K/Sr Dual-Doping Enables Simultaneous Enhancement of Ionic Conductivity and Phase Stability for β-PbF2-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-Pb0.88Sr0.08K0.04F1.96 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 SrF2 and PbF2 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 Sr2+. Through reconfiguration of the local coordination environment of F–, the electrostatic interaction between F– and Pb2+ 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 PbF2-based solid electrolytes featuring both high ionic conductivity and exceptional phase stability.