Capacity Fading Mechanisms of Ni-Rich Cathodes in All-Solid-State Batteries: Quantitative Approaches

All-solid-state batteries (ASSBs) comprising Ni-rich layered cathode active materials (CAMs) and sulfide solid electrolytes are promising candidates for highly safe next-generation batteries with high energy densities. However, these ASSBs experience significant capacity fading during cycling owing to surface degradation at the CAM/electrolyte interface and severe lattice volume changes in the CAM, resulting in loss of contact between the particles in the CAM or between the CAM and the electrolyte. In this study, we specifically quantified the capacity fading factors of Ni-rich Li[Ni_xCo_yAl_1-x-y]O₂ (NCA) composite ASSB cathodes as functions of Ni content. Surface degradation at the CAM/electrolyte interface was found to be the main cause of capacity fading in a CAM with an 80% Ni content, whereas inner-particle isolation and detachment of the CAM from the solid electrolyte plays a significant role in capacity fading as the Ni content increases to and exceeds 85%. Although coating the surface and modifying the CAM morphology significantly mitigate surface degradation and internal particle isolation, detachment remains a challenge owing to severe lattice volume changes experienced by Ni-rich CAMs. A comprehensive understanding of the capacity fading mechanisms in ASSBs comprising Ni-rich CAMs is expected to shed light on future research strategies that overcome their limitations.