Beyond Electrostatic Interactions: Ring Strain-Driven Weakening of Solvent-Li+ Binding Enables Facile FSI Anion Reduction and Inorganic-Rich SEI Formation in Fluorinated Ether Electrolytes

Fluorinated ether solvents are widely recognized as effective materials for enhancing the performance of Li metal batteries. In particular, 2,2,3,3-tetrafluoro-1,4-dimethoxybutane (FDMB) has attracted significant attention for its dramatic improvement of the electrochemical stability of Li metal anodes. FDMB functions by effectively regulating the Li⁺ solvation sheath, increasing the contribution of bis(fluorosulfonyl)imide (FSI) anions to the solvation sheath, and promoting the formation of a stable anion-derived solid−electrolyte interphase (SEI). However, the molecular-level mechanism for why FDMB solvents exhibit superior performance has not yet been elucidated. Herein, first-principles calculations reveal that the incorporation of fluorine into the solvent causes a large repulsion in the seven-membered ring structure formed by FDMB−Li⁺ coordination, leading to a significant reduction in Li⁺ solvation ability, which in turn enhances the contribution of FSI anions to the solvation sheath. The FSI moiety is also identified as the reduction site within the Li⁺ solvation sheath, and its chemical reactivity is highest in the FDMB-containing solvation structure. The reductive decomposition of the FSI moiety via S−F and S−N bond cleavages is thermodynamically and kinetically most favorable in the Li⁺ solvation sheath incorporating FDMB, suggesting that the FDMB-based electrolyte significantly promotes FSI decomposition and facilitates the formation of a stable inorganic-rich SEI.