Layered oxide cathodes for sodium-ion batteries: origins of microcracks and countermeasures

Layered transition metal oxide cathodes (Na _x TMO₂) for sodium ion batteries have stood out among various cathode materials owing to their advantages in cost and electrochemical performance. However, their further practical application is hindered by deep-seated structural degradation, interfacial instability, and poor air stability. Among these issues, microcracks serving as a critical bridge between intrinsic defects in Na _x TMO₂ and macroscopic performance decay have emerged as a central focus of current research. Accordingly, this review systematically summarizes the origins of microcrack formation in Na _x TMO₂ and the corresponding mitigation strategies. The influence of primary structural defects introduced during material synthesis is first examined. Subsequently, the structural and chemical roots of microcrack formation during electrochemical processes are elucidated. Finally, crack formation pathways induced by environmental factors and interfacial evolution, particularly those associated with air exposure and cathode-electrolyte interfaces, are discussed. Building on these mechanistic insights, we critically compare current suppression strategies, including electronic structure regulation, lattice strain engineering, and interfacial engineering. The failure mechanisms and mitigation approaches related to microcracks summarized in this review are expected to contribute to the overall performance enhancement of Na _x TMO₂ and to provide important guidance for their industrial application in sodium-ion batteries.