Atomic-scale understanding of fast Li ion diffusion in amorphous nLiCl-GaF3 solid electrolytes

Despite the distinct advantages of a wide electrochemical window and high moisture stability, fluoride solid electrolytes have limitations in practical applications because of their very low ionic conductivity. This ab initio study investigated the dynamic behavior of Li ions in amorphous fluoride-based n LiCl-GaF 3 solid electrolytes ( n = 2 and 3) that exhibit exceptionally high conductivity of over 1 mS cm −1 . The cation−anion bond strength in n LiCl-GaF 3 increases in the order of Li−Cl < Li−F < Ga−Cl < Ga−F bonds. Ga ions flexibly adjust their charge state depending on the number of surrounding F and Cl ions, leading to the formation of various Ga-centered polyhedra, including Ga@FCl 3 , Ga@F 2 Cl 2 , Ga@F 3 Cl 2 , and Ga@F 4 Cl. These polyhedra are structurally robust because of strong Ga–anion bonds, allowing Li ions to diffuse relatively freely in the space between them. This results in high conductivities of 4.82 and 4.76 mS cm −1 when n = 2 and 3, respectively. The diffusion of Li ions proceeds through repeated vibrational and translational motions in F-rich and Cl-rich local environments, respectively. This study provides atomic-level insights into how Li ions can rapidly diffuse in fluoride-based solid electrolytes capable of strongly capturing Li ions.