Decoding Multiscale Degradation of Layered Cathodes in All‐Solid‐State Lithium Batteries: An Advanced Diagnostics‐Driven Framework

Abstract

All‐solid‐state lithium batteries (ASSLBs) with layered oxide cathodes such as LiNi 1‐x‐y Mn x Co y O 2 (NMC), LiNi 1‐x‐y Co x Al y O 2 (NCA), LiNiO 2 (LNO), and lithium‐rich layered oxides (LLOs) offer superior energy density and safety, yet their performance is severely limited by complex, coupled degradation mechanisms at the cathode/inorganic solid‐state electrolyte (SSE) interface and within the cathode bulk. This review establishes a Diagnosis‐Mechanism‐Mitigation (D‐M‐M) framework that decodes these multiscale degradation phenomena by cross‐validating multimodal diagnostic evidence from electronic structure to the cell level. Two categories are dissected: interfacial degradation, encompassing chemical decomposition, space‐charge layer formation, and resistive interphase growth; and bulk cathode degradation, including structural phase transitions, cation disorder, oxygen loss, and microstructural cracking. Drawing on advanced spectroscopy, microscopy, 3D imaging, and in situ/operando methods, each degradation cascade is traced from its diagnostic signature to its mechanism and to the validated mitigation it informed, from interfacial coatings to dopant engineering and microstructure optimization. Machine learning and artificial intelligence are assessed for interpreting complex diagnostic datasets and guiding predictive cathode design. The framework is further extended to practically relevant conditions (thick electrodes, low stack pressure, and large‐format cells), and the field's unresolved controversies are delineated together with the experiments that would decide them.