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Understanding the Improved High-Temperature Cycling Stability of LiNi0.5Mn0.3Co0.2O2/Graphite Cell with Vinylene Carbonate: A Comprehensive Analysis Approach Utilizing LC-MS and DART-MS Yi-Hung Liu, Sahori Takeda, Ikue Kaneko, Hideya Yoshitake, Takashi Mukai, Masahiro Yanagida, Yuria Saito, and Tetsuo Sakai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10391 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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The Journal of Physical Chemistry

Understanding the Improved High-Temperature Cycling Stability of LiNi0.5Mn0.3Co0.2O2/Graphite Cell with Vinylene Carbonate: A Comprehensive

Analysis

Approach

Utilizing

LC-MS

and

DART-MS Yi-Hung Liu,*,†,§ Sahori Takeda,† Ikue Kaneko,‡ Hideya Yoshitake,‡ Takashi Mukai,† Masahiro Yanagida,† Yuria Saito,† and Tetsuo Sakai†,‡ †

Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial

Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan ‡

Innovation Center for Organic Electronics, Yamagata University, 1-808-48, Arcadia, Yonezawa,

Yamagata 992-0119, Japan §

Department of Greenergy, National University of Tainan, Tainan 70005, Taiwan

＊

Corresponding author E-mail: [email protected] Tel.: +886-6-2606186

Fax: +886-6-2602205

1

ACS Paragon Plus Environment

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ABSTRACT An analysis approach utilizing liquid chromatography mass spectrometry (LC-MS) and direct analysis in real time mass spectrometry (DART-MS) is applied to discern the influence of vinylene carbonate (VC) on the cycling performance of a LiNi0.5Mn0.3Co0.2O2 (NMC)/graphite cell at elevated temperature. The VC-containing cell exhibits much improved cycling performance against the elevated temperature rather than the VC-free one. Based on the LC-MS results, more decomposition compounds including carbonate oligomers and organophosphates are present in the electrolyte without VC after the initial cycling, while they are less identifiable in the VC-containing electrolyte. On the other hand, the DART-MS results show that a thermally resistant film, which is primarily composed of the cyclic organophosphates, favors to form on the graphite anode surface of the VC-containing cell rather than the VC-free one, preventing further electrolyte decomposition. Moreover, a reaction scheme is proposed to reasonably explain the formation of the decomposition compounds, in which it is understood that VC can trap the free alkoxide anions and meanwhile allow more EC to react with POF3 during the initial cycling, thus reducing the decomposition compounds in the electrolyte and facilitating the formation of thermally stable organophosphates. Consequently, the cell’s cycling performance is improved at elevated temperature.

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1. INTRODUCTION Ensuring a long cycle life of a lithium ion battery (LIB) is always a pursued target for various electronic applications, from portable ones like laptops to large-scale ones such as electric vehicles (EVs), hybrid electric vehicles (HEVs) as well as other stationary battery storage systems. For these large-scale applications, extension of the battery’s cycle life, particularly at elevated temperature, is highly required because the large-scale batteries are more often utilized under an environment with high temperature. Moreover, continuous operation of thermally unstable batteries would eventually lead to a thermal runaway, thus threatening our life seriously. To improve the thermal stability of LIBs, various development strategies are thus toward seeking the thermally stable active materials, binders, separators, substrates and electrolytes. Since the commonly used electrolytes, which are mainly composed of LiPF6 salt and organic solvents, are more likely to decompose at elevated temperature than the other battery components, development of thermally stable electrolytes should be put in high priority. Regarding this issue, the use of electrolyte additives is one of the most effective and economical ways to enhance the battery’s performances in terms of cycle life as well as rate capability.1 Among many electrolyte additives, vinylene carbonate (VC) is the most well-known one, which has been shown to have positive effects on the improvement of electrochemical performances for either cathode2-5 or anode side.6-11 For example, in a previous research of Aurbach’s group,8 it has been reported that VC can react with the cathode material of LiNiO2 and LiMn2O4, and the anode

