First-Principles Calculations on Diffusion and Association Behavior of Fluoride Ions in Ba-Doped LaF3 Solid Electrolyte

Abstract

Fluoride ion batteries, which generate electrical energy through the fluorination/de-fluorination reaction of electrode active materials, are expected to be the next-generation storage batteries with high energy density. LaF3 with a tysonite-type structure is known as one of the representative solid-state fluoride ion conductors. Substitutional solid solution of divalent cations such as Ba or Sr ions at La sites forms fluoride ion vacancies as charge compensation, resulting in fluoride ion conductivity. The main objective of this study is to analyze the formation behavior of point defects in LaF3 through quantitative evaluation of their formation energies using first-principles calculations, and to elucidate the conduction mechanism of fluoride ions in LaF3.The plane-wave basis PAW method implemented in VASP code was used for the electronic structure calculations. The electron correlation treatment uses the PBEsol-type potential based on the generalized gradient approximation. Calculations for given point defects were performed. The cutoff energy of the plane wave basis sets was set to 420 eV, and the structure optimization calculations were performed until the Hellmann-Feynman force acting on each atom was less than 0.02 eV/Å.There are three fluoride ion sites (12g, 4d, and 2a) in the unit cell of LaF3. Using the supercell model, we calculated the defect formation energies of fluoride ion vacancies at each site. The formation energies at the 12g, 4d, and 2a sites are 0.17 eV, 0.52 eV, and 0.52 eV, respectively. According to the defect formation energies, fluoride ion vacancies are most likely to form at the 12g sites.To elucidate the elementary processes of fluoride ion transfer between 12g sites by the vacancy mechanism, we performed first-principles calculations and transition state search using the Nudged Elastic Band method. From symmetry, four different pathways exist in the lattice of undoped LaF3. The pathway with the highest energy barrier is shown to have very low transfer energies, 0.17 eV, and the energy barriers of the other pathways are less than 0.1 eV. These results suggest that LaF3 has a potential for very fast fluoride ionic conductivity by the vacancy mechanism. In the presentation, the mechanism of fluoride ion conduction in Ba-doped LaF3 will also be reported. This work was supported by the RISING2 (JPNP16001) and the RISING3 (JPNP21006) projects from the New Energy and Industrial Technology Development Organization (NEDO), Japan.