Regulation of Solvation and Interfacial Chemistry via Monofluorinated Cations Enables High‐Voltage and Safe Lithium Metal Batteries

The fluorine-rich electrode electrolyte interphase, chemically sourced from fluorinated anions and solvents, plays a pivotal role in improving the cycling stability of lithium metal batteries (LMBs) equipped with Ni-rich cathodes. To prestore fluorine source on cations, here a novel monofluorinated cationic skeleton has been designed and synthesized. Its role is first investigated in the regulation of solvation structure and evolution in both bulk and interface regions. The monofluorinated cation can compete with lithium ions for coordinating electrolyte molecules, which improves the oxidative stability of solvents on the cathode surface and prevents the undesirable transition from the anion-rich to anion-deficient structure at the anode interface induced by the interfacial electric field. By leveraging this ionic liquid architecture carrying fluorine in both cation and anion, A localized moderate-concentration ionic liquid electrolyte (LMCILE) is developed that exhibits exceptional compatibility with lithium metal anodes and superior safety characteristics. LiNi_0.8Co_0.1Mn_0.1O₂|LMCILE|Li (4.5 V) cells display excellent cycle stability with a good capacity retention of 82.9% over 950 cycles. The Ni-rich LiNi_0.9Co_0.05Mn_0.05O₂|LMCILE|Li (4.5 V) system also delivers good electrochemical performance with high capacity retention of 91.4% after 300 cycles and 90.3% after 200 cycles, even at 60 °C.