Tuning lithium diffusion by anionic substitution in a trigonal halide superionic conductor

Halide solid electrolytes have emerged as a key enabler for solid-state batteries, offering exceptional electrochemical and mechanical compatibility with cathodes. Among them, Li₂ZrCl₆ stands out as a cost-effective alternative, unlike most reported halide electrolytes that rely on scarce and expensive elements such as Y, Er, Ho, Sc and Yb. However, its relatively low ionic conductivity (∼10⁻⁴ S cm⁻¹) remains a critical limitation for practical applications. In this study, we employ a theoretical approach to unravel the lithium diffusion mechanisms in trigonal Li₂ZrCl₆ and systematically evaluate the effect of partial anion substitution on ionic transport. Our findings reveal that even minimal amount of anion (e.g., sulfur) substitution can drastically enhance lithium percolation pathways, significantly accelerating lithium diffusion kinetics. This enhancement stems from a unique in-plane diffusion mechanism, where electrostatically restricted vacant sites at pathway intersections – previously inaccessible – become activated through anion substitution. In particular, sulfur mitigates cationic repulsion and enlarges intermediate sites at these intersections, unlocking multiple new diffusion pathways. Guided by this insight, we experimentally demonstrate that even a minimal level of sulfur substitution (∼3 %) increases ionic conductivity by a factor of three compared with pristine Li₂ZrCl₆. Further tuning achieves a remarkable conductivity of ∼1.33 $\times$ 10⁻³ S cm⁻¹ at 25 °C, the highest reported for Li₂ZrCl₆ electrolytes $\ge$ 90 % Zr content. These findings highlight the structural adaptability of trigonal Li₂ZrCl₆ for anion substitution, presenting a new and effective strategy to achieve high ionic conductivity in cost-effective halide solid electrolytes.