Precision Chemical Routes to Achieve Superior Oxyhalide Solid Electrolytes in Advanced All-Solid-State Batteries

Conspectus Oxyhalide solid-state electrolytes (SSEs) represent a strategically important subclass of halide-based materials that offer a promising solution to critical challenges in all-solid-state batteries (ASSBs), such as poor interfacial stability and mechanical fragility. By incorporating oxygen into halide frameworks, these materials preserve the wide electrochemical stability and cathode compatibility of halide SSEs while simultaneously enabling ionic conductivities exceeding 10 –2 S cm –1 and enhanced thermal resilience through carefully designed oxygen incorporation routes. This unique combination makes them a frontier material class for next-generation energy storage. The precise control of oxygen content is central to optimizing oxyhalide performance. Techniques including targeted substitution reactions, nanoscale oxide additions, and the use of oxygen-rich precursors have enabled the creation of novel SSE architectures. These methods allow for meticulous defect engineering and phase purity control, which are essential for tuning bulk ionic transport and managing interfacial behavior, particularly against reactive lithium metal anodes and high-voltage cathodes operating above 4.8 V vs Li + /Li. As global efforts such as the HELENA Project and multiple academic breakthroughs converge on the development of safer and more scalable battery chemistries, oxyhalide SSEs stand out as a frontier platform with significant implications for future electric vehicles, grid storage, and aerospace energy systems. This Account provides a timely overview of the synthesis–structure–property relationships in oxyhalide SSEs. It meticulously charts the tailored chemical pathways employed in their synthesis, offering a comprehensive understanding of the design principles that govern their exceptional performance. We embark on a detailed examination of the crucial role played by diverse oxygen sources, spanning the spectrum from fundamental alkali metal oxides to sophisticated metal oxychlorides, in building the unique structural frameworks of these electrolytes. Unraveling the structure–property relationships is paramount, and this Account provides critical insights into how these oxygen sources profoundly influence the mechanisms of ion transport and ultimately dictate the overall ionic conductivity, a key metric for battery efficiency. Furthermore, we illuminate the essential characterization methodologies utilized to probe the structural, morphological, and electrochemical attributes of these fascinating materials, providing a toolkit for researchers in the field. Beyond fundamental synthesis and characterization, this Account casts a forward-looking lens onto the promising applications of oxyhalide SSEs in next-generation energy storage devices. By understanding the precise interplay between synthesis, structure, and performance, we aim to accelerate the development and implementation of safer, more energy-dense, and longer-lasting batteries for a sustainable future, impacting everything from electric vehicles to grid-scale storage.