Structure design of a BiOF solid electrolyte with remarkably outstanding fluoride ion diffusion performance induced by Ga doping

Abstract

BiOF is considered as a potential excellent solid electrolyte for fluoride ion batteries due to its special layered structure. However, its ionic conductivity is far from meeting the practical standards. Herein, we have proposed optimal structure regulation at the atomic scale to achieve the rapid promotion of its fluoride ion conductivity from the Ga doping effect. After a chemical potential search and calculations of formation energies, it is noted that Bi_1−xGa_xOF shows thermodynamic stability with the Ga concentration (x) varying from 0 to 0.25. Furthermore, Bi_0.875Ga_0.125OF can not only maintain excellent electrical insulation performance but also has the highest fluoride ion conductivity (1.63 × 10⁻³ S cm⁻¹ at 300 K), far exceeding that of BiOF (2.76 × 10⁻⁷ S cm⁻¹ at 300 K). This is attributed to the optimal Ga regulation effect on the diffusion channel for the fluoride ion, weakening the strong constraining effect between fluoride ions and Bi–Bi layers. Besides, Bi_0.875Ga_0.125OF has the best ductility, and it can maintain a wide stable voltage platform (1.34–5.52 V), when Ga is selected as the anode material. Therefore, our results provide a novel idea and approach for developing BiOF solid-state electrolytes with superior ionic conductivity.