Investigating the Reduction of Fluoroethylene Carbonate and Vinylene Carbonate in Lithium-Ion Cells with Silicon-Graphite Anodes

The electrolyte additives fluoroethylene carbonate (FEC) and vinylene carbonate (VC) improve the lifetime of lithium-ion batteries with silicon-containing anodes by their reduction yielding a more stable solid electrolyte interphase (SEI). However, the reductive decomposition mechanism of FEC and VC has not yet been fully clarified. For this purpose, we investigate the electrolyte decomposition in LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622)/silicon-graphite pouch cells containing either 1 M LiPF₆ in FEC:dimethyl carbonate (DMC) or 1 M LiPF₆ in VC:DMC using high-performance liquid chromatography, gas chromatography, X-ray photoelectron spectroscopy, and inductively coupled plasma optical emission spectrometry. Based on the molar consumptions of FEC and VC, and the cumulative irreversible capacities, we show that three electrons are consumed for every reduced FEC molecule, and that one electron is consumed for every reduced VC molecule. Based on the results, reactions of the FEC reduction are proposed yielding LiF, Li₂CO₃, Li₂C₂O₄, HCO₂Li, and a PEO-type polymer. Furthermore, the reaction of the VC reduction is proposed yielding lithium-containing, polymerized VC. During formation, the capacity loss of the cells is induced by lithium trapping in Li_xSi_y/Li_xSiO_y under the SEI and by lithium trapping in the SEI. During subsequent cycling, only lithium trapping in the SEI triggers the capacity loss.