Unlocking Superionic Conduction by Modulating Electrostatic Interactions in a Zirconium-Based Trigonal Halide Solid Electrolyte

Abstract

Halide solid electrolytes have recently emerged as promising candidates for solid-state batteries, combining high ionic conductivity, favorable cathode compatibility, and mechanical softness. Among them, the zirconium-based halide Li₂ZrCl₆ has attracted attention due to its cost advantages over rare-metal-based counterparts, but its intrinsic ionic conductivity remains insufficient for practical applications. While aliovalent substitution has been explored to improve the performance, the mechanisms governing lithium transport and dopant interactions remain unclear. In this study, we systematically investigate how aliovalent cation substitution modulates lithium diffusion within the trigonal Li₂ZrCl₆ framework. By integrating computational and experimental approaches, we show that reducing electrostatic repulsion between cations and lithium ions not only facilitates local lithium diffusion near substituted sites but also unexpectedly generates additional diffusion pathways beyond conventional doping effects, resulting in a more robust percolated diffusion network. Building on these insights, we propose a simpler, dopant-free strategy, i.e., vacancy-mediated substitution, Li_2.3Zr_0.925Cl₆, promotes rapid hopping kinetics and enables efficient lithium incorporation. It leads to a significantly enhanced ionic conductivity, along with improved rate capability and stability. This work reveals a clear composition-path dependency on structurally tolerant hailde solid electrolytes, offering a viable route toward high-performance, cost-efficient solid electrolytes.