Thermal Behavior of Lithiated Graphite with

Journal of The Electrochemical Society, 153 共2兲 A329-A333 共2006兲

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0013-4651/2005/153共2兲/A329/5/$15.00 © The Electrochemical Society

Thermal Behavior of Lithiated Graphite with Electrolyte in Lithium-Ion Batteries Qingsong Wang,a Jinhua Sun,a,z Xiaolin Yao,b and Chunhua Chenb,* a State Key Laboratory of Fire Science, and bDepartment of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China

A C80 calorimeter was used to study the thermal behaviors of LixC6 and LixC6 in 1.0 M LiPF6 /EC + DEC electrolyte. C80 results show that LixC6 alone shows one exothermic peak, which is attributed to the solid electrolyte interphase 共SEI兲 decomposition. Four exothermic peaks were detected in LixC6-1.0 M LiPF6 /EC + DEC electrolyte samples. These four peaks are attributed to SEI decomposition, Li-electrolyte reaction as well as new SEI film formation, new SEI film decomposition, and Li with PVDF/other products reactions. The apparent activation energy of LixC6 and LixC6-electrolyte at different states of charge was calculated, and it was found that with intercalated lithium increasing, the activation energy shows a decreasing trend. © 2005 The Electrochemical Society. 关DOI: 10.1149/1.2139955兴 All rights reserved. Manuscript submitted August 22, 2005; revised manuscript received September 27, 2005. Available electronically December 30, 2005.

The lithium-ion battery fulfills many of the demands made within the areas of portable electronics and electrical vehicles, and is superior in many ways to the more common nickel-cadmium and nickel-metal hydride batteries.1-5 However, there are also potential safety problems in their use due to the occurrence of thermal runaway in abuse cases.6 The safety is mainly related to the thermal reactivity of the cell materials. The exothermic reactions of materials in battery applications can cause thermal runaway in the cell, and thereby constitute a safety hazard. It has been shown that the thermal stability of anode is critical to the thermal runaway of battery.6-10 Graphite remains the material of choice for anodes in rechargeable lithium-ion batteries because of its high capacity 共372 mAh g−1兲, flat voltage, and low cost.11 During the first charge, graphite-based electrolytes are reduced at the negative electrode. As a result, a surface film is formed consisting of a variety of solvent and salt reduction products.12,13 This film functions as an ionic conductor that allows Li+ ion to be transported through the film during the subsequent intercalation and deintercalation processes. The film is also an electronic insulator, which will prevent the continuous reduction of electrolyte as the film thickness reaches a certain limit. The film then functions as a passivating layer on the graphite surface. It is most often referred to as a solid electrolyte interphase 共SEI兲.11-15 Using ac impedance, the formation process of SEI film on graphite electrode during initial cycles was studied.15-19 The results show that the SEI formation takes place through two major stages. The first stage takes place at voltages above 0.25 V 共before lithiation of graphite兲, during which a loose and highly resistive film is formed. The second stage occurs at a narrow voltage range of 0.25–0.04 V, which proceeds simultaneously with lithiation of graphite electrode. In the second stage, a stable, compact, and highly conductive SEI film is produced.11-15 Studies by means of differential scanning calorimetry 共DSC兲共Ref. 10 and 20-31兲 and accelerated rate calorimetry 共ARC兲共Ref. 26 and 32-38兲 have shown that the thermal stability of graphite electrodes is critically dependent on the type of carbon the choice and presence of the electrode,10,26,29,35 electrolyte,20,22,39,40 particularly the lithium salt used in the electrolyte,10,23,33,36,39-41 state of charge,20,22,27,33 specific surface area,27,33,35 binder level in graphite,22,24,26,32 cycle number,27 and so on. The general results have been reviewed by Roth et al.:22 SEI layer decomposition 共typically 90–130°C兲; reaction of intercalated lithium with electrolyte solvent 共90–290°C兲; electrolyte decomposition 共200–300°C兲; positive active material decomposition and reaction with solvent 共150–500°C兲. Unfortunately, in these studies

