Solvation Structure and Derived Interphase Tuning for High-Voltage Ni-Rich Lithium Metal Batteries with High Safety Using Gem-Difluorinated Ionic Liquid Based Dual-Salt Electrolytes

Stabilizing electrolytes for high-voltage lithium metal batteries (LMBs) is crucial yet challenging, as they need to ensure stability against both Li anodes and high-voltage cathodes (above 4.5 V versus Li/Li⁺), addressing issues like poor cycling and thermal runaway. Herein, a novel gem-difluorinated skeleton of ionic liquid (IL) is designed and synthesized, and its non-flammable electrolytes successfully overcome aforementioned challenges. By creatively using dual salts, fluorinated ionic liquid and dimethyl carbonate as a co-solvent, the solvation structure of Li⁺ ions is efficiently controlled through electrostatic and weak interactions that are well unveiled and illuminated via nuclear magnetic resonance spectra. The as-prepared electrolytes exhibit high security avoiding thermal runaway and show excellent compatibility with high-voltage cathodes. Besides, the solvation structure derives a robust and stable F-rich interphase, resulting in high reversibility and Li-dendrite prevention. LiNi_0.6Co_0.2Mn_0.2O₂/Li LMBs (4.5 V) demonstrate excellent long-term stability with a high average Coulombic efficiency (CE) of at least 99.99 % and a good capacity retention of 90.4 % over 300 cycles, even can work at a higher voltage of 4.7 V. Furthermore, the ultrahigh Ni-rich LiNi_0.88Co_0.09Mn_0.03O₂/Li system also delivers excellent electrochemical performance, highlighting the significance of fluorinated IL-based electrolyte design and enhanced interphasial chemistry in improving battery performance.