EIS and XPS Investigation on SEI Layer Formation during First Discharge on Graphite Electrode with a Vinylene Carbonate Doped Imidazolium Based Ionic Liquid Electrolyte

Adding organic carbonates (e.g., vinylene carbonates, VC) into ionic liquid based electrolyte generally improves the electrochemical performances of graphite electrode (Cgr) in lithium ion batteries. In this study, step-by-step electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS) were performed during the first reduction cycle to follow the formation of the solid electrolyte interphase (SEI) in lithium/graphite (Li/Cgr) cell using (1-hexy1-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) [C(1)C(6)Im][NTf2] doped with vinylene carbonate (VC). The EIS spectra evolution, recorded at different cutoff voltages, indicated a two-step SEI thin film formation. These results were supported by XPS measurement at the same cutoff voltage. Hence, we pointed out that the first film results from the VC decomposition at 0.8 V to form an organic layer constituted of lithium alkylcarbonates. This film was developed up to 0.2 V. From 0.8 to 0.2 V, we detected a slight decomposition of IL solvent. This process was driven by a progressive sulfone decomposition reaction through the formation of polyoxysulfone, Li2S, Li3N, and LiF. This process was at the origin of the formation of the second film of inorganic nature beyond 0.2 V. This SEI film was stable up to 0.01 V; the composition probed by XPS remained unchanged.