New cost-effective halide solid electrolytes for all-solid-state batteries. Mechanochemically prepared Fe${}^{3+}$-substituted Li${}_2$ZrCl${}_6$

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 Li${}_3$YCl${}_6$ 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) Li${}_2$ZrCl${}_6$ and Fe${}^{3+}$-substituted Li${}_2$ZrCl${}_6$, derived via a mechanochemical method. Conventional heat treatment yields cubic close-packed monoclinic Li${}_2$ZrCl${}_6$ with a low Li${}^{+}$ conductivity of 5.7 × 10${}^{-6}$ S cm${}^{-1}$ at 30 °C. In contrast, hcp Li${}_2$ZrCl${}_6$ with a high Li${}^{+}$ conductivity of 4.0 × 10${}^{-4}$ S cm${}^{-1}$ is derived via ball-milling. More importantly, the aliovalent substitution of Li${}_2$ZrCl${}_6$ with Fe${}^{3+}$, 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${}_{2.25}$Zr${}_{0.75}$Fe${}_{0.25}$Cl${}_6$. The superior interfacial stability when using Li${}_{2+x}$Zr${}_{1-x}$Fe${}_x$Cl${}_6$, as compared to that when using conventional Li${}_6$PS${}_5$Cl, is proved. Furthermore, an excellent electrochemical performance of the all-solid-state batteries is achieved via the combination of Li${}_2$ZrCl${}_6$ and single-crystalline LiNi${}_{0.88}$Co${}_{0.11}$Al${}_{0.01}$O${}_2$. (© 2021 Wiley-VCH GmbH)