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.