Dual-Interphase Modulation with a Locally Concentrated Ionic Liquid Electrolyte toward High-Performance Lithium Metal Batteries

The construction of stable electrode/electrolyte interphases (EEIs) is crucial for the development of practical lithium metal batteries (LMBs). In this work, we designed locally concentrated ionic liquid electrolytes (signed as FPB2) based on 1,3,5-trifluorobenzene (3FB) diluent to address this challenge. The appropriate addition of 3FB encourages Li⁺ association with anions, creating a significant amount of anion-dominated aggregate-II solvation structure, which enables profound anion decomposition and rapid Li⁺ transport. In parallel, the low lowest unoccupied molecular orbital of 3FB allows for its own reductive decomposition on the lithium metal anode (LMA) surface. Benefiting from the synergistic decomposition of anions and 3FB, a uniform and stable solid electrolyte interphase (SEI) is formed on the LMA. Additionally, a thin cathode electrolyte interphase (CEI) abundant in LiF is generated on the LiFePO₄ (LFP) cathode, resulting in the effective suppression of electrolyte oxidation. This dual-interphase stabilization endows impressive cycling performance in a Li/LFP full battery under demanding conditions (10.8 mg·cm^–2 LFP, 50 μm Li) to achieve 81.5% capacity retention and 99.4% average Coulombic efficiency over 200 cycles. This work presents a new paradigm for electrolyte design in high-performance LMBs by exploiting a functional diluent to proactively stabilize dual EEIs.