* Electrochemical Society Active Member. z

E-mail: [email protected]

using DSC, solvent leakage at higher temperatures issue was proposed.22,25,27-29,41 The DSC sample holder started leaking at ca. 160°C,29 or 200°C,27 as the sample holders were not designed for a possible gas evolution reaction. Furthermore, ARC is available to research self-heating substances, but is unfit to detect endothermic reactions.42 The thermal stability of the SEI layer, and the graphite electrode in general, is crucial to its use in practical cells. However, the thermal decomposition kinetics is seldom reported; therefore, a Calvet calorimeter C80, with good airtight characteristics and precision, was used to investigate the thermal stability of lithiation graphite with 1.0 M LiPF6 /ethylene carbonate 共EC兲⫹diethyl carbonate 共DEC兲 electrolyte in detail to disclose their thermal stabilities and thermal decomposition kinetics. Experimental The graphite electrode used in this study consists of a mixture of graphite 共Hongyuan Carbon Industry Co., Ltd兲 and polyvinylidene fluoride 共PVDF兲 binder. The graphite electrode is made of 92% graphite material and 8% binder, cast on copper foil. The electrodes were obtained ready for use, dried overnight in vacuum at 70°C, and handled in an argon-filled glove box 共MBraun Labmaster 130, ⬍1 ppm O2 and H2O兲. The graphite electrode was cut as a 14 mm diameter disk of about 400 ␮m thickness to get more mass of sample for the C80 experiment. Electrochemical cycling was performed in button-type coin cells 共CR 2032兲. Coin cells were assembled in an argon-filled glove box with the graphite as positive electrode, lithium as counter electrode, 1.0 M LiPF6 /EC + DEC 共1:1 wt %兲 as the electrolyte, and Celgard 2400 polyethylene as the separator 共20 ␮m thickness兲. The cells were galvanostatically cycled on a multichannel battery cycler 共Neware BTS2300, Shenzhen兲 at 0.2 mA cm−2 current density in the first three formation cycles. The cells were charged to 3.0 V and then discharged to 0 V for Li/graphite cell. The definition of “discharge” here is for the half-cells, namely, the lithiation process for the graphite electrodes. If they are used as the anodes in full cells, this process corresponds to the “charge” step. Then, the cells were discharged to different state of discharge for thermal test. Then, the discharged cells were disassembled in a glove box. To remove the electrolyte from the electrode, the wet charged electrode powder was placed into a bottle. To that bottle a portion of dimethyl carbonate 共DMC兲, a volatile organic solvent, was added and the bottle was then shaken by hand. The sample was then decanted and the DMC rinsing procedure was repeated. After the second decanting, the sample was dried to remove the DMC solvent. As the DMC can be followed by a partial dissolution of the polymeric components of the SEI, which in turn may decrease the integrity and real thermal stability of the SEI, the DMC rinsing procedure is done no more than twice. After drying, the electrode material was scraped,

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Figure 1. The cycling profile of a graphite anode containing PVDF-binder with a lithium foil as a counter electrode.

