New Insight into the Role of Fluoro-ethylene Carbonate in Suppressing Li-Trapping for Si Anodes in Lithium-Ion Batteries

Fluoro-ethylene carbonate (FEC) is widely adopted as a film-forming electrolyte additive in Li-ion batteries and is believed to promote the formation of a stable LiF-rich solid electrolyte interphase (SEI) on silicon electrode surfaces. In this work, different from traditional recognition, it is found that the role of FEC in suppressing Li-trapping is more important than that of stabilizing the SEI film. In FEC-free electrolyte, the growth and rearrangement of the SEI makes up for 47.8% of the capacity loss, and the remaining 52.2% is related to Li-trapping within Si particles. In the presence of 10% FEC, capacity loss related to SEI growth is reduced by 52.9%, while Li-trapping within Si particles is reduced by 82.3%. Microscopic studies demonstrate that LiF-doping in the bulk silicon phase is the main cause which effectively inhibits the formation of crystalline Li15Si4 and thus reduces Li-trapping during electrochemical cycles. This new insight discloses a hidden effect of FEC additive on the Si anode in Li-ion batteries.