XPS study of lithium surface after contact with lithium-salt doped polymer electrolytes

X-ray photoelectron spectroscopy (XPS) is used to probe the surface layer and element composition of Li-metal electrodes before and after contact with polymer electrolytes containing LiN(SO2CF3)2 (LiTFSI) or LiBF4. Native film on as-received metallic lithium was composed of Li2CO3/LiOH in the outer layer and Li2O in the inner layer. LiF was formed during lithium contact with electrolyte due to reaction between the native film and impurities in the electrolyte. The polymer electrolyte containing LiTFSI yielded a very thin film with limited porosity in the inner layers, which was reflected in the limited amplitude dependence of complex impedance spectra. LiBF4 mixed with polymer resulted in a thicker film with high porosity, as was postulated from the greater influence of the amplitude of the oscillation level.