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., Mg²⁺ and Zr⁴⁺). We synthesize Li₂Mg_1/3Zr_1/3Cl₄ in the spinel crystal structure and compare its properties with the high-performing Li₂Sc_2/3Cl₄ that has been reported previously. We find that Li₂Mg_1/3Zr_1/3Cl₄ has lower ionic conductivity (0.028 mS/cm at 30 °C) than the isostructural Li₂Sc_2/3Cl₄ (1.6 mS/cm at 30 °C). We attribute this difference to a disordered arrangement of Mg²⁺ and Zr⁴⁺ in Li₂Mg_1/3Zr_1/3Cl₄, which may block Li⁺ migration pathways. However, we show that aliovalent substitution across the Li_2-z Mg_1-3z/2Zr _z Cl₄ series between Li₂MgCl₄ and Li₂ZrCl₆ can boost ionic conductivity with increasing Zr⁴⁺ 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.

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