Reversible anionic redox activity in Na3RuO4 cathodes: a prototype Na-rich layered oxide

Sodium-ion batteries are attractive for large-scale energy storage due to the abundance of sodium, but the deficient capacity achieved by cathode materials prevents their further applications. Chemical substitution of Na in transition metal layers is a promising solution to utilize both the cationic and anionic redox activities for boosting energy storage. Unfortunately, different from the classic Li-rich Li2MnO3, a pure prototype with anionic redox activity has not been found among the typical Na-rich cathodes. Herein, we originally design a Na-rich layered oxide prototype, namely Na3RuO4 (Ru5+), which delivers a partial reversible capacity solely via the participation of oxygen anions. More importantly, the anionic redox activity is validated by the in situ Raman observation of reversible peroxo-based O-O (de) bonding upon cycling. Our findings not only highlight the multiple electron-transfer strategy for capacity extension, but also broaden the horizon in designing Na-rich electrode materials for high-energy sodium-ion batteries.