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materials of graphite. Despite no profound impact on the cycling behavior of the cathodes has been confirmed, VC substantially improves the cycling stability of the graphite anode together with a reduction of irreversible capacity, particularly at elevated temperature. The high reactivity of VC, corresponding to its polymerizable vinyl group, tends to induce polymerization with other compounds during cell cycling, resulting in the formation of a passivating film on the anode surface.8,9 Accordingly, various polymerization products as well as nonpolymeric species can be produced during the film formation process.8,12,13 Based on a series of XPS analysis of the electrode surfaces for the LiCoO2/graphite and LiFePO4(LFP)/graphite cells, it has been evidenced that there is no interaction between the cathode and anode during the VC polymerization process.14 In addition, different from the LiFePO4/graphite cell, in which no polymeric products were found on the cathode surface, the same VC polymer can be identified on both the cathode and anode surfaces for the LiCoO2/graphite cell as a result of a radical polymerization process. Another study has further pointed out that the improvement of Coulombic efficiency and reduction of charge/discharge endpoint slippage can be achieved by use of VC additive for a Li/graphite half cell, which is more profound at elevated temperature.15 As for a Li/LiNi1/3Mn1/3Co1/3O2 (NMC) cell, VC was found to suppress the parasitic reactions occurred at the cathode surface, corresponding to a better cycle life. According to a recent research study, VC has also been proved to form a polymer at both NMC and graphite surfaces, resulting in more stable and protective solid electrolyte interface (SEI) films.16 Liquid chromatography-based analysis methods17-20 and gas chromatography-mass spectrometry

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(GC-MS)21 have been utilized to detect neutral decomposition compounds, while FTIR8,11 and XPS2,14,16 are commonly used in order to identify the compounds of the films on the electrode surfaces based on their functional groups. Recently, direct analysis in real time mass spectrometry (DART-MS) has been applied as a more direct analysis approach to specify the compounds deposited on the electrode surfaces of LiFePO4/graphite22 and LiFePO4/hard carbon23 cells after cycling at elevated temperature. It is supported from the studies that a combination of LC-MS and DART-MS analysis can allow one to simultaneously identify the degradation compounds in the electrolyte and on the electrode surfaces, benefiting the elucidation of the formation mechanisms of a thermally stable film induced by the VC additive. Since the olivine LiFePO4 has already been known as a thermally stable cathode material, the impact of VC on the thermal stability of a cell composed of layered cathode materials with higher energy density, such as NMC should be more worthy investigating based on the abovementioned analysis approach. In this study, we aim to apply the analysis approach combing LC-MS and DART-MS to comprehensively understand the influence of VC on the thermally resistant film formation for a NMC/graphite cell at 60 °C. The cell’s cycling stability is correlated to the identified decomposition products either in the electrolyte or on the electrode surface. Based on the analysis results, a series of reaction mechanisms of the decomposition compounds is proposed to explain the VC-induced film formation on the electrode surface, offering new insight into the VC contribution to the improved cycling stability at elevated temperature.

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2. EXPERIMENTAL SECTION 2.1 Preparation of NMC and graphite electrodes The LiNi0.5Mn0.3Co0.2O2 (NMC) powders (Nihon Kagaku Sangyo Co., Ltd.) were used to prepare the cathode material. The powders were firstly mixed with a conductive agent of 5 wt% acetylene black (AB) and a 3.5 wt% water-based acrylic/carboxyl methyl cellulose (3.5 wt%/1.5 wt%) binder to form the slurry, followed by casting it on a carbon coated aluminum foil sheet. The coated sheet was then preheated at 80 ºC for 10 min. before it was moved to calendaring. After a final drying treatment was conducted under vacuum at 160 ºC for 6 h, the as-prepared cathode was obtained. As for the anode, a slurry mixing 83.7 wt% graphite (Osaka Gas Chemical Co., Ltd) powders, 9.3 wt% soft carbon, 1 wt% vapor growth carbon fiber (VGCF), 2 wt% acetylene black (AB) and 4 wt% water-based acrylic binder was coated on a copper foil sheet, after which the sheet was subjected to calendaring and heat treatment similar to those for the cathode.

