Study of the Surface Composition of Highly Smooth Lithium Deposited in Various Carbonate Electrolytes Containing HF

We investigated the physical state of the surface film on lithium electrodeposited in various carbonate electrolytes containing HF in order to clarify the interfacial reaction and the electrodeposition process of lithium in nonaqueous electrolyte under a presence of a small amount of HF. The morphology and surface composition of lithium were analyzed with a scanning electron microscope and X-ray photoelectron spectroscopy (XPS). Propylene carbonate, ethylene carbonate, dimethyl carbonate, and diethyl carbonate were selected as solvents, and LiCF3SO3, LiBF4, and LiClO4 were selected as electrolyte salts. A small amount of aqueous hydrogen fluoride solution was used as a source of HF. The lithium electrodeposited in all electrolytes without HF was in a dendritic shape. On the other hand, the morphology of the lithium deposited in all electrolytes containing HF was highly smooth with a hemispherical shape. The surface composition and morphology of electrodeposited-lithium were strongly affected by the presence of HF in carbonate electrolytes rather than by the solvents or salts. Various basic lithium compounds in the surface film were converted to LiF through acid−base reactions with HF. These results suggested that the surface film on lithium and morphology of lithium seemed to be more sensitive to a minor component, HF, in carbonate electrolytes. Moreover, the XPS analysis revealed that lithium electrodeposited in carbonate electrolytes containing HF was covered with a highly stable and ultrathin (20−50 Å) film consisting of a LiF/Li2O bilayer. From these results, it can be concluded that such a stable thin surface film promotes the highly smooth deposition of lithium.