Theoretical Design of Lithium Chloride Superionic Conductors for All-Solid-State High-Voltage Lithium-Ion Batteries

The development of solid electrolytes (SEs) is a promising pathway to improve the energy density and safety of conventional Li-ion batteries. Several lithium chloride SEs, Li₃MCl₆ (M = Y, Er, In, and Sc), have gained popularity due to their high ionic conductivity, wide electrochemical window, and good chemical stability. This study systematically investigated 17 Li₃MCl₆ SEs to identify novel and promising lithium chloride SEs. Calculation results revealed that 12 Li₃MCl₆ (M = Bi, Dy, Er, Ho, In, Lu, Sc, Sm, Tb, Tl, Tm, and Y) were stable phase with a wide electrochemical stability window and excellent chemical stability against cathode materials and moisture. Li-ion transport properties were examined using bond valence site energy (BVSE) and ab initio molecular dynamics (AIMD) calculation. Li₃MCl₆ showed the lower migration energy barrier in monoclinic structures, while orthorhombic and trigonal structures exhibited higher energy barriers due to the sluggish diffusion along the two-dimensional path based on the BVSE model. AIMD results confirmed the slower ion migration along the 2D path, exhibiting lower ionic diffusivity and higher activation energy in orthorhombic and trigonal structures. For the further increase of ionic conductivity in monoclinic structures, Li-ion vacancy was formed by the substitution of M³⁺ with Zr⁴⁺. Zr-substituted phase (Li_2.5M_0.5Zr_0.5Cl₆, M = In, Sc) exhibited up to a fourfold increase in ionic conductivity. This finding suggested that the optimization of Li vacancy in the Li₃MCl₆ SEs could lead to superionic Li₃MCl₆ SEs.