2.2 Electrochemical evaluation and material analysis For the electrochemical evaluation, a coin-type test cell (CR2032) was used to accommodate the as-prepared electrodes together with a glass-fiber filter used as the separator in between the electrodes. In addition, the electrolyte used was 1 M LiPF6 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) at the ratio of 1:1 (v/v) (Kishida Chemicals) with or without 1 wt% VC

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(Kishida Chemicals). The VC concentration was adopted in order to produce better electrode performances of graphite15 and to avoid possible oxidative polymerization on the cathode1 particularly under higher voltage and elevated temperature operation. The electrolyte volume in each test cell was 150 L, allowing us to obtain sufficient amount of electrolyte for analysis. The value of moles of VC to the specific surface area of graphite, which is calculated as an estimated relevant VC amount, is 8.43  10-6 mole g m-2. This value is close to that of a previous study15 using graphite with specific surface area of 0.8 m2 g-1. A charge-discharge machine (BLS series, Keisokuki Center Co., Ltd.) was applied to galvanostatically carry out the charge/discharge cycling tests for which the current density was 75 mA g-1; cutoff voltage range was over 2.5–4.2 V; operating temperature was at 60 ºC. With regard to the material analysis, the cycled cells were decomposed in a dry room with an ultra-low humidity environment to prudentially collect the samples of electrolytes and electrodes. LC-MS (Nexera, Shimadzu Corp. and Esquire 3000 plus, Bruker) and DART-MS (DART-SVP, AMR Inc. and LCMS-8030, Shimadzu Corp.) were respectively applied to analyze the collected electrolyte and electrode surface. In the DART-MS experiments, the electrode surface was allowed to contact the ion beam ejected from the beam generator at an angle of approximately 45º without shielding the beam. Further, the detection was conducted at various beam temperatures of 150, 250, 350 and 450 °C.

3. RESULTS AND DISCUSSION

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3.1 Effect of VC on high-temperature cycling performance of the NMC/Graphite cell Figure 1a shows the cycling performance at 60 °C for the NMC/graphite cells with/without VC additive. It is obvious that the VC-containing cell exhibits better cycling stability with a capacity retention of 79% at the 100th cycle. On the other hand, the capacity of the VC-free cell drastically decays in the initial cycling and then becomes stable with a gradual capacity fading, resulting in a lower capacity retention of 44% at the 100th cycle. This result differs from that for the cells evaluated at 30 °C, which shows only a smaller difference in capacity fading during cycling (see Figure S1). Therefore, it is denoted that the electrochemical behaviors during the initial cycling at 60 °C dominates the thermal stability of the NMC/graphite cells, which is greatly influenced by the VC additive. The 1st and 2nd charge/discharge curves for the two cells mentioned above are shown in Figure 1b. For the VC-containing cell (solid line), its first charge capacity reaches 189 mAh g-1 at 4.2 V though experiencing a plateau at ca. 3.6 V while the discharge capacity reduces to 159 mAh g-1 at 2.5 V, leading to a Coulombic efficiency of 84%. After the 1st cycle, the 2nd charge and discharge profiles look more symmetric and the discharge capacity is almost the same as the 1st one. The reduced irreversible capacity and symmetric profiles in the 2nd cycle offer the evidence of improved electrochemical reversibility. In the case of VC-free (dot line), the 1st charge profile almost overlaps with that of the VC-containing cell, whereas its 1st discharge capacity is lower, thus giving a poorer Coulombic efficiency. As for its 2nd cycle, much reduced capacities in both charge and discharge sides can be observed from the profiles despite the irreversible capacity becomes less. It is therefore

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indicated that VC plays an important role in preventing the capacity fading, which corresponds to the initial irreversible electrochemical reaction at the elevated temperature.

Figure 1. (a) Cycling performance of NMC/graphite cells with/without VC at 60 °C, and (b) corresponding initial charge/discharge curves.