trying not to abrade the SEI film, from the copper current collectors carefully for thermal-testing. To characterize the thermal stability of the lithiated electrodes in the presence of electrolyte, about equal amounts of lithiated electrode material 共including PVDF兲 and 1.0 M LiPF6 /EC + DEC electrolyte were transferred into a high-pressure stainless steel vessel 共8.5 mL in volume兲 of a microcalorimeter 共Setaram C80兲 sealed in argon atmosphere. The weight of each sample 共vessel + sample兲 was measured before and after the experiment to verify that the system was hermetically sealed. The weight was constant in all cases, indicating that there were no leaks during the experiments. The measurements were carried out using a heating rate set at 0.2°C min−1 in the temperature range 30–300°C. The thermal effects of each sample with temperature were thus recorded automatically, and the C80 calculations were based on dry film weight of the electrode material. Results and Discussion Cycling profile of a graphite anode.— Figure 1 shows the first three cycling profiles of a graphite anode with a lithium foil as a counter electrode at a current of 0.2 mA cm−2. A plateau observed near 0.45 V in the first discharge process is associated with the decomposition of the electrolyte on the surface of graphite in the formation of the passivating layer.12,13,28 This plateau disappears in subsequent cycling because the passivating layer suppresses further electrolyte decomposition. The irreversible capacity observed in the first three cycles for graphite electrode are 138, 19, and 13 mAh g−1, respectively, suggesting that after the first three cycles, the formatted SEI film has been formed. Typical thermal behavior profiles.— Figure 2 presents the C80 heat flow curves obtained from the rinsed lithiated graphite anode sample, 1.0 M LiPF6 /EC + DEC electrolyte and their coexisting system. This figure shows that the rinsed lithiated graphite, i.e., Li0.84C6, starts to be exothermic at about 47°C, and reaches the first peak at 181°C, the mild second peak at 248°C with the total heat generation of −1339 J g−1. An endothermic peak at 173°C and an exothermic peak at 192°C are detected in the C80 scan of 1.0 M LiPF6 /EC + DEC electrolyte. Three obvious exothermic peaks are detected at 102, 218, and 252°C, respectively, with total heat generation of −2342 J g−1 共based on Li0.86C6兲 in the C80 experiment of Li0.86C6 coexisting with 1.0 M LiPF6 /EC + DEC electrolyte. The rinsed lithiated graphite contains graphite, intercalated lithium, SEI layer, and PVDF. The SEI layer mainly consists of stable 共such as LiF, Li2CO3兲, and metastable components 关such as polymers, ROCO2Li, 共CH2OCO2Li兲2, and ROLi兴.6,20,21,28,33 The graphite, LiF, and Li2CO3 are stable, and they do not decompose

Figure 2. C80 heat flow curves of lithiated graphite 共Li0.84C6兲, 1.0 M LiPF6 /EC + DEC electrolyte, and Li0.86C6-electrolyte at a heating rate of 0.2°C min−1.

and produce heat below 300°C. The PVDF has a minimal effect on the thermal response of the anode reactions.22 Therefore, the peak of Li0.84C6 in C80 experiment is attributed to the SEI breakdown and its reaction with intercalated lithium as21,28,33 共CH2OCO2Li兲2 → Li2CO3 + C2H4 + CO2 + 0.5O2

关1兴

2Li + 共CH2OCO2Li兲2 → 2Li2CO3 + C2H4

关2兴

Assuming the reaction mechanism is dependent on the Arrhenius law, based on the C80 data, the following equation 3 is obtained as43,44

冉

ln

冊

dH/dt E 1 = − · + ln A ⌬HM 0 R T

关3兴

where dH/dt is over all heat flow, ⌬H is heat of reaction, M 0 is initial mass of reactant, E is activation energy, R is gas constant, T is temperature of system, and A is pre-exponential factor. By plotting the curve of ln共dH/dt/⌬HM 0兲 versus inverse temperature, the E and A can be easily calculated.43 By this method, the SEI thermal decomposition activation energy is calculated as E = 101.3 kJ mol−1, and pre-exponential factor is A = 5.60 ⫻ 109 s−1. With the presence of electrolyte together with Li0.86C6, three exothermic peaks are detected in the C80 experiment. This coexisting system starts to decompose at 69°C, and reaches the first peak at 102°C with heat generation of −495.5 J g−1, which is earlier than the rinsed LixC6 alone. This first exothermic process should be the thermal decomposition of SEI layer judging from previous studies. It is reported23,28 that PF5 was the major species responsible for the destruction of the SEI layer at high temperatures. Because PF5 is a powerful Lewis acid, which is produced by the equilibrium decomposition LiPF6 ⇔ PF5 + LiF, and attacks the electron lone pair or the atom with a large electron density, it is likely to react with the oxygen atom of carbonyl groups 共–CvO兲 and compounds possessing sites of increased electron density. In addition, the dominant component of the SEI is 共CH2OCO2Li兲2, indicating the C–O functional group. Hence, it is reasonable to propose that PF5 damages the SEI in a LiPF6 system at 69°C.23,28 Using the above method, the activation energy is calculated as E = 78.6 kJ mol−1, and the preexponential factor is A = 5.14 ⫻ 106 s−1. The breakdown of SEI layer makes the electrolyte easy to reach the lithiated graphite surface, furthermore, the intercalated Li ions move from the inner structure to the edge of graphite particles at high temperatures,23,45 and PF5 accelerates the Li movement by re-


