XPS ANALYSIS OF LITHIUM SURFACES FOLLOWING IMMERSION IN VARIOUS SOLVENTS CONTAINING LIBF4

The surface film on lithium immersed in various electrolytes was analyzed by x-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy, and the potential sweep method. The surface film formed on lithium immersed in propylene carbonate or gamma-butyrolactone containing 1.0 mol dm(-3) LiBF4(LiBF4/PC or LiBF4/gamma-BL) for 3 days consists of LiF and a small amount of organic compounds. On the other hand, the surface film on lithium immersed in tetrahydrofuran (THF) containing 1.0 mol dm(-3) LiBF4(LiBF4/THF) consists of a large amount, of organic compounds and LiF. LiF and organic compounds are formed by the chemical reaction of LiOH, Li2CO3, and Li2O with HF involved in the electrolyte plus the direct reaction of the solvent with the lithium metal, respectively. The amount of organic compounds produced was influenced by the kind of solvent. For the formation of organic compounds, solvents have to permeate the LiF layer on the lithium. Probably, the permeability of the solvent is related to the formation of organic compounds. The permeability of the electrolyte is estimated quantitatively from the surface tension and viscosity. The surface tension and viscosity were obtained using the capillary rise method and Ostwald's viscometer, respectively. The surface tension and viscosity of LiBF4/THF were much smaller than those of LiBF4/PC or LiBF4/gamma-BL. This indicates that THF is more permeable than PC and gamma-BL. THF may easily reach the lithium metal surface to form a large amount of organic compounds. From these results, it can be concluded that the surface reaction of the lithium dose not only depends on the chemical properties of the lithium surface and electrolyte, but also on the physicochemical properties of the electrolyte.