From Additive to Cosolvent: How Fluoroethylene Carbonate Concentrations Influence Solid-Electrolyte Interphase Properties and Electrochemical Performance of Si/Gr Anodes

Silicon-containing anodes perform best when the solid- electrolyte interphase (SEI) accommodates the high volume changes of silicon particles, as this reduces side reactions and extends the cell lifetime. With this work, we investigate the influence of different fluoroethylene carbonate (FEC) electrolyte concentrations on the SEI composition and thickness and correlate these SEI properties to the electrochemical performance. Three electrolytes (i.e., 2FEC:98LP30, 20FEC:80DMC, and 50FEC:50DMC) are cycled with 9% Si/Gr anodes, and their SEIs are characterized postmortem using photoelectron spectroscopy (XPS). We propose a fitting model for the XPS results in which FEC decomposition yields -C-O, DO (1,3-dioxolan-2-one),-CO2Li, Li2CO3, and LiF. -C- O, DO, and -CO2Li are most probably incorporated in a cross-linked polymeric network. Due to its distinct chemical environments, detecting DO can be unambiguously linked to the presence of FEC decomposition products in the SEI. The presence of DO-type species in the C 1s spectra is correlated to the electrochemical performance: A higher retention in silicon activity was observed for the 20 and 50 vol % FEC-containing electrolytes, where FEC decomposition products (i.e., DO) were present even after 100 cycles. By contrast, when cycling in the 2FEC:98LP30 electrolyte, the silicon activity cannot be retained, and FEC decomposition products are barely detected after 100 cycles. We suggest that the presence of the -C-O-, DO-, and-CO2Li-containing polymeric network positively influences the SEI during silicon volume changes. Additionally, we show that the interaction of FEC and LiPF6 plays an important role in the formation of SiOxFy species.