Stabilizing Anionic Redox and Tuning Its Extent in Na-Rich Cathode Materials through Electronic Structure Engineering

The capacity of sodium-ion batteries (SIBs) can be enhanced by incorporating anionic redox into Na-rich cathode materials. However, excessive participation of oxygen in the redox process during the cycling often leads to several undesired issues including oxygen release. In this study, using first-principles computational methods through a systematic investigation and detailed analysis, we demonstrate an electronic structure tuning strategy through the aliovalent doping method to tune the amount of anionic redox in SIBs. Furthermore, we provide a method for achieving reversible anionic redox and emphasize that reversible anionic redox is not solely dependent on the covalent interaction between the transition metal and oxygen but is influenced by multiple factors that govern the electronic structure of the material. Using the aforementioned strategy, we identify an Al-doped Na-rich material, Na2Ru0.5Al0.5O3, which exhibits reversible cationic and anionic redox. Additionally, we rationalize the dominance of cationic redox in pristine Na2RuO3.