Surface State Control of Lithium for Highly Smooth Electrodeposition

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Lithium metal which has a theoretically highest energy density, cannot be used in practical batteries, because of the dendritic morphology of lithium deposits. In this study, a highly smooth deposition of lithium was realized by a novel surface state control of lithium.(1)When HF or (C_2H_5)_4NF(HF)_4 was added into nonaqueous electrolytes, the dendrite formation was successfully suppressed.(2)In this case, the surface film consisted of a tight bi-layr structure of LiF/Li_2O.(3)The dynamic observation for the surface film formation was carried out by using in situ FTIR measurement.(4)The effect of HF depended on the electrodeposition current and the concentration of HF.(5)The surface film formed during the first deposition was destroyed during the first dissolution process, leading to a direct attachment of electrolyte with lithium metal. The new surface film was formed at the destroyed part during the second charge process.(6)The destroyed surface film was accumulated during discharge and charge cycles. This surface film was so porous that its resistance was very low.(7)However, this surface film prevented a adequate supply of HF to form a tight bi-layr structure of LiF/Li_2O,leading to the formation of lithium dendrite.(8)The nano-distribution of surface potential was measured by using Atomic Force Microscopy and Surface Potential Detection Method. The LiF/Li_2O surface film was highly uniform.