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.