An Intermediate Solvation Regime to Enable Efficient Quasi-Solid-State Conversion in Li–S Batteries

Lithium–sulfur (Li–S) batteries are considered next-generation energy storage systems owing to their ultrahigh theoretical energy density and low materials cost. However, practical deployment is impeded by polysulfide shuttle, sluggish redox kinetics, and high polarization, particularly under lean electrolyte conditions. Quasi-solid-state conversion (QSSC) electrolytes suppress polysulfide dissolution but often exhibit poor ionic transport and low-rate capability. Here, we report a solvation-structure–engineered weakly solvating electrolyte (WSE) based on tetraglyme (G4) diluted with 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether (TFETFE). By systematically comparing G2 vs G4 glymes and TFETFE vs 1-(1,1,2,2-tetrafluoroethoxy) propane (TFEPE), we show that solvent chain length and diluent choice critically tune the Li+ solvation shell. Spectroscopic, impedance, and molecular dynamics results reveal that G4–TFETFE establishes a partially localized first solvation shell enriched with uncoordinated oxygen while maintaining a delocalized secondary shell. This balance minimizes soluble polysulfide formation while retaining sufficient mobility for fast redox kinetics. Consequently, G4–TFETFE cells exhibit ultra-low polarization, excellent rate performance, and stable long-term cycling (74% retention after 300 cycles, average CE ≈ 99.3%). In contrast, G2-based systems or TFEPE-containing electrolytes suffer from sluggish kinetics and rapid fading. This work establishes solvation structure optimization as a powerful design principle for WSEs, bridging fundamental ion–solvent interactions with practical Li–S battery performance.