Enhancing Na-ion storage in Na₃V₂(PO₄)₃/C cathodes for sodium ion batteries through Br and N co-doping

Na₃V₂(PO₄)₃ (NVP) features a three-dimensional open structure, which allows fast migration of Na⁺ ions, and thus, NVP is a superior cathode material for sodium ion batteries. However, the application of the NVP cathode is limited by its low electronic conductivity. To overcome this limitation, we developed an NVP nanocomposite cathode by doping carbon with bromine (Br) and nitrogen (N) during the sol-gel assisted hydrothermal reaction. The Br-and N-doping leads to the formation of amorphous carbon (a-C) on the surface of NVP particles. The nanocomposite possesses multilevel structures containing both pores with different sizes and aggregated grains (200-300 nm). Since the doped N provides electrons to the carbon matrix, the electronic conductivity of the nanocomposite is enhanced. Br-Doping leads to the drop of activation energy for V⁴⁺/V⁵⁺ redox reaction. The nanocomposite cathode exhibits a high reversible capacity of 41 mA h g^-1 at 10C. Both the superior rate and cycling performances could be attributed to the synergistic effect of the multilevel structure and amorphous carbon.