose its thermal stability, it was studied by using C80 micro calorimeter in this paper.
E - d : sunjh @ustc. edu .cn
Wmg Q ,S .et id. , Thermal stabiity of LiPFJEC+ DMC + EMC electrolyte for lithium ion batteries
2.
Experimental
The C80 Cdvet calorimeter was used in this study, which is a heat flux”calorimeter manufactured by Setaram in France. It is capable to operate in scanning, or as the case of this work, in isothermal constant rising mode. The sensors of the calorimeter are two fluxmeters with a detection limit in power of 2 pW and a calorimetric resolution of 0.1 pW, which are assembled inside of a calorimetric block with a temperature control of at least 0.001 “c range from room temperature to 300 “c [ 12-131. The organic solvents are commercially available products made by Zhangjiagang Guotai-Huarong c o . , Ltd. The salt LiPF6 was produced by Tianjin Jinniu Co. Ltd . Several solutions, i . e . , 1.0 mol*L-’ LiPF6 in EC, DMC, EMC and EC+DMC+EMC ( 1 : l : l w / w ) were prepared in argon filled glove box. In this study, the samples were placed in an 8.5 ml standard high pressure vessel, and were heated from ambient temperature to 300 “c under a dry argon condition, and 1.0 mol * L-’ LiPF6/EC + DMC + EMC was sealed in a high pressure transducer fitted vessel ( V = 3.5 cm’) for C80 experiment. The weights of the samples were about 500 mg and the heating rate is 0.2 “camin-’.
*
3.
Results and discussion
3.1. Thermal behavior of EC Fig. 1 presents the heat flow curves of 1 .o mol*L-’ LiPF6/EC and pure EC in argon filled vessel at elevated temperature. It clearly indicates that the existence of LiPF6 impacts the thermal behavior of EC . The pure EC expresses itself a weak exothermic peak at 198 “c between the range 158 - 241 “c with a heat of reaction of - 43.8 J * g-’, followed by an increasing heat flow trend. With the addition of LiPF6, two exothermic peaks is observed, a pronounced sharp one between 192 and 226 “c and the peak temperature is 212 “c with a heat of reaction - 355.4 J g- . The following broad and lower one is between about 225 and 280 “c and the peak temperature is 253.7 “c with a heat of reaction - 226.8 J. g-’ .
’
-
160
-
&
120-
5 5
100:
F.
60-
8
404
2 X
‘
A
140 -
+
!II I
80 -
95
1.0 mo1.L’ LiPFdEC
in argon filled vessel
+EC in argon filled vcssel
- 102 -
160
200
260
300
Temperature/%
Fig.1. CSO heat flow curves of 1.0 mol * L-’ LiiFk/EC and EC in argon filled vessel.
At the elevated temperature, before the boiling point 238 “c , EC is stable and with little heat generation. After the temperature is over 260 “c , EC decomposes and generate gases such as COz, 0 2 , and H2[14-151. It is generally thought LiPF6 decompose as L i P F 6 ( s ) e L i F ( s ) + PF5(g)[16], when the temperature is over 192 ‘32, LiPF6 acts as a Lewis acid with EC solvents to generate transesterification products, poly ethylene oxide (PEO) polymers and COz[ 171. Under the attack of PF5 in higher temperature, the ring is broken[ 151 . Furthermore, Li’ will react with EC in rising temperature, too. Mogi et at!.[ 181 detected the thermal decomposition products such as CO, CzH4 and so on by pyrolysis-gas ( 300 “c ) chromatography-mass spectroscopy (Py-GC-MS). Then the possible processes are proposed as following[ 18-191 : CHzOCOOCHz(EC)t 2Li’ + 2e--)(CHz0Li), + co (1) CH2OCOOCHz ( EC ) + Li’ + eCHzCH20COzLi (2) 2CH&Hz0CO2Li LiOC02CHzCHzOC02Li+ C2H4 (3)
-+
-+
3.2. Thermal behavior of DMC Fig. 2 shows that DMC has an endothermic peak temperature at 91 “c in argon filled vessel, which is according with its boiling point 90 “c with the endothermic heat I . 5 J g-’ . DMC
RARE MEYXS
%
16
-+
, Vol. 25, Spec. Issue,
2CH4 2CH30C02CH3 + 2e2CH30COzLi + 2CH4
1 .O m o l V LiPF,/DMC in argon field vessel
+
2Li’
Oct 2006
+
(5) H2
+
(6)
3.3. Thermal behavior of EMC
(-104
, 60
I
, 100
.
