A paradigm shift in all-solid-state lithium batteries: halide cathode materials streamlined for multi-electron reactions

Halide cathode active materials (CAMs) are emerging as a transformative platform for all-solid-state lithium batteries (ASSLBs), offering intrinsic high ionic/electronic conductivities, multi-electron transfer capability, and the unique potential for single-phase electrode architectures that eliminate inactive components. Despite these advantages, critical challenges regarding their reaction mechanisms, interfacial stability, and structural evolution during cycling remain inadequately addressed. In this review, we systematically trace the evolution of halide CAMs from liquid-electrolyte systems to ASSLBs, focusing on recent breakthroughs in single-phase interface engineering and multi-electron reaction mechanisms. We highlight how elemental and structural units govern ionic/electronic transport pathways and reversibility, while critically evaluating the dilemma between energy density and efficiency in multi-electron reaction. Looking forward, a roadmap for next-generation halide CAM development is outlined, encompassing high-throughput material screening, controllable design of integrated all-in-one architectures, and strategies to overcome voltage hysteresis and conversion kinetics limitations. By bridging fundamental insights with practical engineering, this review aims to provide actionable guidance for realizing high-energy, cost-effective ASSLBs.