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.