Interfacial modulation engineering for sodium layered oxide cathode: air stability, ion-transfer kinetics, and phase evolution

Sodium-ion batteries (SIBs) have garnered significant attentions for grid-scale energy storage due to the low cost and abundant sodium resources. Among the various cathode materials, sodium layered transition metal oxides (Na_xTMO₂) are considered highly promising for practical applications of SIBs relying on their high theoretical capacities and facile syntheses. However, the poor air stability, sluggish interfacial kinetics, and detrimental phase transitions of Na_xTMO₂ commonly result in unsatisfactory cycling stability as well as inferior rate capability. In this review, recent achievements and progress in interfacial regulations aimed at improving the air stability and electrochemical performances of Na_xTMO₂, such as organic/inorganic coating, interfacial-coating-doping, and heterogeneous phase designing are summarized. Such approaches can not only enable the in-situ conversion of residual alkali and/or enhance the interfacial stability, but also improve the electrochemical reaction kinetics and mitigate phase evolutions. The structural stability enhancement mechanisms of Na_xTMO₂ layered oxides resulted from surface reconstructions are profoundly discussed and the influences on their electrochemical properties are concluded in this work. Finally, we outlook the novel interfacial modification strategies like of layered-tunnel heterostructure building and organic-inorganic co-coating. The state-of-the-art characterization techniques and artificial intelligence are also elaborated to develop high-performance Na_xTMO₂ cathodes in the future. We believe that the insights presented in this review can serve as meaningful guidance for the interfacial modulations of Na_xTMO₂ cathodes.