Li+ conduction in aliovalent-substituted monoclinic Li2ZrCl6 for all-solid-state batteries: Li2+xZr1-xMxCl6 (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 Li2+xZr1-xInxCl6) 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 LiNi0.88Co0.11Mn0.01O2 cathode.