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

P2-type Na<sub><i>x</i></sub>Ni<sub><i>y</i></sub>Mn<sub>1–<i>y</i></sub>O<sub>2</sub> cathodes\nhave attracted attention due to their excellent stability and low\ncost, making them promising for sodium-ion batteries. However, their\npractical application is limited by a low capacity at lower voltages\nand severe phase transitions at higher voltage. To address these challenges,\nwe report a material Na<sub>0.6</sub>Ni<sub>0.3</sub>Mn<sub>0.7</sub>O<sub>2</sub>–OVs (NNMO-OVs) with significantly slowed phase\ntransitions at high voltage by introducing oxygen vacancies OVs into\nthe P2/P3 mixed phase cathode Na<sub>0.6</sub>Ni<sub>0.3</sub>Mn<sub>0.7</sub>O<sub>2</sub> (NNMO). Such a modification effectively broadens\nthe Na<sup>+</sup> diffusion pathways and enhances anionic redox reactions\n(ARR). As a result, an improved capacity of 173 mAh·g<sup>–1</sup> at 1C is obtained by the desirable cathode within a voltage range\nof 1.5–4.3 V. Even at 10C, it exhibits a capacity of 60.1 mAh·g<sup>–1</sup> between 2 and 4.1 V, maintaining 70.1% capacity retention\nafter 700 cycles, showcasing impressive rate performance and cycling\nstability. Additionally, the P2/P3 mixed phase exhibits the presence\nof an OP4 intermediate phase during charging to high voltages, while\nthe introduction of oxygen vacancies (OVs) further suppresses the\nP–O phase transition, maintaining only the P2-OP4 phase transition\nprocess.