Anionic Redox Chemistry for Sodium-Ion Batteries: Mechanisms, Advances, and Challenges

The increasing reliance on energy demands has called for continual improvement of sodium-ion batteries (SIBs) due to the abundant Na resources and low cost. Na-based layered transition metal oxides (TMOs) are promising cathode materials due to their unique structural characteristics that provide two-dimensional (2D) ion diffusion channels and maintain structural integrity during the extraction and insertion of Na-ions. However, the energy density of conventional TMO cathodes is mainly limited by cationic redox reactions. Activating anionic redox (O2–/On–) for additional capacity in TMO cathodes is promising to improve the energy density of SIBs, though the chemistry of anionic redox reaction (ARR) is still unclear. This overview focuses on the underlying science, development, and latest advances of ARR in SIBs. The challenges and possible approaches are discussed in light of ARR reversibility and voltage hysteresis. This review will provide insights on improving the energy density of advanced TMOs cathodes with ARR activity.