Solid Electrolyte Interphase Formation Reactions on Lithium Metal with Ether-Based Electrolytes

Lithium metal batteries offer exceptional theoretical energy density but suffer from safety issues caused by electrolyte degradation and dendrite formation. Formation of a stable solid electrolyte interphase (SEI) is crucial for mitigating these problems. However, the low abundance of SEI species complicates quantitative analysis. This study compares static SEI formation on lithium foil with a dynamic shaking method that continuously abrades and regenerates the SEI, enriching electrolyte-soluble and gaseous SEI products in 1,2-dimethoxyethane (DME)- based lithium bis(fluorosulfonyl)imide (LiFSI) systems. The dynamic approach facilitates quantitative analysis of SEI-derived species that remain undetectable under static conditions. A multimethod investigation combining gas-, liquid-, and solid-phase analyses in this work establishes a mechanistic framework linking the formation of methane, ethane, ethylene, and electrolyte-soluble DME/LiFSI adducts to interfacial lithium−electrolyte reactions. Gas evolution in pure DME is nearly tenfold higher than in 1 M LiFSI in DME (LCE), and increasing LiFSI concentration to 4 M (HCE) reduces methane generation by a factor of five, indicating enhanced SEI stability. Only trace gas formation is observed in LiFSI:DME:TTE (1:1.2:3 n/n, LHCE), demonstrating even stronger SEI stabilization. Quantification of Li2S via H2S release confirms its relevance as an SEI constituent, implicating the co-formation of Li2O and Li3N. Elevated LiFSI concentrations yield higher inorganic SEI content, yet similar LiFSI concentrations in LCE and LHCE electrolytes produce different gas amounts, suggesting additional stabilization pathways. Decomposition of TTE is proposed to generate LiF-rich inorganic phases that strengthen the SEI and suppress gas evolution. These findings provide new mechanistic insights into electrolyte-dependent SEI chemistry and highlight the critical role of fluorine-containing species in achieving safer, more stable lithium metal batteries.