K/Sr Dual-Doping Enables Simultaneous Enhancement of Ionic Conductivity and Phase Stability for β-PbF ₂ -Based Solid-State Fluoride-Ion Electrolyte

All-solid-state fluoride-ion batteries (ASSFIBs) integrate high theoretical energy density, high safety characteristics, and low manufacturing costs, making them a promising candidate for next-generation energy storage technologies. However, the intrinsic shortcomings of high-performance fluoride-ion solid electrolytes severely limit their practical application. Herein, a potassium/strontium dual-doping strategy is designed to synthesize the fluoride-ion conductor β-Pb 0.88 Sr 0.08 K 0.04 F 1.96 with a fluorite structure via a straightforward solid-state reaction method. It has been found that KF doping can introduce abundant fluorine vacancy defects to effectively enhance vacancy concentration, while SrF 2 and PbF 2 can form a continuous solid solution that not only disperses fluorine vacancies generated by K + doping and suppresses defect aggregation to significantly enhance phase stability, but also induces moderate lattice distortion through the incorporated Sr 2+ . Through reconfiguration of the local coordination environment of F –, the electrostatic interaction between F – and Pb 2+ can be weakened, thereby reducing the ionic migration energy barrier. Owing to the higher vacancy concentration and lower migration barrier, the as-prepared material exhibits a high ionic conductivity of 4.77 × 10 –4 S·cm –1 at room temperature, exceeding most reported fluoride ion conductors. ASSFIBs assembled with the fluoride ion conductor as the solid electrolyte present a discharge specific capacity of approximately 132.5 mAh·g –1 over 400 cycles and a capacity retention rate of 97.4%. Therefore, this work offers unique insights into the rational design of PbF 2 -based solid electrolytes featuring both high ionic conductivity and exceptional phase stability.