Ethylene bis-carbonates as telltales of SEI and electrolyte health, role of carbonate type and new additives

The ethylene bis-carbonate compounds formation is responsible for the earliest change in electrolyte composition which can be one of the reasons for battery performance decay. In this study, liquid GC/MS technique is used to detect their formation in electrolytes based on solvent mixtures of EC and different linear carbonates (DMC, DEC and EMC), after the first cycle in full cells composed of synthetic graphite powder/commercial positive films. These compounds stem from linear carbonate electrochemical reduction leading to alkoxide compounds and can be quantified using a selective bicyclic boron ester Lewis acid as an electrolyte additive. Moreover, a quantitative study on ethylene bis-carbonate compounds for which the generation profile is different depending on the linear carbonate type, shows that either in batteries or in a simple chemical mixture of electrolyte and lithium alkoxide, their formation stops when it reaches a threshold concentration due to the thermodynamic equilibrium. The overall information is useful for investigating the passivation ability and the dissolution of the Solid Electrolyte Interphase (SEI) that is formed on the negative electrode material. Finally, the passivation property of the SEI freshly formed with four additives - Vinylene Carbonate (VC), Vinyl Ethylene Carbonate (VEC), Fluoro Ethylene Carbonate (FEC) and 1,3-Propane Sultone (1,3-PS)- is studied.