X-ray Photoelectron Spectroscopy Studies of Lithium Surfaces Prepared in Several Important Electrolyte Solutions. A Comparison with Previous Studies by Fourier Transform Infrared Spectroscopy

Abstract

Li electrodes prepared in situ in solutions and then stored in them for different periods were studied by X-ray photoelectron spectroscopy (XPS) including depth profiling performed by argon sputtering followed by XPS. A set of solvents, propylene carbonate (PC), ethylene carbonate (EC)−dimethyl carbonate (DMC) mixtures, and 1,3-dioxolane, and a set of salts, LiAsF6, LiBF4, LiPF6, LiN(SO2CF3)2, and LiC(SO2CF3)3, were investigated with respect to the effect of storage time. The results of this study were compared with previous studies of Li electrodes in the same solutions by in situ and ex situ Fourier transform infrared spectroscopy. Basically, the results thus obtained are in line with the previous studies. The Li surface chemistry is dominated by solvent reactions. However, all the above salt anions are also reduced to form insoluble species which also contribute to the build-up of the surface films (e.g., the salt anions of the type MFy- (M = As, P, B) are reduced to LiF and species of the LixMFz type). The surface reactions of these solvents and the salts on Li are discussed in detail. Depth profiling of the surface films formed on Li in solutions indicates that they have a multilayer structure. The concentration of the organic salts in the surface layers decreases as the layer is closer to the Li−film interface.