Enhancing Ionic Transport and Interfacial Stability in Lithium Metal Batteries via Dual-Anion Ionic Liquid Electrolytes

Lithium metal batteries (LMBs) are promising candidates for high-energy-density storage, but their practical use is limited by unstable Li deposition, continuous electrolyte consumption, and safety concerns. Ionic liquid electrolytes offer intrinsic thermal and electrochemical stability; however, conventional systems suffer from low ionic conductivity and poor wettability, which increase interfacial resistance. Here, we present a dual-anion locally concentrated ionic liquid electrolyte (D-LCILE) that combines FSI⁻ and TFSI⁻ anions with a fluorinated diluent to improve ion transport and Li-metal interfacial stability. Raman spectroscopy and molecular dynamics simulations reveal that the dual-anion/diluent design promotes aggregate-rich solvation structures and enhances Li-ion mobility. D-LCILE exhibits improved separator wettability, high ionic conductivity of 1.053 mS cm⁻¹, and a high Li⁺ transference number of 0.879. Interfacial analyses using SEM, XPS depth profiling, and ToF-SIMS demonstrate that D-LCILE induces dense and uniform Li deposition and forms a fluorine-rich inorganic solid electrolyte interphase. These results confirm increased LiF content and a more stable SEI, while ab initio molecular dynamics indicates that FSI⁻ preferentially contributes to LiF formation and TFSI⁻ improves interfacial stability through its higher decomposition resistance. In thin-Li symmetric cells, D-LCILE suppresses short-circuit behavior and remains stable at high current density, with a critical current density up to 5 mA cm⁻². Furthermore, 20 μm Li||LiFePO₄ full cells using D-LCILE deliver stable cycling over 200 cycles at 1C with an average Coulombic efficiency above 99.90% and 99.93% capacity retention. This work demonstrates that dual-anion solvation engineering is an effective strategy for overcoming the conductivity–stability trade-off in ionic liquid electrolytes and advancing safe, long-life LMBs.