New Cost-Effective Halide Solid Electrolytes for All-Solid-State Batteries: Mechanochemically Prepared Fe³⁺-Substituted Li₂ZrCl₆

Owing to the combined advantages of sulfide and oxide solid electrolytes (SEs), that is, mechanical sinterability and excellent (electro)chemical stability, recently emerging halide SEs such as Li₃YCl₆ are considered to be a game changer for the development of all-solid-state batteries. However, the use of expensive central metals hinders their practical applicability. Herein, a new halide superionic conductors are reported that are free of rare-earth metals: hexagonal close-packed (hcp) Li₂ZrCl₆ and Fe³⁺-substituted Li₂ZrCl₆, derived via a mechanochemical method. Conventional heat treatment yields cubic close-packed monoclinic Li₂ZrCl₆ with a low Li⁺ conductivity of 5.7 × 10⁻⁶ S cm⁻¹ at 30 °C. In contrast, hcp Li₂ZrCl₆ with a high Li⁺ conductivity of 4.0 × 10⁻⁴ S cm⁻¹ is derived via ball-milling. More importantly, the aliovalent substitution of Li₂ZrCl₆ with Fe³⁺, which is probed by complementary analyses using X-ray diffraction, pair distribution function, X-ray absorption spectroscopy, and Raman spectroscopy measurements, drastically enhances the Li⁺ conductivity up to ≈1 mS cm⁻¹ for Li_2.25Zr_0.75Fe_0.25Cl₆. The superior interfacial stability when using Li_2+xZr_1−xFe_xCl₆, as compared to that when using conventional Li₆PS₅Cl, is proved. Furthermore, an excellent electrochemical performance of the all-solid-state batteries is achieved via the combination of Li₂ZrCl₆ and single-crystalline LiNi_0.88Co_0.11Al_0.01O₂.