Aliovalent Substitution in Li2ZrCl6 Halide Electrolytes: Linking Lithium Content, Phase Stability, and Ionic Transport

Abstract

Aliovalent substitution was investigated as a strategy to enhance lithium transport in the halide solid electrolyte Li2ZrCl6. Here, Zr4+ was partially replaced by divalent (Mg2+, Zn2+) and trivalent (Fe3+, Ga3+) cations to form Li2+yZr1−xMxCl6 compositions prepared by mechanosynthesis. Structural analysis is consistent with dopant-dependent incorporation mechanisms, including octahedral substitution (Mg2+, Fe3+) and tetrahedral interstitial occupation (Zn2+, Ga3+), and reveals the formation of secondary phases beyond solubility limits. High-frequency impedance spectroscopy (1 MHz−3 GHz) enables separation of the bulk and grain boundary contributions to ionic transport, thereby allowing comparison of intrinsic lithium transport across the different substitution series. Notably, all substituted compositions with a theoretical nominal lithium content between 2.1 and 2.3 exhibit higher ionic conductivity than the pristine Li2ZrCl6, with bulk room-temperature conductivities reaching a maximum of 1.24 mS cm−1 for Li2.25Zr0.75Fe0.25Cl6 without applied pressure during measurement. Symmetric Li|electrolyte|Li cells with substituted compositions demonstrated stable stripping−plating behavior over extended cycling. Proof-of-concept Li-metal full cells employing the Li2+yZr1−xMxCl6 electrolytes and a high-voltage oxide NMC90505 cathode show functional compatibility, albeit without optimization of electrode architecture or interfaces. These results demonstrate how aliovalent substitution provides an effective route to enhance ionic transport in Li2ZrCl6-based halide electrolytes with dopant chemistry influencing phase stability and accessible compositional space. The findings identify nominal lithium content as a key compositional descriptor for optimizing transport in Li2ZrCl6-derived systems and underline the impact of multiphase coexistence on macroscopic electrochemical performance.