Disentangling Interfacial Degradation and SOC-Dependent Transport Constraints in Sulfide All-Solid-State Battery Cathodes

Sulfide solid electrolyte based all-solid-state batteries incorporating layered oxide cathodes exhibit complex interfacial degradation and transport limitations that are not adequately captured by conventional electrochemical diagnostics. In particular, the relationship between apparent lithium transport parameters and resistance growth in composite solid-state cathodes remains insufficiently resolved, hindering mechanistic interpretation of performance decay. In this presentation, we introduce a correlative electrochemical framework that combines state-of-charge (SOC) resolved galvanostatic intermittent titration technique with in situ electrochemical impedance spectroscopy and distribution of relaxation times analysis to probe interfacial and transport phenomena in sulfide-based composite cathodes. By systematically varying the Ni content in layered oxide cathodes and the chemistry of the conductive additive, this approach enables separation of cycle-dependent interfacial degradation from SOC-dependent kinetic constraints that emerge during operation. Using this framework, we show that transport parameters extracted from transient electrochemical measurements are strongly influenced by the evolution of interfacial accessibility and impedance, rather than intrinsic bulk diffusion alone; an effect that becomes particularly pronounced at low state of charge. The results reveal how interfacial processes govern the apparent lithium transport response during cycling and provide a physically consistent interpretation of SOC-dependent kinetic limitations in sulfide all-solid-state batteries. More broadly, this work highlights the importance of SOC-resolved, correlative diagnostics for meaningful assessment of transport behavior and offers general insight into interfacial design strategies for mitigating performance decay in sulfide-based all-solid-state battery cathodes.