Solvation-Equilibrium Locking Enables Ultrawide-Temperature Operation of High-Voltage Lithium-Metal Batteries.

High-energy lithium metal batteries are promising for space, polar, deep-sea, and other extreme environments, but their operation is constrained by temperature-sensitive ion transport, Li+ solvation, and interfacial chemistry. A solvation-equilibrium-locked electrolyte (SELE) is reported by combining dimethyl sulfite (DMS), a low-freezing weakly solvating solvent, with Pyr13DFOB (1-methyl-1-propylpyrrolidinium difluoro(oxalato)borate) as an ionic-liquid co-solvent. Cooperative ion-dipole and weak C-H⋯O interactions between DMS and Pyr13 + regulate the free-solvent reservoir, while TFSI- (bis(trifluoromethanesulfonyl)imide) and DFOB- enrich the primary Li+ coordination shell. Variable-temperature 7Li NMR, Raman, and molecular dynamics simulations show that solvent-separated ion pairs, contact ion pairs, and aggregates remain nearly invariant from -60°C to 60°C, unlike the strongly temperature-responsive Pyr13DFOB-free electrolyte. This locked anion-rich solvation structure reduces solvent coordination around Li+, mitigates solvent-derived interfacial decomposition and gas evolution, and promotes LiF- and borate-rich interphases on Li metal and NMC811. Consequently, Li||NMC811 cells deliver discharge capacity at -85°C and operate up to 120°C, while pouch cells remain functional after cooling to -122.9°C. A 6.6 Ah Li||Ni90 pouch cell further achieves 565 Wh kg-1 under lean-electrolyte conditions, establishing solvation-equilibrium locking as a molecular design principle for lithium metal batteries operating over a broad temperature range.