New Insight into Electrochemical Differences in Cycling Behaviors of a Lithium-ion Battery Cell Between the Ethylene Car
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New Insight into Electrochemical Differences in Cycling Behaviors of a Lithium-ion Battery Cell Between the Ethylene Carbonate- and Propylene Carbonate-Based Electrolytes Ken Tasaki,a Alexander Goldberg,b Jian-Jie Liang,b and Martin Winterc a Mitsubishi Chemical USA, 410 Palos Verdes Blvd., Redondo Beach, CA 90277. b Accelrys Software Inc., 10188 Telesis Ct., San Diego, CA 92121. c Institut für Physikalishe Chemie, Westfälishe Wilhelm-Universität Münster, Münster, Germany. ABSTRACT Density functional theory (DFT) calculations and classical molecular dynamics (MD) simulations have been performed to gain insight into the difference in cycling behaviors between the ethylene carbonate (EC)-based and the propylene carbonate (PC)-based electrolytes in lithium-ion battery cells. DFT calculations for the ternary graphite intercalation compounds (Li+(S)iCn: S=EC or PC), in which the solvated lithium ion Li+(S)i (i=1~3) was inserted into a graphite cell, suggested that Li+(EC)iCn was more stable than Li+(PC)iCn in general. Furthermore, Li+(PC)3Cn was found to be energetically unfavorable, while Li+(PC)2Cn was stable, relative to their corresponding Li+(PC)i in the bulk electrolyte. The calculations also revealed severe structural distortions of the PC molecule in Li+(PC)3Cn, suggesting a rapid kinetic effect on PC decomposition reactions, as compared to decompositions of EC. In addition, MD simulations were carried out to examine the solvation structures at a high salt concentration: 2.45 mo kg-1. The results showed that the solvation structure was significantly interrupted by the counter anions, having a smaller solvation number than that at a lower salt concentration (0.83 mol kg-1). We propose that at high salt concentrations, the lithium desolvation may be facilitated due to the increased contact ion pairs, so that a stable ternary GIC with less solvent molecules can be formed without the destruction of graphite particles, followed by solid-electrolyte-interface film formation reactions. The results from both DFT calculations and MD simulations are consistent with the recent experimental observations. INTRODUCTION It has long been known that a lithium-ion battery cell having graphite as the anode active material in the ethylene carbonate (EC)-based electrolyte can be cycled, while charging of a similar cell in the propylene carbonate (PC)-based electrolyte only gives rise to continuous decompositions of PC and exfoliation of graphite.1-3 Despite a large number of reports discussing the electrochemical differences between the two electrolytes experimentally as well as theoretically, 1-7 there is no widely accepted single explanation for this observation. Recently, it has been found that a lithium-ion cell having graphite as the anode can be cycled in a PC electrolyte when salt concentrations are high.8 The authors suggested desolvation of lithium played an important step prior to co-intercalation of lithium into graphite. 8 Here, we examine the lithium solvation/desolvation and the ternary graphite intercalation compounds
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