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 LixZr1–x/2Mgx/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 Li1.25Zr0.375Mg0.625Cl4 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.