X-ray photoelectron spectroscopy study of surface films formed on Li electrodes freshly prepared in alkyl carbonate solutions

Li samples were freshly prepared (shearing) and stored (2 days) in dimethyl carbonate(DMC) and ethyl carbonate-diethyl carbonate (EC-DEC), and dry (20 ppm of H2O) and wet (500 ppm of H2O) EC-DMC solutions of LiAsF6 (1 M), and were then studied by X-ray photoelectron spectroscopy (XPS). The XPS analysis, including depth profiling of these surface films, appears to be reliable on the qualitative level only, because both the X-ray beam and sputtering should be suspected as being partially destructive to the surface films on lithium. These studies basically confirm previous conclusions obtained by Fourier transform infrared spectroscopy spectroscopic studies of Li surfaces. Surface films formed on Li in alkyl carbonate solutions of LiAsF6 are comprised of ROCO2Li, Li2CO3, LiF, LixAsFy, and Li oxides. XPS could also detect surface species with Li-C bonds (e.g,, LiCH2CH2OCO2Li). When EC is present, its reduction dominates the surface film formation. The presence of water suppresses both solvent and salt anion reduction and enriches the surface films with Li2CO3 (because of secondary reactions of water with surface species), LiOH, and Li2O. These studies also confirm that the surface films formed on Li have a multilayer structure.