Unveiling Reaction Pathways of Ethylene Carbonate and Vinylene Carbonate in Li-ion Batteries

Ethylene carbonate (EC) and vinylene carbonate (VC) are the archetypical electrolyte solvent and additive in Li-ion batteries (LIBs), respectively. However, our understanding of their reaction pathways remains incomplete. Herein, the reaction pathways of EC and VC are explored by using online electrochemical mass spectrometry complemented by nuclear magnetic resonance analysis. For EC, reduction occurs through two distinct pathways <0.8 V vs Li+/Li, one yielding C2H4 and the other yielding CO, depending on the electrode potential and the EC concentration. The CO-releasing pathway does not contribute to the solid electrolyte interphase formation. For VC, reduction commences at <1.9 V, but CO2 gas evolution proceeds through a chemical step via a nucleophilic attack and VC ring opening. Additionally, VC scavenges H2O and reduced protons via hydrolysis and via proton abstraction from the carbon electrode to form EC. Our study uncovers further reaction pathways and underscores the unique properties of EC and VC, both individually and in combination, and elucidates their roles in influencing the formation process in Li-ion batteries.