Fluoride-Ion Migration Mechanism in Fluoride Battery Electrolyte Material Cs 2 RbBiF 6 with Double-Perovskite Structure

Recently, fluoride-ion batteries have been extensively studied for next-generation rechargeable batteries. In this battery system, fluoride-ion electrolyte material with high fluoride-ion conduction is strongly desired. A limited number of the fluoride-ion electrolyte material were reported. Most of them have Fluorite structure or Tysonite structure and their related crystal structures. If we have new fluoride-ion electrolyte material in another crystal structure, we can expand our choice of material. In this study, we carried out theoretical studies using first-principles static and dynamic calculations for fluoride battery electrolyte material Cs 2 RbBiF 6 with double-perovskite structure. first-principles static and dynamic calculations in this study were performed by projector augmented wave (PAW) methods based on density functional theory (DFT). The generalized gradient approximations parameterized for the solid system (GGA-PBE_sol) were used. Our calculations revealed this material, Cs 2 RbBiF 6 consists of strong covalent bonding BiF 6 octahedron and ionic Cs-F bonding. Phonon calculations showed instability of BiF 6 octahedron rotations. This indicates that BiF 6 octahedron rotations can occur in elevated temperature regions. The fluoride-ion conduction behavior was studied by first-principles molecular dynamics calculations. The fluoride-ion conduction trajectory recorded at 600 K is shown in Fig. 1. In this figure, fluoride-ion mainly migrates around the BiF 6 octahedron. On the other hand, fluoride-ion mainly migrations between BiF 6 octahedrons were not so often observed. Migration energies were determined using the nudged elastic band (NEB) method and were evaluated to be 0.089 eV for the path within the BiF 6 octahedron and 0.212 eV for the path between BiF 6 octahedron. Our calculation results indicate the potential of high fluoride-ion conduction of these materials. Acknowledgments: This work was supported by the RISING3 (JPNP21006) projects from the New Energy and Industrial Technology Development Organization (NEDO), Japan. Figure 1