, 150
.
, 200
.
,
.
260
4 300
Temperature/%
Fig.2. C80 heat flow curves of 1.0 rno1-L-l L i P F a M C and DMC in argon filled vessel.
shows stable characteristics with slow endothermic phenomenon in argon filled vessel at elevated temperature, and a sharp exothermic occurs at 223 T. An endothermic peak appears at 68.5 “c in 1.0 mol. L-’ WF6/DMC solution with the endothermic heat 15.4 J * g - I in Fig. 2 , then, it undergoes a fluctuant process till to 187 “c. The followed zigzag endothermic process reaches to peak temperature at 219 “c the with heat of reaction 41.7 J - g - ’ . The exothermic process of pure DMC may be caused by the cleavage of C-0 bond in higher temperature[ 151 . The molecule of DMC possesses two active centers (alkyl and carbonyl carbons), whose reactivity can be tuned with the temperature. In particular, two distinct pathways can be recognized in the reaction of DMC with a generic anionic nucleophile ( Y - ) [ 201. When’ PFs- nucleophile attacks at carbony1 carboq of DMC , the cleavage of acyl-oxygen bond generates a methoxycarbonyl product at lower temperature ( < 100 T ) [ 21 3 . When temperature is over 187 “r: , PF6- nucleophile attacks at DMC and undergoes a methylation reaction to generate methoxide and CO, . At the elevated temperature, the endothermic processes show that Lit also react with DMC and the possible reactions as follows[ 19, 221 : CH30C02CH3+ 2e + 2Li + +2CH30Li + CO (4) CH30CO2CH3+ 2e + 2Li + + H2 Li2C03 + +
Fig. 3 shows the heat flow curves of 1 . 0 mol L-’ LiPF6/EMC and pure EMC in argon filled vessel at elevated temperature. In argon atmosphere, EMC shows an exothermic peak temperature at 164 “c with a heat generation of - 2 5 . 8 Jog-’. At 236 T , EMC starts to release heat again, and reaches to peak temperature at 243 “c . After LiPF6 dissolves in EMC, the solution starts its endothermic process at 154 “c, and reaches to the first endothermic peak temperature at 191 ‘C with the heat of absorption 163. 7 J g-’. Later, an exothermic peak was detected at 208 “c with the hat of generation - 148- 3 J g- . At last, another endothermic peak temperature was detected at 258 T with the heat of absorption 45.6 Jog-’.
’
2o
-m-
1 .O mo1.L-l LiPFJEMC in argon filled vessel
. -0- EMC in argon tilled vessel
10-
; E
0:
Y
g
-10-
2
-20
C
I1 30
60
I00
160
200
250
300
Ternperature/”C
Fig.3. CsO heat flow curves of 1.0 mol*L-’ LiPF&MC and EMC in argon fded vessel. Yoshida et a l . [ 191 ever used Fouriertransform infrared (FT-IR) technique to speculate the exothermic process of EMC at elevated temperature. The trans-esterification is shown in Eq. ( 7 ) . That the sharp exothermic process of EMC appears at 230 “c may be the polyrneric or decomposition reaction[ 191 . 2EMCzDMC + DEC (7) L i F 6 produces a strong Lewis acid, PFs , and PF, attacks the electron lone pair of oxygen of a solvent molecule of EMC , which causes the
Wung Q .S .et al . , Thermal stability of LiPFJEC + DMC + EMC electrolyte for lithium ion batteries
97
sively, with the endothermic heat and exothermic heat of 29.5 and - 494.8 J *g-' , respectively. In order to disclose the reaction process of 1.0 mol * L-' LiPFJEC + DMC t EMC electrolyte at elevated temperature, the experimental result of 1.0 rnol-L-' LiPF,/EC, 1.0 mol*L-' LiPF,/DMC and 1 .0 mol L- LiPF6/EMC were averaged and the result was shown in Fig. 5 . It can be seen in Fig. 5 that the average result is similar with the experimental result. Endothermic and exothermic processes were found successively in the two curves, and they show close endothermic peak temperature, but the exothermic peak temperature is delayed 8 "c in averaged curve. By comparing the heat flows of 1 . 0 mol. L-' LiPF6/EC + DMC + EMC, 1 . 0 mo1.L-l LiPF6/EC, 1.0 mol. L-' LiPF,/DMC, 1.0 mol. L-' LiPF,/EMC and their averaged result, it can be speculated that the endothermic process is mainly contributed by EMC and the exothermic process is mainly contributed by EC . Then, it is concluded that LiPF6 may be reacting with EC , DMC and EMC respectively.
