Redox‐Active Halide Catholytes for Solid‐State Lithium Batteries

All‐solid‐state Li batteries that incorporate halide solid‐state electrolytes (SEs) promise the safety, thermal stability, and high energy density needed for electric vehicles and grid‐scale renewable energy storage. However, a substantial fraction of these electrochemical inert SEs is used as catholytes within the composite cathodes to sustain continuous Li‐ion pathways, adding “dead weight” that does not contribute to the overall capacity. This review highlights the emerging redox‐active halide catholytes, specifically Li containing transition metal halide that combine reversible redox chemistry with efficient mixed ionic–electronic conductivity. When integrated into composite cathodes, these materials contribute to an additional 20–50% reversible capacity, and simultaneously lowers the electronic transport tortuosity. Drawing on insights from solid‑state ionics and electronic band structure, the design principles for achieving mixed ionic–electronic conductivity are first outlined, and then summarize recent progress on Fe‐, V‐, and Ti‐based redox‐active halide catholytes, along with their dynamic behavior during cycling. Finally, the conclusion outlined further directions for redox‐active halide catholytes, including the discovery of new materials, anion‑sublattice engineering, and the elucidation of dynamic interface evolution, as well as the exploration of anionic‑redox processes.