Preparation of different phase structure nano sized FePO₄/C cathode material by ultrasonic-assisting and their...

As one of the cathode materials for phosphate-based sodium ion batteries, FePO₄ has received extensive attention due to its excellent theoretical capacity and stability. However, the FePO₄ cathode has the problem of low ionic conductivity and electronic conductivity, which limits its application in sodium-ion batteries. The phase composition and microstructure of FePO₄ are crucial to ensure the excellent electrochemical properties. Therefore, in this paper, the nano-sized amorphous FePO₄·2H₂O/C with a particle size of only 50 nm was prepared by ultrasonic-assisted precipitation. The carbon black oxidized can be uniformly dispersed in samples and form a spatial network structure. Four kinds of crystalline cathode materials were successfully prepared by further treatment of FePO₄·2H₂O/C, including amorphous FePO₄/C, hexagonal FePO₄/C, monoclinic FePO₄·2H₂O/C and monoclinic/orthogonal FePO₄·2H₂O/C. The microstructure, phase composition, particle size distribution and specific surface area of the samples were characterized by XRD, SEM, TEM, EDS, Raman, and BET. The results show that the amorphous FePO₄/C particle size is smaller, and the specific surface area is larger. The electrochemical properties of samples were analyzed by CV and EIS. The results show that the crystal structure affects the specific charge-discharge capacity, Na⁺ diffusion coefficient, and charge transfer resistance of the materials. The amorphous FePO₄/C has excellent electrochemical performance, the specific discharge capacity is 149.8 mA h g^-1, the Na⁺ diffusion coefficient is 2.71 × 10^-16 cm² s^-1, and the charge transfer resistance is 139 Ω. The results show that the amorphous structure is effective for improving the electrochemical performance of FePO₄ cathode materials.