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 LixSiy 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.