3.2 Electrolyte analysis by LC-MS Figure 2 shows the LC-MS chromatograms of the electrolytes collected from the NMC/graphite cells experiencing 2 charge/discharge cycling at 60 °C. According to the analysis results, seven types of compounds, including two carbonate oligomers (C1 and C2) and four organophosphates (P1-P4), are identified and summarized in the right box. Other non-identifying peaks in the chromatograms could be owing to the impurity substances in the sample solution. The electrolyte of case (a) contains 9



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C1, C2 and P3 with relatively low peak intensities. After 100 cycling (b), other types of organophosphates P1, P2 and P4 are formed in addition to the aforementioned compounds. It is also worth noting that, compared with the compounds C1 and C2, the peak intensity of P3 remarkably increases after the long-term cycling, indicating that the formation reaction of the organophosphates is more significant than that of the carbonate oligomers during the cycling. When introducing VC additive into the electrolyte, no compounds can be identified after 2 cycling, except for C1 with a much lower peak intensity, as shown in (c). With further cycling up to 100 cycles (d), despite several carbonate oligomers and organophosphates are found in the electrolyte, they are all within much lower peak intensity compared with the VC-free case (b). Since the organophosphates and the carbonate oligomers should be attributed to the decomposition of LiPF6 and EC/DEC solvent, respectively, it is evidenced from the above analysis that adding VC offers the function of hindering those decomposition reactions at elevated temperature. A previous study reported that, for Li-rich HE-NCM-based lithium ion cells, the use of a glass fiber separator would lead to formation of more decomposition compounds compared with a polypropylene (PP) separator24. Based on their conclusions, electrode potentials higher than 4.2 V and activation of the Li2MnO3 domains of the Li-rich cathode would be the reasons accounting for further formation of decomposition compounds corresponding to POF3. However, compared with our previous report20 using a PP separator, the glass fiber separator tested here does not facilitate the formation of more organophosphates in electrolyte, indicating a limited amount of POF3. Lower cut-off voltage (4.2 V) and non-existence of

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Li2MnO3 domains in cathode material may allow us to eliminate the concern of separator type in this study.

Figure 2. LC-MS chromatograms of various electrolyte samples of VC-free after (a) 2 cycle and (b) 100 cycle charge/discharge tests; VC-containing after (c) 2 cycle and (d) 100 cycle charge/discharge tests. (The deduced chemical formulae for the identified compounds are summarized in the right box)

3.3 Electrode surface analysis by DART-MS To further understand the influence of VC on the initial decomposition reactions, DART-MS is applied to analyze the electrode surfaces. The analysis results for the NMC cathodes of the cells with/without VC are shown in Figure 3. The number for each compound nearby the figures represents the m/z value and its deduced chemical structure is summarized in Table 1. The representative spectra at 450 °C can also be referred to Figures S2 and S3. In spite of the variation in intensity over the detected temperature range, most of the compounds can be referred to the solvents EC (m/z = 89/106/177) and DEC (119) for the VC-free cell (a), except for the minor compound of 11



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organophosphate P1 (153) that is present at higher temperature. The result of the VC-containing cell (b) is similar to the VC-free one, demonstrating EC and DEC are the primary compounds detected. Because most of the detectable compounds are the organic solvents, the NMC cathode surface may remain intact during the initial electrochemical cycling regardless of the presence of VC. This result is in consistent with the previously reported one revealing that no degradation compounds are found on the LiFePO4 cathode surface except for the solvents.17

Figure 3. Summary of DART-MS spectra for NMC cathodes experiencing 2 cycle charge/discharge test. (a) VC-free sample; (b) VC-containing sample.

Similarly, the graphite anodes of the VC-containing and VC-free cells are analyzed by DART-MS and the results are summarized in Figure 4. The representative spectra can be referred to Figures S4 (350 °C) and S5 (450 °C) for the VC-free cell; Figures S6 (350 °C) and S7 (450 °C) for the 12


