Theoretical Investigations on Oxidative Stability of Solvents and Oxidative Decomposition Mechanism of Ethylene Carbonate for Lithium Ion Battery Use

The electrochemical oxidative stability of solvent molecules used for lithium ion battery, ethylene carbonate (EC), propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate in the forms of simple molecule and coordination with anion PF6−, is compared by using density functional theory at the level of B3LYP/6-311++G (d, p) in gas phase. EC is found to be the most stable against oxidation in its simple molecule. However, due to its highest dielectric constant among all the solvent molecules, EC coordinates with PF6− most strongly and reaches cathode most easily, resulting in its preferential oxidation on cathode. Detailed oxidative decomposition mechanism of EC is investigated using the same level. Radical cation EC•+ is generated after one electron oxidation reaction of EC and there are five possible pathways for the decomposition of EC•+ forming CO2, CO, and various radical cations. The formation of CO is more difficult than CO2 during the initial decomposition of EC•+ due to the high activation energy. The radical cations are reduced and terminated by gaining one electron from anode or solvent molecules, forming aldehyde and oligomers of alkyl carbonates including 2-methyl-1,3-dioxolane, 1,3,6-trioxocan-2-one, 1,4,6,9-tetraoxaspiro[4.4]nonane, and 1,4,6,8,11-pentaoxaspiro[4.6]undecan-7-one. The calculation in this paper gives a detailed explanation on the experimental findings that have been reported in literatures and clarifies the mechanism on the oxidative decomposition of EC.