N-doped carbon decorated Na3V2(PO4)3@NC composite derived from melamine as cathode material for high-rate and ultralong-life sodium-ion batteries

Abstract

Sodium superionic conductor (NASICON) Na3V2(PO4)3 (NVP) is one of the most promising cathode materials for Sodium ion batteries (SIBs). However, its low electronic conductivity makes the cycle and rate performance less ideal. Increasing the electronic conductivity of NVP by carbon coating has been more and more confirmed, especially for N-doped carbon. Here, in-situ melamine-derived N-doped carbon-coated Na3V2(PO4)3@NC (NVP@NC) by hydrothermal-assisted sol-gel method is prepared. The introduction of N has no effect on the original NASICON structure of NVP@C, but the melamine addition amount affected the intensity of the diffraction peak from different NVP@NC samples. The NVP@NC shows superior cycling and rate performance. In the NVP@NC-2 sample with nC3N6H6/nNVP = 2, its discharge capacity maintains 106.82 mAh·g−1 at the 500th cycle at 0.2 C. The reversible capacities at 10 and 20 C are 90.5 and 85.0 mAh·g−1 with capacity retention of 81.2% and 76.3%, respectively. The zero-band gap of NVP@NC-2 through first-principles calculations means the ionic conductivity of NVP@C is greatly enhanced after N doping. The addition of the N element to the carbon layer contributes a pair of lone pairs of electrons to the matrix carbon and introduces certain active sites and defects. The reason for the improved electrochemical performance is that the N-doped carbon layer (NC) enables the expansion and deformation of the crystal structure to be controlled, increasing the Na+ diffusion coefficient and the electrical conductivity of NVP.