Theoretical Insights on the Comparison of Li-Ion Conductivity in Halide Superionic Conductors Li₃MCl₆, Li₂M_2/3Cl₄, 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 Li₂Sc_2/3Cl₄ 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 Li₂M_2/3Cl₄, and LiMCl₄. It is revealed that halospinel Li₂M_2/3Cl₄ 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, Li₂Sm_2/3Cl₄ 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.