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.

Batteries play a vital role in the ongoing energy transition, driving the demand for safer, energy denser and higher performing energy storage solutions. This has propelled research of solid-state batteries. Halide electrolytes, with high ionic conductivities and high oxidation stabilities, have attracted tremendous interest. Currently, the main challenge is that most promising halide solid electrolytes are reliant on expensive and scarce metals, hindering their application at an industrial scale. Therefore, it is of great significance to develop cost-effective halide electrolytes. Zr-based electrolytes show great promise due to their cost-effectiveness (ZrCl₄ = 12.5 USD/kg) and high abundance in the earth's crust (165 mg/kg). However, so far their conductivity have been unsatisfactory, falling below 1 mS/cm. Anion doping with elements like Cl, Br, I, and O has demonstrated to be effective in improving the conductivities of sulfide solid electrolyte. In this work, the O-doping effect is investigated in Zr-based halide solid electrolytes with Li_2xZrCl₄O_x. By using various analysis techniques such as X-ray diffraction, electrochemical impedance spectroscopy, and cyclic voltammetry, this study explores the relationship between compositions, conductivities, and phases within the Li_2xZrCl₄O_x system. The findings reveal that, for each Zr-based oxyhalide composition, varying ball milling times result in different phases, with the most amorphous phase displaying the highest ionic conductivity. Specifically, for x = 1, Li₂ZrCl₄O reaches 1.60 mS/cm after 17.2 hours of ball milling, characterized by a structure featuring 61% amorphous content. Additionally, it demonstrates good performance as an all-solid-state battery with LiNi_0.8Mn_0.1Co_0.1O₂/ Li₂ZrCl₄O/ Li₆PS₅Cl/Li-In, achieving an initial capacity of 125.6 mAh/g at 0.5C and retaining 67.47% capacity after 1000 cycles. Moreover, the impact of I-doping is further explored in Li₃YCl₃Br_3-xI_x, another cost-effective halide solid electrolyte (YCl₃ = 330 USD/kg and 33 mg/kg). The Li₃YCl₃Br_3-xI_x electrolyte displays tunable conductivity and stability characteristics with an excellent conductivity of 3.55 mS/cm for x = 1 compared to 1.94 mS/cm for x = 0 but with a trade-off in oxidation potential of 3.474 V to 3.59 V. This study provides insights into novel cost-effective electrolytes and exhibits the potential of anion doping in enhancing and tuning both conductivity and stability. These electrolytes hold a serious potential as a solid electrolyte in solid-state batteries.