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VC-containing cell. For the VC-free graphite anode, the detected compounds are composed of the organic solvents EC and DEC together with the organophosphate P1. This is similar to the result of NMC cathode of Figure 3a, whereas P1 on the anode side is detected in a wider temperature range with a stronger intensity than that on the cathode side. On the other hand, the VC-containing case (b) reveals several types of organophosphates labelled from P5-P13 and carbonate oligomers C1 at beam temperature higher than 350 °C, showing a very different result from that of (a). Also, this result is different from that of our previous LFP/graphite cell where only P9 and P12 were detected except for EC. This finding suggests that NMC should be more electrochemically active than LFP to form the decomposition compounds induced by VC. Among these compounds, P6 (169), P9 (253/270) and P12 (275/292) exhibit relative strong detection intensity, denoting that they are more likely to form on the graphite surface. It is also worth noting that, unlike the other organophosphates, they all consist of cyclic phosphate group, which should be more structurally stable against the elevated temperature. More importantly, compared with the NMC cathodes (see Figure 3), most of the decomposition compounds are detected on the graphite surface of the VC-containing cell, and they are different from those found in the electrolyte (see Figure 2), indicating these compounds could be much less soluble in the electrolyte. This suggests that adding VC substantially facilitates the formation of the compounds, mainly composed of the cyclic organophosphates, constituting a thermally resistant film on the graphite surface during the initial cycling, which corresponds to the reduced decomposition compounds in the electrolyte, as mentioned in Figure 2c. That is, further

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electrolyte decomposition can be prevented by forming the thermally resistant film;17 consequently, the capacity fading can be alleviated from the initial cycling for the VC-containing cell, as discussed in Figure 1.

Figure 4. Summary of DART-MS spectra for graphite anodes experiencing 2 cycle charge/discharge test. (a) VC-free sample; (b) VC-containing sample.

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Table 1. Summary of the compounds identified by DART-MS [M+H]+ /[M+NH4]+/[2M+H]+/

Notation

Molecular weight

EC

88

89/106/177

DEC

118

119

C1

206

213

P1

152

153

P5

126

144

P6

168

169

P7

98

197/214

P9

252

253/270

P10

254

255/272

P11

270

271

P12

274

275/292

P13

334

352

[2M+NH4]+/[M+Li]+

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Suggested structure


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3.4 Influence of VC on the formation mechanisms of decomposition compounds It is well known that LiPF6 dissolves in carbonate-based solvents and readily decomposes into LiF and PF5, reaching an equilibrium state. Once a trace amount of water or alcohol (ROH) is present in the electrolyte, PF5 further reacts with them to form phosphoryl trifluoride (POF3) that is the precursor for the organophosphates. Figure 5 shows the proposed formation mechanisms for the primary cyclic organophosphates, as shown in Figure 4b. All the products P6, P9 and P12 are initiated from OPF2OC2H2F that is formed by the reaction of POF3 and EC, accompanied with the release of CO2, as proposed by Campion et al.25 A subsequent reaction of OPF2OC2H2F, either esterification with EC or hydrolysis, leads to the formation of P6 or P9. As for P12, it is derived from a further reaction of P6 and cyclic OPF2OC2H2. According to the reaction mechanisms, it is known that EC plays a very important role in triggering the formation of these organophosphates.

Figure 5. Proposed formation mechanisms for the primary cyclic organophosphates formed on the graphite anode surface of the VC-containing cell.

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On the other hand, EC also tends to react with alkoxide anion (C2H5O¬ ), which is from the common electrochemical reduction product lithium alkoxide,26 to form the other decomposition compounds as detected by LC-MS. The probable reaction mechanisms for the compounds detected in the electrolyte are also summarized in Figure 6. These compounds are all induced by the reactions associated with either the alkoxide anion (C2H5O¬ ) or the intermediate (C2H5OCO2C2H4O¬ ) that is derived from the reaction of EC and alkoxide anion. It has been reported that VC is capable of trapping the alkoxide anion, prohibiting further corresponding reactions.27 Based on this viewpoint, once the VC is added into the EC-based electrolyte, the alkoxide anion is trapped and the route to generate the intermediate is hindered, thus suppressing the formation of the compounds C1, C2, and P1-P4. Despite different reaction mechanism involving DEC and POF3 was previously reported to explain the formation of P322, this reaction scheme satisfactorily agrees with the analysis results of Figure 2. Further, since EC is less involved in the reaction associated with the alkoxide anions, it should be, instead, more likely to react with POF3, facilitating the formation of cyclic organophosphates, as discussed in Figure 5. This is considered the main reason for the induced thermally resistant film formation on the graphite anode surface and the reduced decomposition compounds in the electrolyte, which is greatly influenced by VC particularly at elevated temperature. The proposed reaction scheme can also support the explanation for the LC-MS and DART-MS results of our previous studies22,23 where the correlation between reduced decomposition compounds in the electrolyte and on the electrode surface may not be sufficiently elaborated.

