Decoupling Chemical and Mechanical Contributions to Capacity Fading in Ni‐Rich Cathodes for Sulfide‐Based All‐Solid‐State Batteries

ABSTRACT All‐solid‐state batteries (ASSBs) promise safer energy storage and the potential to outperform current lithium‐ion batteries. Ni‐rich cathodes deliver high energy density, whereas sulfide solid electrolytes offer exceptional Li + conductivity and inherent mechanical compliance, making their combination particularly attractive. Yet their capacity fades rapidly because (electro)chemical and mechanical degradation occur concurrently within the cathode. Here, we propose an approach to decouple these two factors in LiNi 0.8 Co 0.1 Mn 0.1 O 2 |Li 6 PS 5 Cl 0.5 Br 0.5 |Li metal cells by monitoring the state‐of‐charge window. Uncoated cathodes experience capacity fading primarily due to (electro)chemical degradation: interfacial reactions form resistive interphases consisting of NiO‐like rock salt, SO x , PO x , and related species that hinder Li + extraction, resulting in pronounced charge fading. A conformal LiNbO 3 coating on the cathode surface suppresses interfacial reactions, significantly mitigating charge fading but revealing dominant discharge fading arising from mechanical degradation, such as particle cracking and interfacial delamination. While the protective coating stabilizes the interface and improves initial performance, the increased capacity intensifies electrode volume changes, underscoring the need for integrated electro‐chemo‐mechanical design strategies. Evaluating charge and discharge fading outlines the interplay between (electro)chemical and mechanical degradation and offers a new diagnostic framework to guide the development of more durable, high‐energy‐density ASSBs.