Morphology and chemical compositions of surface films of lithium deposited on a Ni substrate in nonaqueous electrolytes

Abstract

The chemical compositions of surface films of lithium deposited on a Ni substrate in γ-butyrolactone (γ-BL) or tetrahydrofuran (THF) containing 1.0 mol dm−3 LiClO4, LiAsF6, LiBF4, or LiPF6 were analyzed by X-ray photoelectron spectroscopy. The morphology of lithium deposited on these electrolytes was examined with a scanning electron microscope. The relationship between the lithium surface film formed during electrochemical deposition in γ-BL electrolytes and the morphology of the lithium were deduced. Dendrite formation was suppressed when LiPF6 + γ-BL, which includes a small amount of HF as the decomposition product of PF−6 ions, was used as the electrolyte. A similar surface film was obtained when a small amount of HF was added to LiClO4 + γ-BL. This suggests that lithium dendrite formation is suppressed in the presence of a small amount of HF which may provide a thin compact surface film. Suppression of lithium dendrites was also observed when LiAsF6 + THF was used as the electrolyte. However, it did not suppress dendrite formation completely.