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Figure 6. Proposed reaction mechanisms of the compounds detected in the electrolyte and the VC influence on the reaction pathways.

4. CONCLUSIONS The cycling stability of a NMC/graphite cell at 60 ºC is investigated via an analysis approach incorporating the LC-MS and DART-MS techniques. The VC-containing cell exhibits much improved cycling performance against the elevated temperature rather than the VC-free one. According to the LC-MS analysis, more decomposition compounds of carbonate oligomers and organophosphates are identified in the electrolyte without VC, while the formation of these compounds can be suppressed by introducing 1 wt% of VC during the initial cycling. On the other hand, from the DART-MS analysis results, it is known that a thermally resistant film, which is mainly consisted of cyclic organophosphates, prefers to form on the graphite anode rather than the 18


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NMC cathode surface in the VC-containing cell rather than the VC-free one. Moreover, the formation mechanisms of the detected compounds are elucidated, allowing us to satisfactorily explain the reduced decomposition compounds in the electrolyte and the formation of the thermally resistant film induced by VC. That is, VC can trap the free alkoxide anions and meanwhile allow more EC to react with POF3, leading to the formation of the thermally stable organophosphates that is beneficial to the improved cycling performance at elevated temperature. As an extension of this research, future works regarding the effects of water content and VC quantity would be required to quantitatively evaluate the electrolyte decomposition process.

Supporting Information Cycling test results at 30 C Original data of DART-MS spectra

AUTHOR INFORMATION Corresponding Author Prof. Yi-Hung Liu* Department of Greenergy, National University of Tainan, Tainan 70005, Taiwan E-mail: [email protected] Author Contributions

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Yi-Hung Liu designed the experiments, performed the electrochemical evaluations and carried out DART-MS measurements together with the data analysis. Sahori Takeda carried out the LC-MS measurements and analyzed the data. Takashi Mukai performed the electrochemical measurements. The manuscript was written by Yi-Hung Liu. All the authors discussed and checked the results, giving approval to the final version of the manuscript.

ACKNOWLEDGMENTS The authors would like to appreciate Ms. Miki Okano of National Institute of Advanced Industrial Science and Technology (AIST) for the technical support on the experiment. Also, the authors would like to greatly thank financial support from ASIT.

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REFERENCES (1) Zhang, S. S. A Review on Electrolyte Additives for Lithium-Ion Batteries. J. Power Sources 2006, 162, 13791394. (2) Ouatani, L. E.; Dedryvère, R.; Siret, C.; Biensan, P.; Reynaud, S.; Iratçabal, P.; Gonbeau, D. The Effect of Vinylene Carbonate Additive on Surface Film Formation on Both Electrodes in Li-Ion Batteries. J. Electrochem. Soc. 2009, 156, A103−A113. (3) Stojković, I. B.; Cvjetićanin, N. D.; Mentus, S. V. The Improvement of the Li-Ion Behaviour of Li1.05Cr0.10Mn1.85O4 in an Aqueous Medium upon Addition of Vinylene Carbonate. Electrochem. Commun. 2010, 12, 371−373. (4) Burns, J. C.; Sinha, N. N.; Coyle, D. J.; Jain, G.; VanElzen, C. M.; Lamanna, W. M.; Xiao, A.; Scott, E.; Gardner, J. P.; Dahn, J. R. The Impact of Varying the Concentration of Vinylene Carbonate Electrolyte Additive in Wound Li-Ion Cells. J. Electrochem. Soc. 2012, 159, A85−A90. (5) Takamatsu, D.; Orikasa, Y.; Mori, S.; Nakatsutsumi, T.; Yamamoto, K.; Koyama, Y.; Minato, T.; Hirano, T.; Tanida, H.; Arai, H.; Uchimoto, Y.; Ogumi, Z. Effect of an Electrolyte Additive of Vinylene Carbonate on the Electronic Structure at the Surface of a Lithium Cobalt Oxide Electrode under Battery Operating Conditions. J. Phys. Chem. C 2015, 119, 9791−9797. (6) Jeong, S.-K.; Inaba, M.; Mogi, R.; Iriyama, Y.; Abe, T.; Ogumi, Z. Surface Film Formation on a Graphite Negative Electrode in Lithium-Ion Batteries: Atomic Force Microscopy Study on the

