Fluoroethylene carbonate (FEC) is an effective electrolyte additive that can significantly improve the cycling ability of silicon and other anode materials. However, the fundamental mechanism of this improvement is still not well understood. Based on the results obtained from 6Li NMR and X-ray photoelectron spectroscopy studies, we propose a molecular-level mechanism for how FEC affects the formation of solid electrolyte interphase (SEI) film: 1) FEC is reduced through the opening of the five-membered ring leading to the formation of lithium poly(vinyl carbonate), LiF, and some dimers; 2) the FEC-derived lithium poly(vinyl carbonate) enhances the stability of the SEI film. The proposed reduction mechanism opens a new path to explore new electrolyte additives that can improve the cycling stability of silicon-based electrodes.
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