Engineering Anion-Solvent Interactions to Modulate Ion Pairing and Accelerate Desolvation in High-Voltage Lithium-Metal Batteries.

The development of high-energy-density lithium-metal batteries (LMBs) is critically hindered by sluggish Li+ desolvation kinetics and unstable interfacial chemistry at both the lithium-metal anodes (LMAs) and high-voltage cathodes. Herein, we propose an electrolyte design strategy using synthesized fluorinated ether solvents─3-ethoxy-1,1,2,2,2-pentafluoropropane (E5FP, -CF3 terminus) and 3-ethoxy-1,1,2,2-tetrafluoropropane (E4FP, -CHF2 terminus)─characterized by weak Li+ interactions, to investigate the role of anion-solvent interactions beyond the conventional Li+-solvent/anion binary coordination. Spectroscopic analysis, molecular dynamics simulations, and density functional theory calculations reveal that the partially positive hydrogen atom in the -CHF2 group of E4FP enhances dipole-ion interactions with anions, thereby weakening Li+-anion coordination within the solvation sheath and promoting contact ion pair (CIP)-favored solvation structure. This CIP-favored configuration significantly reduces Li+ desolvation energy and improves ionic conductivity, Li+ transference number, and interfacial charge transfer kinetics. Consequently, the CIP-favored solvation environment facilitates the formation of robust and uniform solid electrolyte interphase/cathode electrolyte interphase layers, enables uniform lithium deposition, and suppresses parasitic side reactions, transition metal dissolution, and structural degradation of Ni-rich Li[Ni0.83Co0.11Mn0.06]O2 (NCM83) cathodes. Overall, this study highlights anion-solvent interactions as an important design parameter influencing solvation structure evolution and identifies CIP-favored solvation as an effective pathway toward stable high-voltage LMBs.