Spontaneously formation of SEI layers on lithium metal from LiFSI/DME and LiTFSI/DME electrolytes

Lithium is a highly reactive metal that can react with most of the organic electrolytes. Here, we report the spontaneous formation of solid electrolyte interphase (s-SEI) on the surface of lithium metal. We focused on investigating the differences in the properties of s-SEI layers, formed in the electrolyte 1M LiFSI (lithium bisfluorosulfonimide)/DME (dimethyl ether) or 1M LiTFSI [lithium bis((trifluoromethyl)sulfonyl)azanide]/DME with the extension of soaking time. Morphology, structural peculiarities and mechanical properties of the s-SEI were investigated by AFM. At the same time, in-depth XPS profile analysis revealed that the s-SEI layers were similar to the electrochemically reduced SEI layers, mainly composed of inorganic components, such as LiF, Li3N, organic components of alkyl lithium carbonate (ROCO2Li) and lithium alkylates (ROLi). Cu/Li and Li/Li symmetric cells showed that the thickened s-SEI layers could increase the polarization potential of plating/stripping, while Li3N riched s-SEI has the effect of reducing polarization potential.