Solid-state batteries are a promising next-generation energy storage technology due to their improved safety and potential for higher energy density--especially when paired with high-capacity anodes such as lithium metal. However, many solid electrolytes suffer from instability at the low operating potentials of these next-generation anodes, leading to irreversible capacity loss. This challenge is particularly relevant for halide electrolytes, which, despite their good cathodic stability and high ionic conductivity, often exhibit poor anodic stability. The incorporation of zirconium has been shown to enhance ionic conductivity, but its influence on low-potential electrochemical stability remains insufficiently explored.

In this work, we address this gap by engineering zirconium(IV)-based halide electrolytes and studying their behaviour across three distinct chemical environments: (i) an isolated zirconium system Li2ZrCl6, (ii) an aliovalently substituted compound (Li2.5In0.5Zr0.5Cl6), and (iii) a multi-cation high-entropy compound (Li2.75MCl6, M = Sc, Lu, Yb, Zr). Using density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations, we predict electrochemical behaviour at low voltages and validate our predictions experimentally. We distinguish between the intrinsic electrochemical stability window, where no redox activity occurs, and an extended lithiation/delithiation region, where redox activity can proceed without structural decomposition.

Our findings show that Li2ZrCl6 exhibits such reversible redox activity beyond its intrinsic stability window, offering enhanced compatibility with low-voltage anodes and additional storage capacity. However, this beneficial behaviour does not translate to the other systems: Li2.5In0.5Zr0.5Cl6 undergoes decomposition via the formation of metallic indium, while the multi-cation compound exhibits severe capacity loss in practice, despite being computationally predicted to resist destructive reduction. This discrepancy between theoretical predictions and experimental outcomes highlights the challenges of modelling complex chemistries and underscores the need for rigorous experimental validation.

Overall, this work lays the groundwork for understanding how zirconium influences redox behaviour in halide electrolytes and reveals the complex interplay between composition, structure, and electrochemical stability--guiding future strategies for the design of reduction-tolerant solid electrolytes.