Li⁺ conduction in aliovalent-substituted monoclinic Li₂ZrCl₆ for all-solid-state batteries: Li_2+xZr_1-xM_xCl₆ (M = In, Sc)

Newly emerging halide superionic conductors with excellent (electro)chemical oxidation stability and deformability are considered as the enabler for high-performance all-solid-state batteries. Compared to close-packed monoclinic Li₃InCl₆ or Li₃ScCl₆, despite the same structural framework, the lower ionic conductivity of Li₂ZrCl₆ is intriguing. Herein, the structural evolution and Li⁺ migration of aliovalent-substituted Li₂ZrCl₆ with In³⁺ (or Sc³⁺) are investigated. A monoclinic crystal structure over the entire range of substitution (0 ≤ x ≤ 1.0 in Li_2+xZr_1-xIn_xCl₆) is identified by the Rietveld refinement of neutron diffraction. By the aliovalent substitution, the Li⁺ conductivity of Li₂ZrCl₆ is increased drastically from 7.1 × 10^-6 to max. 2.1 × 10^-3 S cm⁻¹ at 30 °C. It is revealed that the aliovalent substitution results in anisotropic lattice volume expansion and redistribution of Li in the lattice. Specifically, the increased concentration of Li⁺ in the (0 0 2) plane renders the Li⁺ migration more favorable. The bond valence energy level calculations also disclose two dimensionally (2D) preferable 3D Li⁺ migration channels, which emphasizes a tetrahedral Li site in the (0 0 2) plane as the key for facile Li⁺ migration. Furthermore, the excellent electrochemical performance of all-solid-state batteries using In-substituted Li₂ZrCl₆ is demonstrated for single-crystalline LiNi_0.88Co_0.11Mn_0.01O₂ cathode.