How electrolyte additives work in Li-ion batteries

Although electrolyte additives have been extensively used in modern Li-ion batteries, the practice remained a "dark-art" with little rationale understanding. In this work, using two representative additives that have been extensively for superior electrochemical performances, i.e., vinylene carbonate (VC) and LiPO2F2 (LPF), we thoroughly investigated the fundamental chemistry and electrochemistry involved in Li-ion battery environment and explored the underneath mechanism. The analyses performed on bulk electrolyte and surfaces of both graphitic anode and transition metal oxide cathode reveal complicated reaction cascades among these additives with electrolyte components as well as electrode materials. It was found that the effectiveness of these additives lies not only in how they participate the interphasial chemistries, but also in how they suppress the major side reactions between the bulk electrolyte solvents, the trans-esterification. The correlation between the additive chemistries and electrochemical performances provide valuable guidelines to rational selection, design and synthesis of future additives of higher effectiveness and for new battery chemistries.