DFT Study of Reduction Mechanisms of Ethylene Carbonate and Fluoroethylene Carbonate on Li⁺-Adsorbed Si Clusters

Silicon anodes show great potential but have severe practical limitations resulting from swelling effects during lithiation and from solid-electrolyte interphase (SEI) layers formed at the electrode surface due to electrolyte electrochemical instability. Aiming to understand the anode SEI reactions at ultra-low degrees of lithiation, we investigate reductive decomposition mechanisms of ethylene carbonate (EC) and fluoroethylene carbonate (FEC) on Li + -adsorbed Si clusters using density functional theory. The Li location in these clusters is very different than those found on Li x Si y alloy surfaces and this difference yields important variations in the reduction mechanisms compared with those of higher lithiated Si surfaces. EC decomposition at ultra-low lithiation conditions may undergo one- or two-electron transfer processes depending on the local interfacial density of EC and Li + ions. One-electron transfers are preferred at high EC concentration while two-electron transfers are favored at high Li + density on the Si surface. FEC exhibits more adsorption modes than EC, and the ring opening may occur in multi-sites, leading to more reduction pathways. Overall, the two-electron transfer process is thermodynamically and kinetically favorable for FEC reductive decomposition. Similarities and differences between EC and FEC reduction processes for this low-lithiated surface are discussed and compared to those in higher lithiated surfaces.