'
ii1'
9
-
1 .O mol-L-' LiPFJEC+DMC+EMC in argon filled vesscl 5 0 - -EC+DMC+EMC F
-
-Experimental result of 1 .O mol L-' LiPF,/EC+ /\ 40 DMC+EMC -Average result of 30 1 .O rnol L-' LiPF,/EC 1.0 rnol L-' LiPFlDM 1 .O rnol L-' LiPFJEM
j:
-10
I
- 2 o J , 100
.
, 150
.
,
.
200
,
.
250
I
300
Temperature/"C
Fig.5. Comparison of 1.0 mo1.L-l LiF,JEC + DMC + EMC With the average of 1.0 mol-L-' LipFd EC, 1 . 0 moleL-' LiFflMC and 1 . 0 mol . L - ' LiPFaMC in argon fdled vessel.
-10 - 2 0 , . , 60 90
.
, . ~. 120
150
, . , 180
210
,
, . , . 240
270
300
TemperaturdC
Fig.4. CSO heat flow curves of 1. 0 mol * L-' LPF&C +DMC + EMC and EC + DMC + EMC in argon filled vessel.
3.5. Pressure behavior of LiPF6/EC
DMC + EMC
+
It is thought that electrolyte decompose to gas at elevated temperature [ 221 . The pressure was detected by a high pressure transducer and
RARE iUEZ4L.S , Vol. 25, Spec. Issue,
98
'
Fig.6 shows the result of 1.0 mol L- LiPF6/ EC + DMC + EMC at elevated temperature. It can be seen that the pressure is increasing with temperature increasing. At about 150 T , the pressure starts to increase and get sharp increase at 178 "c , which is corresponding to the endothermic process in the heat flow curve. With temperature increasing, the pressure increases sharply and reaches 15 MPa after the exothermic process, which indicates that a lot of gases have been produced. The gases generation rate can be determined by Eq. ( 13) [ 16,
25
I
dP
-= dt
k,P"
maybe consist of C2Hz, CH,, CO , C02 etc . according to the above researches. In sealed lithium ion batteries, once the electrolyte decomposs, coupling with the reaction between electrolyte and electrodes, a lot of gases cause the pressure increasing till the battery explosion. Therefore , the electrolyte thermal stability plays a key role for lithium ion battery safety.
0.0 -
3
-0.5 -
a
4.0:
-
-1.5-
f
Taking natural logarithm of Eq .( 13) : dP h(-) = n h P + Ink, dt
Oct 2006
-2.0,
1 I
-2.5
-3.01, 0.3
20
# 1.0 rnc4.t' UPF@C+DMC+
.
,
.
0.6
, 0.9
.
, 1.2
.
, 1.5
.
6
In P
Fig.7. In(dPldt) versus InP for calculation of n and R , , the dash line is fitted Line.
4. -20-1 50
,
, 100
.
, 150
.
,
.
200
, 250
. c -2 300
Temperantre/'C
Fig. 6 . Heat flow and pressure changing curves of 1 .O mole L - * LipF&C + DMC + EMC at elevated temperature.
