Surface chemistry engineering of layered oxide cathodes for sodium‐ion batteries

Sodium‐ion batteries (SIBs) have attracted extensive attention to be applied in large‐scale energy storage due to their low cost and abundant storage resources. Among cathode materials for SIBs, layered oxide cathodes are considered one of the most promising candidates for practical application owing to their high theoretical capacities, simple synthesis routes, and environmental friendliness. However, poor air stability, complicated interfacial reaction, and irreversible phase translation of layered oxide cathodes pose problems for the long‐term cycle as well as rate performance. In this review, the recent achievements and progress in surface engineering chemistry strategies to improve the electrochemical performance of SIBs have been summarized including mechanical mixing, in‐situ coating methods, and designing unique interfacial structures. Moreover, inspired by previous studies, we propose an innovative concept of interface conversion reaction with bulk penetration doping integration, which is expected to deal with both interfacial and intrinsic issues synchronously through heat treatment. It could not only eliminate residual sodium compounds on the surface and improve air stability but also suppress the dissolution and the migration of transition metal and the phase transformation. The insights that came up in this review can be considered as a guide for surface engineering on layered oxide cathode for SIBs. The practical application of layered oxide cathodes for sodium‐ion batteries has been blocked by the dissatisfied long‐term cycling performance caused by the surface failure including dissolution of transition metal ions, gas release, side reaction, and crack generations. In this review, we propose an innovative concept of interface conversion reaction with bulk penetration doping integration, which is expected to deal with both interfacial and intrinsic issues synchronously.