Sodium and oxygen site doping strategies for layered transition metal oxide cathode materials in sodium-ion batteries

Layered transition metal oxides (LTMO) are among the most promising materials for sodium-ion battery (SIB) cathodes, owing to good electrochemical performance and a variety of modification options for enhancement and refinement. However, drawbacks include irreversible structural and chemical changes, capacity decay, air and moisture instability, and mechanical and chemical incompatibility at the cathode electrolyte interface. Many ingenious solutions have been offered to alleviate these issues, including coating and doping strategies and morphological manipulation. Recently, sodium (Na) site substitution has attracted attention as an attempt to address these issues. Elements with similar ionic charge or radius compared to Na (Ca²⁺, Mg²⁺, K⁺, Li⁺) have been successfully inserted into the Na sites to act as stable structural and chemical pillars during desodiation. These "pillars" can increase structural stability and decrease capacity decay during cycling, especially in higher voltages, enhance Na kinetics, and alleviate detrimental redox reactions. The main drawback is decreased initial capacity and synthesis complications. In this perspective article, we explore the effects of the Na and O site doping in LTMO cathode materials in SIBs.