Figure 3. C80 heat flow curves of graphite sample with different amount of intercalated lithium at a heating rate of 0.2°C min−1. In LixC6, x denotes the level of intercalation.

moving electrons from graphite. Then, the intercalated lithium easily reacts with electrolyte directly. A mild heat generation continues from 125°C until an exothermic peak appears at 218°C in C80 experiment of Li0.86C6-electrolyte in Fig. 2. This peak is attributed to the reaction of lithiated graphite with the electrolyte.24,46 Several studies have concluded that intercalated lithium reacts with the electrolyte to form a new stable SEI film after SEI decomposition at the edge planes.24,27,28,33 The formation of secondary SEI film reactions stops when either the new film is thick enough to insulate reactants or all the intercalated Li has been consumed. This new SEI film prevents the electrolyte from continued reaction with intercalated lithium. As this new SEI film could decompose with rising temperature, therefore, the peak at 233°C is contributed by new SEI film decomposition process. ARC results35 disclosed that between 90 and 243°C, Li0.81C6 and EC/DEC react to form Li-alkyl carbonates, which are consistent with the coupled earlier exothermic processes in our experiments. It is reported22,28 that PVDF has a minimal effect on the thermal response of the anode reactions; then, the peak at 252°C is attributed to overlapped reactions of PVDF, LixC6, Lialkyl carbonates, and EC/DEC with the formation Li2CO3. Some researchers24,26,27 attributed this peak to the reaction of lithiated graphite with PVDF, Yamaki et al.24 suggested that the sharp exothermic peak at 280°C 共corresponding to the peak at 252°C in our experiments, as the heating rate, 0.2°C min−1, is lower than Yamaki’s, 5°C min−1兲 was the direct reaction of the lithiated graphite with electrolyte due to SEI breakdown. We believe that the salts and/or EC/DEC are also involved in these reactions.28 The reaction heat of intercalated lithium with electrolyte 共second exothermic process兲 is −1090.6 J g−1, and the activation energy is calculated as E = 64.3 kJ mol−1, and pre-exponential factor is A = 5.9 ⫻ 102 s−1. LixC6 and LixC6-electrolyte thermal behavior.— Figure 3 presents the C80 heat flow curves obtained from lithiated graphite anode samples at a 0.2°C min−1 heating rate from 30 to 300°C. One main exothermic peak is detected in each sample. With the amount of intercalated lithium increasing in the graphite, the onset temperature presents a decreasing trend; however, the exothermic peak swing instead of adhering strictly to this decreasing trend. At x = 0.22, 0.52, and 0.76, small exothermic process presents before the main large exothermic peaks. These processes are according to the SEI decomposition in theirs coexisting system; at x = 0.84, a small peak at 139°C is observed too. In other situations, these processes are absent or not obvious. However, the exothermic curves are not smooth, which may be the overlapped results of different reactions, including SEI decomposition. These may be attributed to

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Figure 4. Onset temperature, exothermic peak, and overall heat generation of graphite samples with different amounts of intercalated Li.

