Intrinsic effects of electrolytes on lithium metal deposition and dissolution investigated through a separator-free cell†

Lithium metal batteries are a significant promise for next-generation energy storage due to their high energy density. However, challenges persist in their commercialization stemming from issues during the lithium deposition/dissolution processes, such as low Coulombic efficiency, dendrite formation, and dead-lithium formation. Addressing these challenges requires careful electrolyte design to enhance the reversibility of the lithium metal negative electrode by modifying solvation structures and engineering interfaces. The Coulombic efficiency of lithium deposition/dissolution is one of the most crucial factors in evaluating the performance of electrolytes toward lithium metal, although this is influenced by various factors. In this study, a separator-free cell is adopted to minimize extraneous influences and focus on assessing the intrinsic effects of electrolytes on lithium deposition/dissolution. 48 different electrolytes based on three salts of Li[PF₆], Li[FSA] and Li[TFSA] varying in solvents were investigated with or without additives. Moreover, Raman spectroscopy and X-ray photon spectroscopy enhance the discussion by revealing variations in the major species of solid electrolyte interphase components under different electrolyte conditions.