Oxidative Decomposition Mechanism of Ethylene Carbonate on Positive Electrodes in Lithium-Ion Batteries

Abstract

Abstract For extending the lifetime and improving the safety of lithium-ion batteries, the decomposition mechanism of electrolytes in lithium-ion batteries was kinetically and stereospecifically investigated in simplified reaction systems, which were in contact with the charged positive electrodes including Li1−xCoO2 as an active material removed from batteries. By identifying the products, mainly vinylene carbonate (VC) was detected by gas chromatography as an oxidation product of ethylene carbonate (EC). The kinetic isotope effects of the reaction were examined using EC and deuterium-labeled EC-D4. The kH/kD was found to be 2.9 suggesting the C–H bond cleavage step was irreversible and corresponds to the rate-determining step of the overall process in the reaction. Moreover, Arrhenius and Eyring plots and stereospecific studies using syn-substituted EC-D2 indicated that the transition state has a rigid structure and that the elimination of hydrogens from EC proceeds mainly via syn stereochemistry. Upon a change in the charge potential of Li1−xCoO2 from 4.5 V to 4.1 V, the rate of formation of VC decreased. PF6−, PO3F2−, and PO2F2− relating to LiPF6 promoted the generation of VC.