By plotting the curve of l n ( d P / d t ) versus 1nP from the onset temperature to the end of reaction, the reaction order ( n ) and the rate constants of reaction k, can be easily calculated. Fig.7 shows its plot, where the slope of the fitted line is the reaction order n = 1.83, then the pressure rate constants k, = 6. 49 x kpa -0.63min-' . The larger reaction order of 1 . 0 mol * LLiPFdEC + DMC + EMC decomposition indicates that the reaction is greatly affected by the concentration, and the higher pressure rate constants indicates that the reaction goes on rapidly, which produce a lot of gases and causes the pressure increasing rapidly. The gases
'
Conclusions
The C80 calorimeter measurements were used to evaluate the thermal properties of 1.0 mol. L-' LiPF6/EC + DMC + EMC in argon sealed vessel from ambient temperature to 300 T . By studying the high-temperature thermal properties of LiPF6/EC, LiPF6/DMC, LiPFJ EMC, and LiPF6/EC + DMC + EMC one by one, it is shown that in 1.0 mol*L-' LiPF6/EC + DMC + EMC electrolyte, LiPF6 maybe reacts with EC , DMC and EMC separately. Furthermore, it is found that LiPF6 influences the thermal behavior remarkably, with more heat generation and lower onset temperature. The pressure data were taken to determine the reaction order and pressure rate constants of LiPF6/EC+ DMC + EMC decomposition. The results show that this electrolyte can decompose severely and the gases is the main hazards to lithium ion battery safety. ,
References [ 1 ] Kennedy B . , Patterson D. , and Camilleri S. ,
Wung Q .S .et ul ., Thermal stability of LeFJEC + DMC + EMC electrolyte for lithium ion batteries Use of lithium-ion batteries in electric vehicles. J . Power Sources, 2000, 90: 156. Hyung Y . E . , Moon S.I., Yum D . H . , et a l . , Fabrication and evaluation of 100 Ah cylindrical lithium ion battery for electric vehicle applications. J . Power Sources , 1999, 81 82 : 842. Spotnitza R . , and Franklin J . , Abuse behavior of high-power, lithium-ion cells. J . Power Sources, 2003, 113: 81. Biensan Ph., Simon B . , Peres J . P . , et a l . , On safety of lithium-ion cells. J . Power Source s , 1999, 81 -82: 906. Tobishima S . , Takei K . , Sakurai Y . , et a l . , Lithium ion cell safety. J . Power Sources, 2000, 90: 188. Wang Q . S . , Sun J . H . , Yao X.L., et a l . , 4isopropyl phenyl diphenyl phosphate as flame-retardant additive for lithium-ion battery electrolyte. Electrochem. Soli&State Lett. , 2005, 8 (9) : A467. Wang Q . S . , Sun J . H . , Yao X . L . , et a l . , Thermal behavior of lithiated graphite with electrolyte in lithium ion batteries. J . Electrochem . S o c . , 2006, 153(2): A329. Yamauchi T., Mizushima K . , Satoh Y . , et a1 . , Development of a simulator for both property and safety of a lithium secondary battery. J. Power Sources, 2004, 136: 99. Wang Q . S . , Sun J . H . , Yao X . L., et a l . , Thermal stability of LiPF,j/EC t DEC electrolyte with charged electrodes for lithium ion batteries. % m c h i r n . Acta, 2005, 437(1 -2): 12. Ravdel B . , Abraham K . M . , Gitzendanner R . , et a l . , Thermal stability of lithium-ion battery electrolytes. J Power Sources , 2003, 119 121: 805. Wang Q . S . , Sun J . H . , Yao X . L., et a l . , C80 calorimeter studies on the thermal behavior of LiPF6 solutions. J . Solution Chemistry, 2006, 35(2): 179. Sun J . H . , Li X . , Hasegawa K . , et a l . , Thermal hazard evaluation of complex reactive substance using calorimeters and Dewar vessel. J %rm A n a l . Cal . , 2004, 76(3) : 883. Sun J.H. , Li Y . F. , and Hasegawa K . , A study of self-accelerating decomposition temperature (SADT) using reaction calorimetry. J . Loss Prevent. Process Id., 2001, 14: 331. [14] Botte G. G . , and Bauer T. J . , MRSST a new
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[16]
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[21]
[22]
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method to evaluate thermal stability of electrolytes for lithium ion batteries. J. Power Source s , 2003, 119-121: 815. Moshkovich M . , Cojocaru M . , Gottlieb H. E . , et a l . , The study of the anodic stability of alkyl carbonate solutions by in situ FTIR spectroscopy, EQCM, NMR and MS. J . Ekctroanal. C h m . , 2001, 497: 84. Wang Q . S . , Sun J . H . , Lu S . X . , et a l . , Study on the kinetics properties of lithium hexafluorophosphate thermal decomposition reaction. Solid State Zonics , 2006, 177 ( 1 - 2 ) : 137. Sloop S.E., Kerr J.B. , and Kinoshita K. , The role of Li-ion battery electrolyte reactivity in performance decline and self-discharge. J . Power Sources, 2003, 119 121: 330. Mogi R . , Inaba M . , Iriyama Y . , et a l . , Study on the decomposition mechanism of alkyl carbonate on lithium metal by pyrolysis-gas chromatography-mass spectroscopy. J . Power Sources , 2003, 119- 121: 597. Yoshida H. , Fukunaga T. , Hazama T. , et a l . , Degradation mechanism of alkyl carbonate solvents used in lithium-ion cells during initial charging. J . Power Sources, 1997, 68: 311. Tundo P., New developments in dimethyl carbonate chemistry. Pure Appl . Chem . , 2001, 73: 1117. Ono Y . , Dimethyl carbonate for environmentally benign reactions. Pure Appl. Chem . , 1996, 68: 367. Kumai K . , Miyashiro H. , Kobayashi Y. , et al , Gas generation mechanism due to electrolyte decomposition in commercial lithium-ion cell. J. PowerSources, 1999, 81-82: 715. Gnanaraj J. S . , Zinigrad E . , Asraf L. , et a l . , A detailed investigation of the thermal reactions of LiPF6 solution in organic carbonates using ARC and DSC. J . Ekctrochem . Soc . , 2003, 150(11): A1533. Kawamura T. , Kimura A. , Egashira M . , et a1 . , Thermal stability of alkyl carbonate mixedsolvent electrolytes for lithium ion cells. J . Power Sources, 2002, 104: 260. Song S.M. , Wang Z. L., and Li W. B. , Physical Chemistry, Part B , third ed. , High Education Press, Beijing, 1993.
