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.,
Mg2+ and Zr4+). We synthesize Li2Mg1/3Zr1/3Cl4 in the spinel crystal
structure and compare its properties with the high-performing Li2Sc2/3Cl4 that has been reported previously.
We find that Li2Mg1/3Zr1/3Cl4 has lower ionic conductivity (0.028 mS/cm at 30 °C)
than the isostructural Li2Sc2/3Cl4 (1.6 mS/cm at 30 °C). We attribute this difference to a disordered
arrangement of Mg2+ and Zr4+ in Li2Mg1/3Zr1/3Cl4, which may block Li+ migration pathways. However, we show that aliovalent substitution
across the Li2–zMg1–3z/2ZrzCl4 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.