Investigating the effect of calcination temperature on the electrochemical properties of Na4MnV(PO4)3/NC@CNTs cathode materials for sodium ion batteries

Manganese-based NASICON-type Na4MnV(PO4)3 (NVMP) is used as an alternative material to the cathode materials (Na3V2(PO4)3) for sodium-ion battery (SIBs). By selecting the cheap and easily available and environmentally non-polluting Mn element to replace part of the highly toxic and expensive V element, it has the advantages of higher voltage platform, lower cost and weaker toxicity. However, the low intrinsic conductivity and poor cycling stability of NVMP severely limit its application in SIBs. In this paper, N-doped double carbon layer-coated NVMP composites (NVMP/NC@CNTs) are prepared by adding additional urea as a nitrogen source and CNTs as a carbon source using a simple and feasible sol-gel synthesis method, which improves their electronic conductivity and structural stability. On this basis, the effects of calcination temperature on the lattice structure, specific surface area, carbon disorder, Na+ diffusion coefficient (DNa+) and electrochemical properties of NVMP/NC@CNTs composites are mainly investigated. The results show that the optimal annealing temperature is 650 °C, and the NVMP/NC@CNTs material has the highest discharge capacity (105.6 mAh g−1 at 0.1C), the best rate performance (75.7 mAh g−1 at 5C), and the most stable cycling stability (79.4 %/89.3 % for 500 cycles at 1C / 5C, respectively). Even at high rate of 10C, an impressive cycling stability (77.5 % capacity retention over 1500 cycles). The sodium storage mechanism, electrochemical performance and pseudo-capacitance contribution of NVMP/NC@CNTs-650 are further investigated by kinetic analysis, GITT test, ex-situ XPS, XRD and EIS characterization. Finally, the assembled NVMP/NC@CNTs-650 || hard carbon (HC) full cell manifests a high capacity of 87.2 mAh g−1 with capacity retention of 92.5 % over 50 cycles at 1C. The results reveal that the appropriate annealing temperature can help to prepare electrochemically excellent and structurally stable NVMP composites, while the co-modification of N-doped dual-nanocarbon can produce a large number of defects and active sites, which is expected to facilitate the commercialization of SIBs.