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[ 181
[23]
[24]
[25]
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Thermal stability of LiPFJEC + DMC + EMC electrolyte for lithium ion batteries WANG Qingsong‘), SUN Jinhua” , and CHEN Chunhuu” State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, China 2) Department of Materials Science & Engineering, University of Science and Technology of China, Hefei 230026, 1)
China (Received 2006-08-20)
Abstract : The thermal stability of lithium-ion battery electrolyte could substantially affect the safety of lithium-ion battery. In order to disclose the thermal stability of 1 .O mol L- LiPF6/ethylene carbonate (EC) + dimethyl carbonate (DMC) + ethylmethyl carbonate (EMC) electrolyte, a micro calorimeter C80 micro calorimeter was used in this paper. The electrolyte samples were heated in argon atmosphere, and the heat flow and pressure performances were detected. It is found that LiPF6 influences the thermal behavior remarkably, with more heat generation and lower onset temperature. LiPF6/EC shows an exothermic peak at 212 “c with a heat of reaction - 355.4 J * g - . DMC based LiPF6 solution shows two endothermic peak temperatures at 68.5 and 187 9: in argon filled vessel at elevated temperature. EMC based LiPF6 solution shows two endothermic peak temperatures at 191 and 258 “c in argon filled vessel. 1.O mol*L-’ LiPF6/EC + DMC + EMC electrolyte shows an endothermic and exothermic process one after the other at elevated temperature. By comparing with the thermal behavior of single solvent based LiPF6 solution, it can be speculated that LiPF6 may react with EC, DMC and EMC separately in 1 .O m0l.L-I LiPFs/EC + DMC + EMC electrolyte, but the exothermic peak is lower than that of 1 - 0 mol * L- LiPF6/EC solution. Furthermore, The 1 . 0 mol ’ LLiPFs/EC + DMC + EMC electrolyte decomposition reaction order was calculated based on the pressure data, its value is n = 1.83, and the pressure rate constants k, = 6.49 x lO-’kPa. -0.83*min-’. Key words: lithium ion battery ; electrolyte; thermal stability; C80 micro calorimeter
[ This study was jimncially supported by ‘‘ 100 Talents Project” of Chinese Academy of Sciences , Nature Science Funds of Anhui Province ( No .050450403) , and Youth Funds of USTC are also appreciated. ]
1.
Introduction
Lithium ion batteries are currently widely used in many portable electronic devices and in recent years the high performance lithium ion batteries are used for electric vehicle (EV) and other large-sized equipment[ 1-21. The extensive applications of the lithium ion batteries generate increasing safety concerns and many studies were concern on the safety of lithium ion batteries[ 3-81 . It is thought that safety is related mainly to the thermal reactivity of the mateCorresponding author: SUN Jinhua
rials in the battery. As the electrolyte links to anode and cathode, its thermal stability is a key element for the safety of lithium ion batteries [ 9111. Therefore, knowledge of the thermal behavior of the electrolyte is essential for designing safer and higher performance batteries. 1.0 mol*L-’ LiPF6/ethylene carbonate (EC) + dimethyl carbonate (DMC) + ethylmethyl carbonate (EMC) electrolyte is widely used in the commercial lithium ion battery, and in order to disclose its thermal stability, it was studied by using C80 micro calorimeter in this paper.
E - d : sunjh @ustc. edu .cn
Wmg Q ,S .et id. , Thermal stabiity of LiPFJEC+ DMC + EMC electrolyte for lithium ion batteries
2.
