A Fluorine and Lithium Donation Strategy to Enable the Heterostructure and Defect-Rich Fluoride Solid Electrolyte with High Conductivity and Air Stability for Solid-State Batteries

Halide-based solid-state electrolytes (SSEs) have been considered to be promising for solid-state lithium metal batteries (SSLMBs) due to their favorable scalability. Currently, the poor humid air stability and incompatibility with the lithium anode are two major suspensive issues. Fluoride SSEs are promising to solve these problems, in view of their exceptional chemical and electrochemical stabilities. Nevertheless, they still face the dilemma of inadequate room-temperature ionic conductivity. Herein, we propose a fluorine and lithium donation strategy to enable the heterostructure and defect-rich fluoride SSE with in situ-constructed bulk-phase and interfacial dual fast Li⁺ diffusion pathways. It is synthesized via in situ fluorination of LaCl₃ by the ionic conductor Li₃GaF₆, which acts as a fluorine and lithium donor, and the defective Li₃GaF₆ enriched in lithium vacancies, Cl doping, and/or fluorine vacancies is formed. The resultant Li₃GaF₆-LaF₃ heterostructure SSE exhibits a high ionic conductivity of 2.24 × 10^-4 S/cm at 30 °C and outstanding humidity tolerance. The interfacial incompatibility between the halide SSE and lithium metal is solved by introducing a biphenyl-complexed Li interlayer. The symmetric cell exhibits a long lifespan of over 1000 h at 0.1 mA/cm². The LiFePO₄-based SSLMBs demonstrate a high reversible capacity of ∼150 mAh/g with a high capacity retention of 93.6% after 100 cycles. This work provides a versatile defect, doping, and second-phase modulation pathway for designing inorganic ion conductors with high conductivity and air stability.