Abstract: Sodium-ion batteries have emerged as a significant alternative to lithium-ion batteries, offering a cost-effective and safe solution with promising potential in energy storage. Among these, P2-type Ni/Mn based oxides possess the advantages of high theoretical capacity and wide operating voltage. However, the P2-O2 phase transition and Jahn-Teller aberration under high pressure significantly impact the cycling reversibility and structural stability. To address the issues above, in this study P2-type Na_0.8Ni_0.33Mn_0.67-xAl_xO₂ with different contents of Al doping using a high-temperature solid-phase method was prepared, and employed as cathode material for sodium-ion batteries. The morphology, composition, elemental valence, and structural features of the material were characterized. It was observed that the introduction of Al doping resulted in strengthening of the metal-oxygen bonds (M-O bonds) and an expansion of the Na layer distance, thereby facilitating Na⁺ diffusion and enhancing structural stability. The electrochemical properties demonstrate that Al doping can impede the high-voltage phase transition, stimulate the electrochemical activity of Mn, and diminish the charge transfer resistance, thereby enhancing the electrochemical properties of the materials. Among the examined materials, the Na_0.8Ni_0.33Mn_0.62Al_0.05O₂ cathode displayed the optimal cycling performance with a capacity retention of approximately 87.3% after 200 cycles at 0.1C (1C = 200 mA·g^-1) in the range of 2-4.2 V, and superior rate performance with a capacity of 100.9 mAh‧g^-1 at 2C in the range of 2-4.2 V.

Key words: sodium-ion battery, Al doping, P2-type cathode, nickel-manganese oxide, electrochemical properties