Expanding the Phase Space for Halide-Based Solid Electrolytes: Li-Mg-Zr-Cl Spinels.

Chloride-based solid electrolytes are intriguing materials owing to their high Li + ionic conductivity and electrochemical compatibility with high-voltage oxide cathodes for all-solid-state lithium batteries. However, the leading examples of these materials are limited to trivalent metals (e.g., Sc, Y, and In), which are expensive and scarce. Here, we expand this materials family by replacing the trivalent metals with a mix of di- and tetra-valent metals (e.g., Mg 2+ and Zr 4+ ). We synthesize Li 2 Mg 1/3 Zr 1/3 Cl 4 in the spinel crystal structure and compare its properties with the high-performing Li 2 Sc 2/3 Cl 4 that has been reported previously. We find that Li 2 Mg 1/3 Zr 1/3 Cl 4 has lower ionic conductivity (0.028 mS/cm at 30 °C) than the isostructural Li 2 Sc 2/3 Cl 4 (1.6 mS/cm at 30 °C). We attribute this difference to a disordered arrangement of Mg 2+ and Zr 4+ in Li 2 Mg 1/3 Zr 1/3 Cl 4 , which may block Li + migration pathways. However, we show that aliovalent substitution across the Li 2- z Mg 1-3 z /2 Zr z Cl 4 series between Li 2 MgCl 4 and Li 2 ZrCl 6 can boost ionic conductivity with increasing Zr 4+ content, presumably due to the introduction of Li + vacancies. This work opens a new dimension for halide-based solid electrolytes, accelerating the development of low-cost solid-state batteries.