Polymer Electrolyte Based All-Solid-State Rechargeable Fluoride Ion Batteries

Rechargeable fluoride ion batteries (FIBs) are one of the most promising energy storage candidates in view of high energy density and low cost. The development of highly F-conductive, safe, and flexible electrolytes is the central task for the construction of high-performance FIBs. Hereby, this work first proposes a polyvinyl alcohol (PVA)-borax-glycerol (PBG) polymer electrolyte. The F⁻ transport along one PVA chain is realized by the interaction between F⁻ and -OH on the PVA chain and the motion of PVA chain would facilitate the migration of F⁻. The B(OH)₄⁻ dissociated from borax can be used as a cross-linking agent, and react with the hydroxyl groups on PVA by a dehydration process to form a polymer with a 3D cross-linked structure. The optimized ionic conductivity (as high as 2.82 × 10⁻⁴ S cm⁻¹ at 30 °C and 1.08 × 10⁻³ S cm⁻¹ at 60 °C) of PBG can be obtained. The flat and soft surface of PBG electrolytes can significantly reduce the activation energy for the interfacial transport process. Benefitting from the high ionic conductivity and easier interfacial transport, the PBG electrolyte makes the all-solid-state FIBs enable reversible cycling at a high current density of 125 mA g⁻¹.