Theoretical Insights on the Comparison of Li-Ion Conductivity in Halide Superionic Conductors Li3MCl6, Li2M2/3Cl4, and LiMCl4 (M = Y, Sc, Al, and Sm)

Recently, halide materials for solid electrolytes have received modest research interest. A variety of new halide electrolytes, such as Li3YCl6, Li3InCl6, and Li2Sc2/3Cl4 halospinel structure, and LiAlCl4, have been experimentally prepared with high Li-ionic conductivities close to 10–3 S/cm and low activation energies. Although much effort (experimental and theoretical) has been devoted to uncovering the best combination of Li–M–X, less attention has been paid to the structural effects on the ionic conductivities and electrochemical stabilities. In this article, DFT and AIMD simulations are performed to do a comparative study on several halide electrolytes with selected structures, including the most common rock-salt Li3MCl6, spinel Li2M2/3Cl4, and LiMCl4. It is revealed that halospinel Li2M2/3Cl4 structures with cubic symmetry are three-dimensionally conducting and mechanically stable superionic conductors. They exhibit an excellent Li-ionic conductivity within the range 0.26–19.0 mS/cm with an activation energy lower than 0.20 eV (0.342–0.195 eV). Among them, Li2Sm2/3Cl4 is predicted to have an outstanding balance between the ionic conductivity and stability. The room-temperature ionic conductivity is calculated to be as high as 15.3 mS/cm, whereas the band gap and electrochemical stability window vs Li/Li+ reach 4.26 V, respectively, making it a promising candidate for practical applications as a superionic conductor in solid-state batteries.