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.