Quantifying Anion Selectivity in SEI Formation for Ether-Based Electrolytes in Lithium Metal Batteries

While current electrolyte design strategies for lithium metal batteries (LMBs) emphasize the importance of an anion-dominated Li+ solvation structure to promote an inorganic-rich solid electrolyte interphase (SEI), solvation structure-SEI composition relationships remain challenging to quantify. Here, we systematically investigate a series of ether/lithium bis(fluorosulfonyl) imide (LiFSI) electrolytes and quantify the lithium fluoride formed from FSI– decomposition using nuclear magnetic resonance spectroscopy. We find that both an anion-dominated electrolyte solvation structure and a reductively stable solvent are necessary to achieve anion-selective SEI formation and establish a framework for anion selectivity in ether-LiFSI electrolytes, guided by density functional theory-derived solvent descriptors, which aligns with existing examples of ether-based Li metal electrolytes. Using this model, we identify an electrolyte formulation of tert-butyl methyl ether and tert-amyl methyl ether that achieves a Coulombic efficiency of 99.3%. Ultimately, this study establishes key electrolyte solvent-interphase-performance relationships to enable high-performance LMBs.