  Fluoroethylene carbonate (FEC) shows promise as an electrolyte additive for
improving passivating solid-electrolyte interphase (SEI) films on silicon
anodes used in lithium ion batteries (LIB). We apply density functional theory
(DFT), ab initio molecular dynamics (AIMD), and quantum chemistry techniques to
examine excess-electron-induced FEC molecular decomposition mechanisms that
lead to FEC-modified SEI. We consider one- and two-electron reactions using
cluster models and explicit interfaces between liquid electrolyte and model
Li(x)Si(y) surfaces, respectively. FEC is found to exhibit more varied reaction
pathways than unsubstituted ethylene carbonate. The initial bond-breaking
events and products of one- and two-electron reactions are qualitatively
similar, with a fluoride ion detached in both cases. However, most one-electron
products are charge-neutral, not anionic, and may not coalesce to form
effective Li+-conducting SEI unless they are further reduced or take part in
other reactions. The implications of these reactions to silicon-anode based LIB
are discussed.
