The Effect of CO2 on Alkyl Carbonate Trans-Esterification during Formation of Graphite Electrodes in Li-Ion Batteries

Apart from the often-described formation of interphases between the electrodes and the electrolyte in Li-ion batteries, changes of the bulk electrolyte also occur during cycling. In this study, we use On-line Electrochemical Mass Spectrometry (OEMS) to measure the gas evolution associated with changes in the electrolyte during the initial cycles of graphite/lithium half-cells in an electrolyte composed of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and the conducting salt LiPF6. The reduction of the electrolyte at the graphite surface within the first cycle is accompanied by the release of lithium alkoxides (LiOR), which initiate the conversion of the co-solvent EMC into the linear carbonates dimethyl carbonate (DMC) and diethyl carbonate (DEC). This trans-esterification can be suppressed by the use of additives such as vinylene carbonate (VC) and vinyl ethylene carbonate (VEC). Upon reduction, VC generates CO2, while VEC generates 1,3-butadiene. The beneficial impact of the additives arises from these gases, which scavenge the highly reactive LiOR species by forming non-reactive products. Furthermore, our results demonstrate the positive effect of CO2 on the cell chemistry and the importance of adjusting the electrolyte volume and additive concentration with respect to the active material mass in Li-ion batteries.