Synergistic Enhancement of Cathode Performance through the Introduction of Oxygen Vacancies in the P2/P3 Mixed Phase.

P2-type NaNiMnO cathodes have attracted attention due to their excellent stability and low cost, making them promising for sodium-ion batteries. However, their practical application is limited by a low capacity at lower voltages and severe phase transitions at higher voltage. To address these challenges, we report a material NaNiMnO-OVs (NNMO-OVs) with significantly slowed phase transitions at high voltage by introducing oxygen vacancies OVs into the P2/P3 mixed phase cathode NaNiMnO (NNMO). Such a modification effectively broadens the Na diffusion pathways and enhances anionic redox reactions (ARR). As a result, an improved capacity of 173 mAh·g at 1C is obtained by the desirable cathode within a voltage range of 1.5-4.3 V. Even at 10C, it exhibits a capacity of 60.1 mAh·g between 2 and 4.1 V, maintaining 70.1% capacity retention after 700 cycles, showcasing impressive rate performance and cycling stability. Additionally, the P2/P3 mixed phase exhibits the presence of an OP4 intermediate phase during charging to high voltages, while the introduction of oxygen vacancies (OVs) further suppresses the P-O phase transition, maintaining only the P2-OP4 phase transition process.