Air Sensitivity and Surface Residual Alkali in Sodium‐Ion Layered Oxide Cathodes: A Review

ABSTRACT Layered oxide cathodes for sodium‐ion batteries are among the most promising candidate materials for large‐scale energy storage systems. However, they are sensitive to humid air during electrode processing and suffer from spontaneous surface degradation primarily manifested as residual alkali species such as NaOH, Na 2 CO 3 , and NaHCO 3 , causing electrochemical performance degradation and delay of industrialization. Although similar surface alkali issues exist in lithium‐ion layered oxide cathodes, it brings distinct and more complex challenges in sodium‐ion batteries due to the unique presence of NaHCO 3 , the severe Na + /H + exchange tendency, and the narrower processing windows, which, however, have received very limited attention in the last few years. This review discusses the residual alkali issue in layered oxide cathodes of sodium‐ion batteries from the formation mechanisms to characterization techniques as well as modification strategies. Particular emphasis is put on the aforementioned sodium‐specific chemistry (e.g., NaHCO 3 ) and derived differences from the well‐established lithium‐ion systems, as well as related research perspectives in the future, aiming to stimulate the development of air‐stable, high‐performance sodium‐ion layered oxide cathodes toward practical applications.