Electro‐Chemo‐Mechanical Coupling in Composite Cathodes of Sulfide‐Based All‐Solid‐State Batteries: Pathways, Degradation, and Design Rules

By replacing flammable organic liquid electrolytes with rigid solid ones, all-solid-state batteries (ASSBs) promise higher pack-level energy density and improved thermal safety than conventional lithium-ion batteries (LIBs). However, their rigidity accompanies elevated significance of mechanical solid-solid contact on charge-transport interfaces during electrochemical operation of cells, which complicates accurate failure diagnosis and obscures rational design rules. This review integrates current understanding of reaction mechanisms and failure pathways of sulfide-based ASSBs, mapping the critical conduction networks, including intra-/inter-CAM transport, CAM|SE interfaces and transport among SE particles in layered oxide composite cathodes and how each is disrupted by electro-chemo-mechanical processes during operation. Beyond cathode volume change during charge and discharge, we highlight degradation of sulfide SEs on cathode active material and carbon surfaces and its direct contribution to contact loss and resistance growth, followed by engineering strategies that raise tolerance to stress accumulation and sustain co-percolation of electronic and ionic transport through cathode morphology/composition control, SE interfacial modification, particle-size engineering, and pressure management. Although our emphasis is on composite cathodes, the design principles extend to full-cell architecture and manufacturing, guiding the development of safe, high-energy-density ASSBs capable of stable operation at practical low-pressure conditions.