Ab Initio Study of the Defect Chemistry and Substitutional Strategies for Highly Conductive Li₃YX₆ (X = F, Cl, Br, and I) Electrolyte for the Application of Solid-State Batteries

The lithium halide Li3YX6 (X = F, Cl, Br, and I) is a promising solid electrolyte candidate because of its outstanding electrochemical stability and superior ionic conductivity. In this work, we apply density functional theory (DFT) to investigate the impact of halide ions and the partial substitution of Y with transition metals (e.g., Sr, Sc, La, and Zr) on the transportation, electrochemical stability, and defect chemistry of Li3YX6. Our result shows that trigonal Li3YCl6 has the lowest activation energy and the highest ionic conductivity in the Li3YX6 series. Such outstanding performance is attributed to the existence of a fast octahedral-to-octahedral Li diffusion pathway and the relatively weaker Coulombic attraction between Li ions and Cl ions. Moreover, we discuss the impact of partially substituting Y with a transition metal (e.g., Sr, Sc, La, and Zr) on the electrochemical stability and transport performance of Li3YCl6. The partial substitution of Y with a larger transition metal such as La reduces the activation energy by increasing the diffusion channel size. Furthermore, partially substituting Y with higher valence transition metal ions such as Zr effectively increases the concentration of vacancies by charge compensation, potentially improving Li conductivity.