Structural and electrochemical investigation of P2-Na_0.67Fe_0.5Mn_0.5O₂ high-performance sodium ion cathode materials

Fe/Mn-based metal oxides have attracted considerable attention as cathode materials for sodium-ion batteries owing to their low cost and high specific capacity. However, the relatively large ionic radius of the sodium ion (1.02 Å) results in inefficient diffusion kinetics, resulting in reduced battery performance. In this study, we enhance the electrochemical performance of P2-Na_0.67Fe_0.5Mn_0.5O₂ by optimizing its crystal structure through controlling calcination time, rather than relying on traditional ion doping methods. The optimized Na_0.67Fe_0.5Mn_0.5O₂ exhibits an initial capacity of 166.1 mAh·g^-1, retaining 73.64 % after 100 cycles at 0.1C (1C = 260 mA·g^-1). Additionally, it demonstrates an initial capacity of 120 mAh·g^-1 at 1C, with 81.25 % of this capacity maintained after 150 cycles, surpassing recently modified materials. The electrochemical properties of Na_0.67Fe_0.5Mn_0.5O₂ were further characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Results indicate that Na_0.67Fe_0.5Mn_0.5O₂ calcined at 900 °C for 12 h exhibits high crystallinity, moderate particle size, and a smooth morphology. Moreover, the cell parameter c is successfully enhanced, thereby expanding the sodium-ion channels and improving sodium-ion diffusion efficiency. XPS results reveal that Fe³⁺ facilitates the oxidation of Mn³⁺ to Mn⁴⁺. Furthermore, the material calcined for 12 h has the highest Mn⁴⁺ content, effectively mitigating the Jahn-Teller effect and improving the stability of the charge-discharge process. These findings indicate that adjusting calcination time is an effective strategy for developing low-cost, high-performance sodium-ion battery cathode materials.