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Effects of Film-Forming Additives in Propylene Carbonate Solutions. Langmuir 2001, 17, 8281−8286. (7) Zhang, X. R.; Kostecki, R.; Richardson, T. J.; Pugh, J. K.; Ross, P. N. Electrochemical and Infrared Studies of the Reduction of Organic Carbonates. J. Electrochem. Soc. 2001, 148, A1341−A1345. (8) Aurbach, D.; Gamolsky, K.; Markovsky, B.; Gofer, Y.; Schmidt, M.; Heider, U. On the Use of Vinylene Carbonate (VC) as an Additive to Electrolyte Solutions for Li-Ion Batteries. Electrichim. Acta 2002, 47, 1423–1439. (9) Oesten, R.; Heider, U.; Schmidt, M. Advanced Electrolytes. Solid State Ionics 2002, 148, 391–397. (10) Herstedt, M.; Rensmo, H.; Siegbahn, H.; Edström, K. Electrolyte Additives for Enhanced Thermal Stability of the Graphite Anode Interface in a Li-Ion Battery. Electrochim. Acta. 2004, 49, 2351−2359. (11) Chen, L.; Wang, K.; Xie, X.; Xie, J. Effective of Vinylene Carbonate (VC) as Electrolyte Additive on Electrochemical Performance of Si Film Anode for Lithium Ion Batteries. J. Power Sources 2007, 174, 538−543. (12) Ota, H.; Sakata, Y.; Inoue, A.; Yamaguchi, S. Analysis of Vinylene Carbonate Derived SEI Layers on Graphite Anode. J. Electrochem. Soc. 2004, 151, A1659−A1669. (13) Wang, Y.; Nakamura, S.; Tasaki, K.; Balbuena P. B. Theoretical Studies to Understand Surface

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Chemistry on Carbon Anodes for Lithium-Ion Batteries: How Does Vinylene Carbonate Play Its Role as an Electrolyte Additive? J. Am. Chem. Soc. 2002, 124, 4408−4421. (14) Ouatani, L. E.; Dedryvère, R.; Siret, C.; Biensan, P.; Gonbeau, D. Effect of Vinylene Carbonate Additive in Li-Ion Batteries: Comparison of LiCoO2∕C, LiFePO4∕C, and LiCoO2∕Li4Ti5O12 Systems. J. Electrochem. Soc. 2009, 156, A468−A477. (15) Xiong, D.; Burns, J. C.; Smith, A. J.; Sinha, N.; Dahn, J. R. A High Precision Study of the Effect of Vinylene Carbonate (VC) Additive in Li/Graphite Cells. J. Electrochem. Soc. 2011, 158, A1431−A1435. (16) Madec, L.; Petibon, R.; Xia, J.; Sun, J.-P.; Hill, I. G.; Dahn, J. R. Understanding the Role of Prop-1-ene-1,3-Sultone and Vinylene Carbonate in LiNi1/3Mn1/3Co1/3O2/Graphite Pouch Cells: Electrochemical, GC-MS and XPS Analysis. J. Electrochem. Soc. 2015, 162, A2635−A2645. (17) Schultz, C.; Kraft, V.; Pyschik, M.; Weber, S.; Schappacher, F.; Winter, M.; Nowak, S. Separation and Quantification of Organic Electrolyte Components in Lithium-Ion Batteries via a Developed HPLC Method. J. Electrochem. Soc. 2015, 162, A629–A634. (18) Schultz, C.; Vedder, S.; Winter, M.; Nowak, S. Qualitative Investigation of the Decomposition of Organic Solvent Based Lithium Ion Battery Electrolytes with LC-IT-TOF-MS. Anal. Chem. 2016, 88, 11160–11168. (19) Tochihara, M.; Nara, H.; Mukoyama, D.; Yokoshima, T.; Momma, T.; Osaka, T. Liquid Chromatography-Quadruple Time of Flight Mass Spectrometry Analysis of Products in Degraded

