Introducing Interlayer Electrolytes: Toward Room-Temperature High-Potential Solid-State Rechargeable Fluoride Ion Batteries

Solid-state fluoride ion batteries (FIBs) promise high specific energy, thermal stability, and safety. Research on FIBs is in its infancy, and a number of issues still need to be addressed to realize its full potential. Progress on FIB strongly depends on developing suitable fluoride-ion-transporting electrolytes at room temperature (RT). BaSnF4 shows high ionic conductivity of 3.5 × 10–4 S cm–1 at RT. However, it has limited electrochemical stability window. Recently, we demonstrated RT rechargeable FIB utilizing BaSnF4 as a solid electrolyte and low electropositive metals, such as Sn and Zn metals, as anodes because of the limited electrochemical stability of BaSnF4, which results in low operating voltages. However, to enable cells with high operating potentials, the electrolyte should be compatible with highly electropositive metals (e.g., La, Ce). Although tysonite-type La0.9Ba0.1F2.9 electrolyte was shown to be compatible with such metals, it has the drawback of low ionic conductivity at RT (0.4 × 10...