Experimental
The C80 Cdvet calorimeter was used in this study, which is a heat flux”calorimeter manufactured by Setaram in France. It is capable to operate in scanning, or as the case of this work, in isothermal constant rising mode. The sensors of the calorimeter are two fluxmeters with a detection limit in power of 2 pW and a calorimetric resolution of 0.1 pW, which are assembled inside of a calorimetric block with a temperature control of at least 0.001 “c range from room temperature to 300 “c [ 12-131. The organic solvents are commercially available products made by Zhangjiagang Guotai-Huarong c o . , Ltd. The salt LiPF6 was produced by Tianjin Jinniu Co. Ltd . Several solutions, i . e . , 1.0 mol*L-’ LiPF6 in EC, DMC, EMC and EC+DMC+EMC ( 1 : l : l w / w ) were prepared in argon filled glove box. In this study, the samples were placed in an 8.5 ml standard high pressure vessel, and were heated from ambient temperature to 300 “c under a dry argon condition, and 1.0 mol * L-’ LiPF6/EC + DMC + EMC was sealed in a high pressure transducer fitted vessel ( V = 3.5 cm’) for C80 experiment. The weights of the samples were about 500 mg and the heating rate is 0.2 “camin-’.
*
3.
Results and discussion
3.1. Thermal behavior of EC Fig. 1 presents the heat flow curves of 1 .o mol*L-’ LiPF6/EC and pure EC in argon filled vessel at elevated temperature. It clearly indicates that the existence of LiPF6 impacts the thermal behavior of EC . The pure EC expresses itself a weak exothermic peak at 198 “c between the range 158 - 241 “c with a heat of reaction of - 43.8 J * g-’, followed by an increasing heat flow trend. With the addition of LiPF6, two exothermic peaks is observed, a pronounced sharp one between 192 and 226 “c and the peak temperature is 212 “c with a heat of reaction - 355.4 J g- . The following broad and lower one is between about 225 and 280 “c and the peak temperature is 253.7 “c with a heat of reaction - 226.8 J. g-’ .
’
-
160
-
&
120-
5 5
100:
F.
60-
8
404
2 X
‘
A
140 -
+
!II I
80 -
95
1.0 mo1.L’ LiPFdEC
in argon filled vessel
+EC in argon filled vcssel
- 102 -
160
200
260
300
Temperature/%
Fig.1. CSO heat flow curves of 1.0 mol * L-’ LiiFk/EC and EC in argon filled vessel.
At the elevated temperature, before the boiling point 238 “c , EC is stable and with little heat generation. After the temperature is over 260 “c , EC decomposes and generate gases such as COz, 0 2 , and H2[14-151. It is generally thought LiPF6 decompose as L i P F 6 ( s ) e L i F ( s ) + PF5(g)[16], when the temperature is over 192 ‘32, LiPF6 acts as a Lewis acid with EC solvents to generate transesterification products, poly ethylene oxide (PEO) polymers and COz[ 171. Under the attack of PF5 in higher temperature, the ring is broken[ 151 . Furthermore, Li’ will react with EC in rising temperature, too. Mogi et at!.[ 181 detected the thermal decomposition products such as CO, CzH4 and so on by pyrolysis-gas ( 300 “c ) chromatography-mass spectroscopy (Py-GC-MS). Then the possible processes are proposed as following[ 18-191 : CHzOCOOCHz(EC)t 2Li’ + 2e--)(CHz0Li), + co (1) CH2OCOOCHz ( EC ) + Li’ + eCHzCH20COzLi (2) 2CH&Hz0CO2Li LiOC02CHzCHzOC02Li+ C2H4 (3)
-+
-+
3.2. Thermal behavior of DMC Fig. 2 shows that DMC has an endothermic peak temperature at 91 “c in argon filled vessel, which is according with its boiling point 90 “c with the endothermic heat I . 5 J g-’ . DMC
RARE MEYXS
%
16
-+
, Vol. 25, Spec. Issue,
2CH4 2CH30C02CH3 + 2e2CH30COzLi + 2CH4
1 .O m o l V LiPF,/DMC in argon field vessel
+
2Li’
Oct 2006
+
(5) H2
+
(6)
3.3. Thermal behavior of EMC
(-104
, 60
I
, 100
.
, 150
.
, 200
.
,
.
