N-doped dual carbon-layer-modified Na3V2(PO4)3 as a high-performance cathode material for sodium-ion batteries

Na3V2(PO4)3 (NVP) is one of the most potential cathode materials for sodium-ion batteries (SIBs), but its actual electrochemical performance is limited by large electron and ion transfer resistance, resulting in lower rate performance and cycle stability. Herein, N-doped dual-carbon decoration strategy is proposed to solve the above problems of NVP. The N-doped double carbon-wrapped NVP (NVP/NC@CNTs) is successfully synthesized via introducing highly conductive CNTs and carbonization of N-rich urea. The results indicate that the NVP/NC@10%CNTs composites exhibit a high initial reversible capacity of 106.5 mAh g−1 at 0.1C, with have high rate capability (94.8 mAh g−1 at 5C) and excellent capacity retention at high rates (98.9% capacity retention at 20C after 2000 cycles). Additionally, the tests of multiple methods (CV, EIS and GITT) exhibit significant improvement in the kinetic properties. More importantly, density functional theory (DFT) calculations confirm that N-doped carbon layers combined with CNTs can significantly reduce the energy bandwidths of the carbon layers and enhance the adsorption energy for Na+, thus increasing the electrical conductivity of the carbon layers and lowering the diffusion energy barrier for Na+. The symmetric full cells are assembled and measured its electrochemical performance. The cells exhibit good compatibility and excellent cycling stability. The excellent performance of NVP/NC@10%CNTs composites can be attributed to the defects and active site generation of N-doped double carbon co-modified NVP composites, resulting in increased electronic conductivity and diffusion rate of sodium ions.