Preparation of Nb⁵⁺ Doped Na₃V₂(PO₄)₃ Cathode Material for Sodium Ion Batteries

Sodium-ion batteries (SIBs) have emerged as a promising alternative to lithium-ion batteries (LIBs) due to the abundance and low cost of sodium resources. Cathode material plays a crucial role in the performance of sodium ion batteries determining the capacity, cycling stability, and rate capability. Na₃V₂(PO₄)₃ (NVP) is a promising cathode material due to its stable three-dimensional NASICON structure, but its discharge capacity is low and its decay is serious with the increase of cycle period. We focused on modifying NVP cathode material by coating carbon and doping Nb⁵⁺ ions for synergistic electrochemical properties of carbon-coated NVP@C as a cathode material. X-ray diffraction analysis was performed to confirm the phase purity and crystal structure of the Nb⁵⁺ doped NVP material, which exhibited characteristic diffraction peaks that matched well with the NASICON structure. Nb⁵⁺-doped NVP@C@Nb_x materials were prepared using the sol-gel method and characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Raman and Brunauer -Emmett-Teller (BET) analysis. First-principles calculations were performed based on density functional theory. VASP and PAW methods were chosen for these calculations. GGA in the PBE framework served as the exchange-correlation functional. The results showed the NVP unit cell consisted of six NVP structural motifs, each containing octahedral VO₆ and tetrahedral PO₄ groups to form a polyanionomer [V₂(PO₄)₃] along with the c-axis direction by PO₄ groups, which had Na1(6b) and Na2(18e) sites. And PDOS revealed that after Nb doping, the d orbitals of the Nb atoms also contributed electrons that were concentrated near the Fermi surface. Additionally, the decrease in the effective mass after Nb doping indicated that the electrons could move more freely through the material, implying an enhancement of the electron mobility. The electrochemical properties of the Nb⁵⁺ doped NVP@C@Nb cathode material were evaluated through cyclic voltammetry (CV), galvanostatic charge-discharge tests, electrochemical impedance spectroscopy (EIS), and X-ray photoelectric spectroscopy (XPS). The results showed that NVP@C@Nb_0.15 achieved an initial discharge capacity as high as 114.27 mAhg^-1, with a discharge capacity of 106.38 mAhg^-1 maintained after 500 cycles at 0.5C, and the retention rate of the NVP@C@Nb_0.15 composite reached an impressive 90.22%. NVP@C@Nb_0.15 exhibited low resistance and high capacity, enabling it to create more vacancies and modulate crystal structure, ultimately enhancing the electrochemical properties of NVP. The outstanding performance can be attributed to the Nb⁵⁺-doped carbon layer, which not only improves electronic conductivity but also shortens the diffusion length of Na⁺ ions and electrons, as well as reduces volume changes in electrode materials. These preliminary results suggested that the as-obtained NVP@C@Nb_0.15 composite was a promising novel cathode electrode material for efficient sodium energy storage.