Layered oxide cathodes for sodium-ion batteries: From air stability, interface chemistry to phase transition

Sodium-ion batteries (SIBs) are considered as a low-cost complementary or alternative system to prestigious lithium-ion batteries (LIBs) because of their similar working principle to LIBs, cost-effectiveness, and sustainable availability of sodium resources, especially in large-scale energy storage systems (EESs). Among various cathode candidates for SIBs, Na-based layered transition metal oxides have received extensive attention for their relatively large specific capacity, high operating potential, facile synthesis, and environmental benignity. However, there are a series of fatal issues in terms of poor air stability, unstable cathode/electrolyte interphase, and irreversible phase transition that lead to unsatisfactory battery performance from the perspective of preparation to application, outside to inside of layered oxide cathodes, which severely limit their practical application. This work is meant to review these critical problems associated with layered oxide cathodes to understand their fundamental roots and degradation mechanisms, and to provide a comprehensive summary of mainstream modification strategies including chemical substitution, surface modification, structure modulation, and so forth, concentrating on how to improve air stability, reduce interfacial side reaction, and suppress phase transition for realizing high structural reversibility, fast Na+ kinetics, and superior comprehensive electrochemical performance. The advantages and disadvantages of different strategies are discussed, and insights into future challenges and opportunities for layered oxide cathodes are also presented.