Interaction between Li, Ultrathin Adsorbed Ionic Liquid Films, and CoO(111) Thin Films: A Model Study of the SolidlElectrolyte Interphase Formation

Aiming at a molecular-level understanding of the processes at the electrodel electrolyte interface (EEI), we investigated the interaction between the battery-releyant ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP](+)[TESI](-)), Li, and CoO(111) thin films on Ru(0001) as a model study of the solidlelectrolyte interphase (SEI) in Li-ion batteries. Employing mainly angle-dependent X-ray photoelectron spectroscopy and scanning tunneling microscopy, in combination with dispersion-corrected density functional calculations for characterization of the CoO (111) surface, we found that vapor deposition of metallic Li on CoO (111) at 300 K results in the conversion of Co2+ to Co-0, together with the formation of Li2O and adsorbed surface Li2O2. The conversion starts in the near-surface region (1-2 nm) and proceeds in the extended-surface region (6-8 nm). If the surface is precovered by molecularly adsorbed [BMP] TFSI] species (solvent/electrolyte), stepwise postdeposition of small amounts of Li results in gradual decomposition of [TEST] and [BMP] (=SEI formation), forming products such as Li3N, Li2S, LiF, LiwCxHyNz, and other Li-bound fragments of the anion (e.g., LiNSO2CF3). For higher amounts of Li deposition, relative to the IL precoverage, IL decomposition is followed by conversion of Co0(111). Hence, the SEI resulting from IL decomposition is permeable for Li, which is essential for the storage of Li in the CoO(111) anode. This study demonstrates the potential of model studies for a molecular-scale understanding of the initial stages of SEI formation at the EEI and its role in Li storage in a CoO(111) model anode.