Sheet-type all-solid-state batteries with sulfidic electrolytes: Analysis of kinetic limitations based on a cathode morphology study

All-solid-state batteries (ASSBs) with sulfide-based solid electrolytes (SEs) promise to boost the energy density of future Li-ion batteries. Still little is known about the influence of cathode morphology and charge transport/transfer processes inside the cathode on battery performance. We report on a morphological investigation of two ASSB cathodes prepared by the industrially relevant sheet-type approach. Both employ state-of-the-art NMC 85|05|10 as cathode active material (CAM) and sulfide-based SEs differing in morphology and intrinsic ionic conductivity, i.e., β-Li₃PS₄ (small mesoporous particles, conductivity: 0.2 mS cm^-1) and 2 Li₃PS₄∙LiI (large nonporous particles, conductivity: 0.8 mS cm^-1). We apply focused ion-beam scanning electron microscopy to obtain high-resolution reconstructions, allowing to differentiate between CAM, SE, and voids and to conduct a morphological analysis of each phase as well as a simulation of ion transport in the SE phase. Based on morphological data, kinetic limitations in the cathodes are analyzed using a transmission-line model, indicating that charge transfer resistance at the CAM-SE interface is the dominating contribution, while resistances due to Li-ion migration in the SE and Li chemical diffusion in the CAM are considerably lower. Reducing charge transfer resistance at the CAM-SE interface is therefore a key to improving ASSB performance.