the effect of DMC, as DMC may dissolve parts of the organic SEI film. It is suggested that the prewashing process damages the SEI by dissolving some of its constituents 共such as polymers and semicarbonates兲 and causing a fast vigorous reaction.20 The heat generation presents an increasing trend, which is clearly exhibited in Fig. 4. Yang et al.28 have proposed that the peak associated with the SEI film decomposition increases with the increasing intercalated lithium, but this trend is not obvious in their experiments. The heat generation of LixC6 and decomposition dynamics parameters were calculated in Table I. With the presence of 1.0 M LiPF6 /EC + DEC electrolyte in the lithiated graphite, in Fig. 5 all the samples show similar thermal behavior at elevated temperature. Obvious peaks can be identified and are designated peaks a, b, and d in the text. A faint peak between peak b and peak d appears uncertain; it is coupled with other process and designated peak c for unification. Judging from the above analysis, a, b, c, and d peaks are attributed to the SEI breakdown, lithium-electrolyte reaction, new SEI film breakdown, and Li2CO3 formation reactions overlapped with PVDF reactions, respectively. The SEI breakdown peaks-remain around 100°C because the SEI layer’s main compositions, LiF, Li2CO3, ROCO2Li, 共CH2OCO2Li兲2, and ROLi, have been formed in the first cycle;20,33,45 therefore, its thermal stability is not influenced by the intercalated degree of lithium. After the breakdown of former SEI layer, the reaction peak b, around 213°C, of electrolyte with intercalated lithium presents little increasing trend with intercalated lithium increasing. Peak c is the breakdown of secondary SEI, and presents increasing trend of heat generation with the increasing intercalated degree of lithium. The breakdown of secondary SEI film permits the rest of the intercalated lithium to react with the electro-

Table I. The thermal characteristics and decomposition dynamics parameters of LixC6. x in LixC6

Onset temperature 共°C兲

Peak 共°C兲

Heat generation 共J g−1兲

Activation energy 共EkJ mol−1兲

0.18 0.22 0.32 0.45 0.52 0.68 0.76 0.84 0.92

136 65 47 50 44 42 41 47 42

207 247 204 166 213 219 234 181 224

−265.3 −603.6 −1280.6 −1302.7 −1469.0 −860.0 −1569.6 −1339.0 −1341.6

115.3 103.8 105.8 86.6 96.5 68.8 82.0 101.3 77.0

Pre-exponential factor A 共s−1兲 1.94 2.75 9.15 8.72 3.43 2.01 3.92 5.60 3.56

⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻

109 107 1010 107 106 105 107 109 106


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Table II. The thermal characteristics and decomposition dynamics parameters of LixC6 − 1.0 M LiPF6 /EC + DEC electrolyte.

x in LixC6 + 1.0 M LiPF6 /EC + DEC 0.19 0.22 0.38 0.46 0.52 0.61 0.73 0.86 0.92

Onset temperature 共°C兲 61 67 59 57 51 64 58 69 59

a 94 96 102 100 103 100 99 102 101

b 211 204 210 211 216 216 214 218 217

c 228 224 228 223 235 231 228 233 234

Activation energy E 共kJ mol−1兲

Heat generation 共J g−1兲

Peaks 共°C兲 d 251 246 241 242 247 243 243 252 249

a −484.1 −478.8 −429.1 −520.2 −593.3 −488.4 −512.2 −495.5 −487.7

b

c

−785.1 −786.3 −848.0 −810.8 −1264.8 −1059.9 −1341.4 −1090.6 −1230.0

lyte or its decomposition products, as the electrolyte decomposes below 192°C 共Fig. 2兲. The electrolyte decomposition products are C2H5OCOOPF4, PF3, C2H4, HF, PF4OH, CO2, etc.47,48 Then, the intercalated lithium will react with these products, which results in the peaks d in Fig. 5. After x 艌 0.52, peaks d become more sharp 共Yang et al.23 reported a sharper peak at x 艌 0.71 using DSC at a 10°C min−1 heating rate兲; this sharper peak corresponds to the peaks of x 艌 0.52 at 0.2°C min−1 heating rate. Aurbach et al.49 and Chung et al.50 noted that EC and LiPF6 can enter into the graphite sheets and react with intercalated lithium to form lithium carbonate and ethylene. This process is similar to the exfoliation in the first cycle of Li intercalation and can bring about significant changes in the surface structure, including the breakdown of the graphite powder into fragments. Therefore, the sharp peak d detected for LixC6 共x 艌 0.52兲-electrolyte samples in Fig. 5 is partly attributed to the collapse of the graphite structure.28 The released Li from the collapsed graphite particles can react with PVDF and electrolyte decomposition products at this high temperature and produce more heat. These reactions can be attributed to the dehydrofluorination of PVDF and formation of LiF and hydrogen, and so on. The thermal parameters and apparent activation energy were summarized in Table II. The activation energy of SEI decomposition 共peak a兲 and Li-electrolyte reaction 共peak b兲 was determined based on Arrhenius laws. The activation energy of SEI decomposition without much deviation with x changing, which indicates the SEI is a stabilized unit formed by LiF, Li2CO3, ROCO2LI, 共CH2OCO2Li兲2,

