Particle Size and Composition Coupling Determine the Interfacial Stability in Sulfide All-Solid-State Battery Composite Cathodes

All-solid-state batteries (ASSBs) have enhanced safety features and potential for high energy density. However, their path to commercialization has obstacles because of the inadequate solid–solid contact and mismatched ionic and electronic transport properties in composite cathodes. In this study, we investigate the effect of Li6PS5Cl (LPSC) solid-state electrolyte particle size and CAM/SSE ratio (using high-Ni NCM90) on interfacial transport kinetics and coupled electrochemical–mechanical stability. Three-dimensional simulations with LAMMPS and multiscale characterization methods reveal a trade-off between ionic and electronic pathways: refined LPSC refined enhances void filling, reduces Li-ion tortuosity, and improves ionic continuity, but also increases the tortuosity of electronic pathways. A volume ratio of 50:50 between CAM and SSE results in a balance between ionic continuity and electronic percolation, stabilizes the growth of CEI, and maintains 65.8% of the capacity after 2C cycling when returning to a low rate. Insufficient cathode electrolyte interphase (CEI) results in interfacial contact degradation, uneven CEI layer formation, and impedance hotspots over extended periods, as well as microcracking and accelerated capacity loss. These findings provide design guidelines for stable, high-performance sulfide composite cathodes.