In this study, polyacrylonitrile and boric acid are used as the nitrogen source and boron source, respectively. The Na₃V₂(PO₄)₂F₃ cathode material with a carbon layer coated by nitrogen and boron was successfully synthesized by using a mechanical ball milling assisted sol–gel method (represented as NVPF@NBC). Simultaneously, Na₃V₂(PO₄)₂F₃ materials with single-atom-doped carbon have been successfully prepared in this study by using the same method. Among the above materials, NVPF@NBC exhibits appreciable electrochemical performance because of the synergistic effect of diatoms in carbon. NVPF@NBC shows an outstanding specific discharge capacity of 127.2 mA h g⁻¹ at 0.2C, and the capacity retention after 100 cycles is 97.6%. In particular, NVPF@NBC exhibits a discharge specific capacity of 106.3 mA h g⁻¹ at 5C, and the capacity retention after 500 cycles is 90.59%. The X-ray diffraction analysis of the synthesized materials shows that the NVPF@NBC material has good crystallinity, indicating that the diatoms of N and B do not change the crystal structure of the material. TEM shows that the NVPF@NBC material exhibits uniform distribution of particles and a uniform carbon coating of about 4 nm. Nitrogen–boron co-doping does not influence the valence state of V³⁺ in the material. The results indicate that the co-doping of nitrogen and boron reinforces the crystallinity of the material and strengthens the sodium storage performance of the material, thereby greatly ameliorating the electrochemical performance of the material.