Fluoroethylene Carbonate Breakdown Mechanisms and Energetics on Two Lithium Silicide Surfaces

Lithium-ion batteries are a leading energy storage technology. One challenge with lithium-ion batteries is the reductive decomposition of electrolyte on the surface, forming a passivating, solid–electrolyte interphase (SEI). The SEI prevents further electrolyte breakdown and consumption, but if not formed properly, may also consume lithium ions and prevent lithium-ion diffusion to the anode. Fluoroethylene carbonate (FEC) is currently one of the best electrolyte additives used to form a more robust SEI on silicon anodes. Herein, we use density functional theory (DFT) to investigate the spontaneous breakdown mechanisms, energies, and charge transfers of FEC on the surface of a silicon anode in a lesser lithiated LiSi and a more lithiated Li15Si4 state. The reductive decomposition of FEC on LiSi and Li15Si4 to F, CO2, and CH2CHO is energetically most favorable on both surfaces, but F, CO, and OCH2CHO can also be formed. The breakdown of FEC via either of the breakdown mechanisms is about 2 times more favorable on the Li15Si4 surface than on the LiSi surface. The Bader charge transferred from the anode to the FEC breakdown products is larger when forming F, CO2, and CH2CHO than when forming F, CO, and OCH2CHO and is also larger on the Li15Si4 surface than on the LiSi surface.