260
4 300
Temperature/%
Fig.2. C80 heat flow curves of 1.0 rno1-L-l L i P F a M C and DMC in argon filled vessel.
shows stable characteristics with slow endothermic phenomenon in argon filled vessel at elevated temperature, and a sharp exothermic occurs at 223 T. An endothermic peak appears at 68.5 “c in 1.0 mol. L-’ WF6/DMC solution with the endothermic heat 15.4 J * g - I in Fig. 2 , then, it undergoes a fluctuant process till to 187 “c. The followed zigzag endothermic process reaches to peak temperature at 219 “c the with heat of reaction 41.7 J - g - ’ . The exothermic process of pure DMC may be caused by the cleavage of C-0 bond in higher temperature[ 151 . The molecule of DMC possesses two active centers (alkyl and carbonyl carbons), whose reactivity can be tuned with the temperature. In particular, two distinct pathways can be recognized in the reaction of DMC with a generic anionic nucleophile ( Y - ) [ 201. When’ PFs- nucleophile attacks at carbony1 carboq of DMC , the cleavage of acyl-oxygen bond generates a methoxycarbonyl product at lower temperature ( < 100 T ) [ 21 3 . When temperature is over 187 “r: , PF6- nucleophile attacks at DMC and undergoes a methylation reaction to generate methoxide and CO, . At the elevated temperature, the endothermic processes show that Lit also react with DMC and the possible reactions as follows[ 19, 221 : CH30C02CH3+ 2e + 2Li + +2CH30Li + CO (4) CH30CO2CH3+ 2e + 2Li + + H2 Li2C03 + +
Fig. 3 shows the heat flow curves of 1 . 0 mol L-’ LiPF6/EMC and pure EMC in argon filled vessel at elevated temperature. In argon atmosphere, EMC shows an exothermic peak temperature at 164 “c with a heat generation of - 2 5 . 8 Jog-’. At 236 T , EMC starts to release heat again, and reaches to peak temperature at 243 “c . After LiPF6 dissolves in EMC, the solution starts its endothermic process at 154 “c, and reaches to the first endothermic peak temperature at 191 ‘C with the heat of absorption 163. 7 J g-’. Later, an exothermic peak was detected at 208 “c with the hat of generation - 148- 3 J g- . At last, another endothermic peak temperature was detected at 258 T with the heat of absorption 45.6 Jog-’.
’
2o
-m-
1 .O mo1.L-l LiPFJEMC in argon filled vessel
. -0- EMC in argon tilled vessel
10-
; E
0:
Y
g
-10-
2
-20
C
I1 30
60
I00
160
200
250
300
Ternperature/”C
Fig.3. CsO heat flow curves of 1.0 mol*L-’ LiPF&MC and EMC in argon fded vessel. Yoshida et a l . [ 191 ever used Fouriertransform infrared (FT-IR) technique to speculate the exothermic process of EMC at elevated temperature. The trans-esterification is shown in Eq. ( 7 ) . That the sharp exothermic process of EMC appears at 230 “c may be the polyrneric or decomposition reaction[ 191 . 2EMCzDMC + DEC (7) L i F 6 produces a strong Lewis acid, PFs , and PF, attacks the electron lone pair of oxygen of a solvent molecule of EMC , which causes the
Wung Q .S .et al . , Thermal stability of LiPFJEC + DMC + EMC electrolyte for lithium ion batteries
97
sively, with the endothermic heat and exothermic heat of 29.5 and - 494.8 J *g-' , respectively. In order to disclose the reaction process of 1.0 mol * L-' LiPFJEC + DMC t EMC electrolyte at elevated temperature, the experimental result of 1.0 rnol-L-' LiPF,/EC, 1.0 mol*L-' LiPF,/DMC and 1 .0 mol L- LiPF6/EMC were averaged and the result was shown in Fig. 5 . It can be seen in Fig. 5 that the average result is similar with the experimental result. Endothermic and exothermic processes were found successively in the two curves, and they show close endothermic peak temperature, but the exothermic peak temperature is delayed 8 "c in averaged curve. By comparing the heat flows of 1 . 0 mol. L-' LiPF6/EC + DMC + EMC, 1 . 0 mo1.L-l LiPF6/EC, 1.0 mol. L-' LiPF,/DMC, 1.0 mol. L-' LiPF,/EMC and their averaged result, it can be speculated that the endothermic process is mainly contributed by EMC and the exothermic process is mainly contributed by EC . Then, it is concluded that LiPF6 may be reacting with EC , DMC and EMC respectively.
