Model Studies on the Formation of the Solid Electrolyte Interphase: Reaction of Li with Ultrathin Adsorbed Ionic-Liquid Films and Co$_{3}$O$_{4}$(111) Thin Films

The formation of the solid electrolyte interphase at the electrode-electrolyte interface is studied by X-ray photoelectron spectroscopy and dispersion-corrected density functional theory calculations. The interaction between an adsorbed ionic liquid, Li, and a Co3O4(111) thin-film model anode reveals bis(trifluoromethylsulfonyl)imide and 1-butyl-1-methylpyrrolidinium decomposition and lithiation of the cobalt oxide. In this work we aim towards the molecular understanding of the solid electrolyte interphase (SEI) formation at the electrode electrolyte interface (EEI). Herein, we investigated the interaction between the battery-relevant ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-TFSI), Li and a Co3O4(111) thin film model anode grown on Ir(100) as a model study of the SEI formation in Li-ion batteries (LIBs). We employed mostly X-ray photoelectron spectroscopy (XPS) in combination with dispersion-corrected density functional theory calculations (DFT-D3). If the surface is pre-covered by BMP-TFSI species (model electrolyte), post-deposition of Li (Li+ ion shuttle) reveals thermodynamically favorable TFSI decomposition products such as LiCN, Li2NSO2CF3, LiF, Li2S, Li2O2, Li2O, but also kinetic products like Li2NCH3C4H9 or LiNCH3C4H9 of BMP. Simultaneously, Li adsorption and/or lithiation of Co3O4(111) to LinCo3O4 takes place due to insertion via step edges or defects; a partial transformation to CoO cannot be excluded. Formation of Co0 could not be observed in the experiment indicating that surface reaction products and inserted/adsorbed Li at the step edges may inhibit or slow down further Li diffusion into the bulk. This study provides detailed insights of the SEI formation at the EEI, which might be crucial for the improvement of future batteries.
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