First-Principles Calculations of Fluoride-Ion Migration in Fluorine-Ion Battery Electrolyte Material K 2 BiF 5

Fluoride-Ion migration in K 2 BiF 5 was studied using first-principles static and dynamic calculations. Due to presence of edge sharing BiF 7 chain along b axis in this structure, one dimensional Fluoride-Ion migrations alongBiF7 chain strongly suggested. Our calculation revealed not only this Fluoride-Ion migrations along BiF 7 chain, but also F ion migrations between BiF7 chain were observed. First-principles calculations of the electronic density of states indicate that K 2 BiF 5 has a large band gap of about 4.8 eV, making it suitable as an ionic conductor. The Fluoride-Ion conduction in the b-axis direction was strongly suggested by the presence of BiF 7 chains sharing a ridge in the b-axis direction in the structure of this material, but the F-ion conduction path analysis by first-principles molecular dynamics shown in Fig. 1 indicates that the F-ions are not only in the b-axis direction and in BiF 7 chains, but are also active betweenBiF 7 The active Fluoride-Ion are determined by the elastic band method. The activation energy determined by the elastic band method is 0.19 eV within a BiF 7 chain, whereas it is 0.25 eV between BiF 7 chains, indicating that F ions can conduct with the same activation energy as within a BiF 7 chain. Acknowledgments This work was supported by the RISING3 (JPNP21006) projects from the New Energy and Industrial Technology Development Organization (NEDO), Japan. Figure 1