'
ii1'
9
-
1 .O mol-L-' LiPFJEC+DMC+EMC in argon filled vesscl 5 0 - -EC+DMC+EMC F
-
-Experimental result of 1 .O mol L-' LiPF,/EC+ /\ 40 DMC+EMC -Average result of 30 1 .O rnol L-' LiPF,/EC 1.0 rnol L-' LiPFlDM 1 .O rnol L-' LiPFJEM
j:
-10
I
- 2 o J , 100
.
, 150
.
,
.
200
,
.
250
I
300
Temperature/"C
Fig.5. Comparison of 1.0 mo1.L-l LiF,JEC + DMC + EMC With the average of 1.0 mol-L-' LipFd EC, 1 . 0 moleL-' LiFflMC and 1 . 0 mol . L - ' LiPFaMC in argon fdled vessel.
-10 - 2 0 , . , 60 90
.
, . ~. 120
150
, . , 180
210
,
, . , . 240
270
300
TemperaturdC
Fig.4. CSO heat flow curves of 1. 0 mol * L-' LPF&C +DMC + EMC and EC + DMC + EMC in argon filled vessel.
3.5. Pressure behavior of LiPF6/EC
DMC + EMC
+
It is thought that electrolyte decompose to gas at elevated temperature [ 221 . The pressure was detected by a high pressure transducer and
RARE iUEZ4L.S , Vol. 25, Spec. Issue,
98
'
Fig.6 shows the result of 1.0 mol L- LiPF6/ EC + DMC + EMC at elevated temperature. It can be seen that the pressure is increasing with temperature increasing. At about 150 T , the pressure starts to increase and get sharp increase at 178 "c , which is corresponding to the endothermic process in the heat flow curve. With temperature increasing, the pressure increases sharply and reaches 15 MPa after the exothermic process, which indicates that a lot of gases have been produced. The gases generation rate can be determined by Eq. ( 13) [ 16,
25
I
dP
-= dt
k,P"
maybe consist of C2Hz, CH,, CO , C02 etc . according to the above researches. In sealed lithium ion batteries, once the electrolyte decomposs, coupling with the reaction between electrolyte and electrodes, a lot of gases cause the pressure increasing till the battery explosion. Therefore , the electrolyte thermal stability plays a key role for lithium ion battery safety.
0.0 -
3
-0.5 -
a
4.0:
-
-1.5-
f
Taking natural logarithm of Eq .( 13) : dP h(-) = n h P + Ink, dt
Oct 2006
-2.0,
1 I
-2.5
-3.01, 0.3
20
# 1.0 rnc4.t' UPF@C+DMC+
.
,
.
0.6
, 0.9
.
, 1.2
.
, 1.5
.
6
In P
Fig.7. In(dPldt) versus InP for calculation of n and R , , the dash line is fitted Line.
4. -20-1 50
,
, 100
.
, 150
.
,
.
200
, 250
. c -2 300
Temperantre/'C
Fig. 6 . Heat flow and pressure changing curves of 1 .O mole L - * LipF&C + DMC + EMC at elevated temperature.
By plotting the curve of l n ( d P / d t ) versus 1nP from the onset temperature to the end of reaction, the reaction order ( n ) and the rate constants of reaction k, can be easily calculated. Fig.7 shows its plot, where the slope of the fitted line is the reaction order n = 1.83, then the pressure rate constants k, = 6. 49 x kpa -0.63min-' . The larger reaction order of 1 . 0 mol * LLiPFdEC + DMC + EMC decomposition indicates that the reaction is greatly affected by the concentration, and the higher pressure rate constants indicates that the reaction goes on rapidly, which produce a lot of gases and causes the pressure increasing rapidly. The gases
'
Conclusions
The C80 calorimeter measurements were used to evaluate the thermal properties of 1.0 mol. L-' LiPF6/EC + DMC + EMC in argon sealed vessel from ambient temperature to 300 T . By studying the high-temperature thermal properties of LiPF6/EC, LiPF6/DMC, LiPFJ EMC, and LiPF6/EC + DMC + EMC one by one, it is shown that in 1.0 mol*L-' LiPF6/EC + DMC + EMC electrolyte, LiPF6 maybe reacts with EC , DMC and EMC separately. Furthermore, it is found that LiPF6 influences the thermal behavior remarkably, with more heat generation and lower onset temperature. The pressure data were taken to determine the reaction order and pressure rate constants of LiPF6/EC+ DMC + EMC decomposition. The results show that this electrolyte can decompose severely and the gases is the main hazards to lithium ion battery safety. ,
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