−22.6 −12.3 −94.6 −161.2 −26.2 −48.7 −257.0 −138.7 −141.0

d −312.0 −544.1 −254.3 −382.9 −296.3 −375.3 −490.3 −617.6 −394.8

Total −1603.8 −1821.5 −1626 −1875.1 −2180.6 −1972.3 −2600.9 −2342.4 −2253.5

a 100.4 115.2 77.4 92.8 83.7 99.1 83.8 78.6 96.2

Pre-exponential factor A 共s−1兲

b

a

93.0 81.2 62.4 83.0 62.5 78.0 68.4 64.3 69.1

⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻

1.09 2.6 3.23 9.01 5.59 4.49 3.58 5.14 2.8

b 10

10 1013 106 108 107 109 107 106 109

1.6 6.7 4.3 6.9 2.9 1.7 4.2 5.9 1.7

⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻

106 104 103 104 102 104 104 102 103

and ROLi. The apparent activation energy of Li-electrolyte reaction is without much changing too, possibly explained as follows. Before x 艋 0.38, Li is consumed completely before the new SEI film completely blocks the surface of edge planes, as Li is very active in high temperature; therefore, the more the Li in LixC6 the lower the activation energy, and no sharp graphite structure collapse was detected in the last exothermic process in LixC6-electrolyte samples. At x = 0.46, the new SEI film prevents the Li from reacting with electrolyte, and there about 0.08 intercalated lithium per C6 stays in the graphite sheet, which is not sufficient to generate a sharp peak from the reaction with PVDF and electrolyte decomposition by-products after the new SEI film breakdown. After x 艌 0.52, the more intercalated Li 共艌0.14 per C6兲 remaining in the graphite, the more intercalated lithium remaining renders the structure more unstable and results in lower activation energy for this structural collapse reaction, which occurs at a lower temperature. The released lithium as a result of the structural collapse reacts with the PVDF binder and electrolyte decomposition by-products to form LiF, hydrogen, and Li2CO3. Conclusions Thermal behaviors of the graphite anode with/without electrolyte at different states of charge are studied using C80 orimeter at a 0.2°C min−1 heating rate in the temperature range 30–300°C. The rinsed LixC6 alone shows one exothermic peak, which is attributed to the SEI decomposition and intercalated lithium reacting with SEI compounds. With the presence of an equal amount of 1.0 M LiPF6 /EC + DEC electrolyte in the LixC6 samples, four exothermic peaks were detected. First, the metastable compounds in SEI film break down when the temperature reaches 61°C 共mean value兲, and this exotherm peaks at about 100°C. Then, the electrolyte can penetrate the broken SEI film and reach the edge planes of the lithiated graphite, and reacts with the lithium diffusing from the inner structure; hence, a secondary SEI film is formed. This reaction does not stop until either the intercalated lithium is consumed completely or products such as Li2CO3 and LiF completely block the surface of edge planes. The secondary SEI decomposes with temperature rising; then, the remaining intercalated lithium in the graphite structure will be released from the graphite structure. The lithium released from graphite particle reacts with PVDF and electrolyte decomposition by-products to generate more heat. Acknowledgments

Figure 5. C80 heat flow curves of LixC6 with the presence of equal amount 1.0 M LiPF6 /EC + DEC electrolyte at a heating rate of 0.2°C min−1. In LixC6, x denotes the level of intercalation.

This study was supported by “100 Talents Project” of Chinese Academy of Sciences and China NKBRSF project, no. 2001CB409600. Financial support from Nature Science Funds of Anhui Province, no. 050450403, is also appreciated. University of Science and Technology of China assisted in meeting the publication costs of this article.

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