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Lithium-Ion Batteries. J. Electrochem. Soc. 2015, 162, A2008–A2015. (20) Takeda, S.; Morimura, W.; Liu, Y. H.; Sakai, T.; Saito, Y. Identification and Formation Mechanism of Individual Degradation Products in Lithium‐Ion Batteries Studied by Liquid Chromatography/Electrospray Ionization Mass Spectrometry and Atmospheric Solid Analysis Probe Mass Spectrometry. Rapid Commun. Mass Spectrom. 2016, 30, 1754−1762. (21) Weber, W.; Kraft, V.; Grützke, M.; Wagner, R.; Winter, M.; Nowak, S. Identification of Alkylated Phosphates by Gas Chromatography–Mass Spectrometric Investigations with Different Ionization Principles of a Thermally Aged Commercial Lithium Ion Battery Electrolyte. J. Chromatogr. A 2015, 1394, 128–136. (22) Liu, Y. H.; Takeda, S.; Kaneko, I.; Yoshitake, H.; Yanagida, M.; Saito, Y.; Sakai, T. An Approach of Evaluating the Effect of Vinylene Carbonate Additive on Graphite Anode for Lithium Ion Battery at Elevated Temperature. Electrochem. Commun. 2015, 61, 70−73. (23) Liu, Y. H.; Takeda, S.; Kaneko, I.; Yoshitake, H.; Yanagida, M.; Saito, Y.; Sakai, T. Formation of Thermally Resistant Films Induced by Vinylene Carbonate Additive on a Hard Carbon Anode for Lithium Ion Batteries at Elevated Temperature. RSC Adv. 2016, 6, 75777−75781. (24) Guéguen, A.; Streich, D.; He, M.; Mendez, M.; Chesneau, F. F.; Nováka, P.; Berg, E. J. Decomposition of LiPF6 in High Energy Lithium-Ion Batteries Studied with Online Electrochemical Mass Spectrometry. J. Electrochem. Soc. 2016, 163, A1095–A1100. (25) Campion, C. L.; Li, W.; Lucht, B. L. Thermal Decomposition of LiPF6-Based Electrolytes for

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Lithium-Ion Batteries. J. Electrochem. Soc. 2005, 152, A2327−A2334. (26) Sasaki, T.; Abe, T. Iriyama, Y.; Inaba, M.; Ogumi, Z. Formation Mechanism of Alkyl Dicarbonates in Li-Ion Cells. J. Power Sources 2005, 150, 208−215. (27) Sasaki, T.; Abe, T.; Iriyama, Y.; Inaba, M.; Ogumi, Z. Suppression of an Alkyl Dicarbonate Formation in Li-Ion Cells. J. Electrochem. Soc. 2005, 152, A2046−A2050.

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Figure Captions Figure 1. (a) Cycling performance of NMC/graphite cells with/without VC at 60 C, and (b) corresponding initial charge/discharge curves. Figure 2. LC-MS chromatograms of various electrolyte samples of VC-free after (a) 2 cycle and (b) 100 cycle charge/discharge tests; VC-containing after (c) 2 cycle and (d) 100 cycle charge/discharge tests. (The deduced chemical formulae for the identified compounds are summarized in the right box) Figure 3. Summary of DART-MS spectra for NMC cathodes experiencing 2 cycle charge/discharge test. (a) VC-free sample; (b) VC-containing sample. Figure 4. Summary of DART-MS spectra for graphite anodes experiencing 2 cycle charge/discharge test. (a) VC-free sample; (b) VC-containing sample. Figure 5. Proposed formation mechanisms for the primary cyclic organophosphates formed on the graphite anode surface of the VC-containing cell. Figure 6. Proposed reaction mechanisms of the compounds detected in the electrolyte and the VC influence on the reaction pathways.

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TOC Graphic

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Understanding the Improved High-Temperature Cycling Stability of

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