Highly Conductive Doped Fluoride Solid Electrolytes with Solidified Ionic Liquid to Enable Reversible FeF₃ Conversion Solid State Batteries

Abstract Solid‐state lithium metal batteries based on fluorinated solid electrolytes have attracted attention due to their safety and stability advantages. However, the fluoride solid electrolytes face the problem of relatively low Li‐ion conductivity due to the lacking of suitable structural prototypes and their corresponding modulation modes. In this work, a chlorine‐doped fluoride solid electrolyte Li₃AlF_5.87Cl_0.13 is developed with high ionic conductivity by thermal fluorination and weak chlorination of mixed ionic liquids. The Cl anion is doped into the cryolite‐like open framework structure, and the electrolyte particle boundaries are decorated by solidified thin‐layer (≈2 nm) ionic liquid. Both the bulk and interface modifications enable the improvement of Li‐ion conductivity to 2 × 10⁻⁴ S cm⁻¹ at 30 °C, which is the highest level among fluoride solid electrolytes. The Li₃AlF_5.87Cl_0.13 with residual solidified ionic liquid shows the excellent stability of interface contact with Li metal, and the corresponding Li symmetric cell exhibits a small overvoltage (≈100 mV) with outstanding cycle life (at least 500 h). For the first time, a conversion‐type solid‐state Li/FeF₃ battery is developed based on this fluoride electrolyte, delivering a reversible capacity as high as 430 mAh g⁻¹. The existence of natural F‐rich interface promotes the conversion reaction reversibility of FeF₃ cathode.