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 × 10–6 S cm–1). To overcome these limitations of the low electrolyte stability and low ionic conductivity, we applied an interlayer electrolyte to build FIB rather than pure electrolytes. A thin layer of La0.9Ba0.1F2.9 was pressed together with a thick layer of BaSnF4. Applying low-conductive La0.9Ba0.1F2.9 as thin layer enhanced the total conductance of the pellet (compared to pure La0.9Ba0.1F2.9), while it physically isolated the less stable and highly conductive electrolyte (BaSnF4) from the anode. This approach allowed the demonstration of relatively high voltage FIBs at RT, which can otherwise not operate either with BaSnF4 electrolyte alone. We optimized the total ionic conductivity of the interlayer electrolyte by altering the thickness of the La0.9Ba0.1F2.9 layer. The total ionic conductivity of interlayer electrolyte was increased to 0.89 × 10–5 S cm–1 for 45 μm thick La0.9Ba0.1F2.9 at RT, which is more than 1 order of magnitude higher compared to the pure La0.9Ba0.1F2.9 (0.4 × 10–6 S cm–1). Finally, we demonstrate the feasibility of operating FIB at RT utilizing the interlayer pellet as an electrolyte, BiF3 as a cathode and Ce as an anode material. The approach described here would enable the design and development of new solid electrolytes with advanced properties with existing electrolytes.