Achieving High‐Capacity Cathode Presodiation Agent Via Triggering Anionic Oxidation Activity in Sodium Oxide

Abstract Compensating for the irreversible loss of limited active sodium (Na) is crucial for enhancing the energy density of practical sodium‐ion batteries (SIBs) full‐cell, especially when employing hard carbon anode with initially lower coulombic efficiency. Introducing sacrificial cathode presodiation agents, particularly those that own potential anionic oxidation activity with a high theoretical capacity, can provide additional sodium sources for compensating Na loss. Herein, Ni atoms are precisely implanted at the Na sites within Na 2 O framework, obtaining a (Na 0.89 Ni 0.05 □ 0.06 ) 2 O (Ni–Na 2 O) presodiation agent. The synergistic interaction between Na vacancies and Ni catalyst effectively tunes the band structure, forming moderate Ni–O covalent bonds, activating the oxidation activity of oxygen anion, reducing the decomposition overpotential to 2.8 V (vs Na/Na + ), and achieving a high presodiation capacity of 710 mAh/g ≈Na2O (Na 2 O decomposition rate >80%). Incorporating currently‐modified presodiation agent with Na 3 V 2 (PO 4 ) 3 and Na 2/3 Ni 2/3 Mn 1/3 O 2 cathodes, the energy density of corresponding Na‐ion full‐cells presents an essential improvement of 23.9% and 19.3%, respectively. Further, not limited to Ni–Na 2 O, the structure–function relationship between the anionic oxidation mechanism and electrode–electrolyte interface fabrication is revealed as a paradigm for the development of sacrificial cathode presodiation agent.