Solid-electrolyte interphase formation during Li metal deposition in LiN(SO2F)2-based solvate ionic liquids

Solvate ionic liquids (SILs) are promising electrolytes for Li metal batteries. In this study, Li plating-stripping reactions in SILs were investigated using electrochemical quartz crystal microbalance measurements and X-ray photoelectron spectroscopy (XPS). SILs were prepared by mixing Li salt and glyme (triglyme or tetraglyme) in a 1:1 molar ratio. During the Li plating-stripping reaction, a reversible mass change was observed in LiN(SO2F)(2) (LiFSA) based SILs. In contrast, a considerably higher mass change than the theoretical value calculated from the Faraday current was observed in the LiN(SO2CF3)(2) (LiTFSA) based SILs, owing to the accumulation of decomposition products of electrolytes on the electrode surface. XPS depth profiling for the deposited Li suggested the formation of thin solid-electrolyte interphase (SEI) in the LiFSA-based SILs while considerably thicker SEI was formed in the LiTFSA-based SILs. The SEI formed in LiFSA-based SILs effectively suppressed the decomposition of electrolytes. Thus, LiFSA-based SILs are favorable for achieving highly reversible charge-discharge of Li electrodes in Li metal batteries.