Harnessing Cation Disorder for Enhancing Ionic Conductivity in Lithium Inverse Spinel Halides

Halides are promising solid-state electrolytes for all-solid-state lithium batteries due to their exceptional oxidation stability, high Li-ion conductivity, and mechanical deformability. However, their practicality is limited by the reliance on rare and expensive metals. This study investigates the Li₂MgCl₄ inverse spinel system as a cost-effective alternative. Molecular dynamics simulations reveal that lithium disordering at elevated temperatures significantly reduces the activation energy in Li₂MgCl₄. To stabilize this disorder at lower temperatures, we experimentally explored the Li _<i>x</i> Zr_1-<i>x</i>/2Mg _<i>x</i>/2Cl₄ system and found that Zr doping induces both Zr and Li disorder at the 16c site at room temperature (RT). This leads to a 2 order-of-magnitude increase in ionic conductivity for the Li_1.25Zr_0.375Mg_0.625Cl₄ composition, achieving 1.4 × 10^-5 S cm^-1 at RT, compared to pristine Li₂MgCl₄. By deconvoluting the role of lithium vacancies and dopants, we reveal that cation disordering to the 16c site predominantly enhances ionic conductivity, whereas lithium vacancy concentration has a very limited effect.