Dual redox reaction of V3+/V4+ and V4+/V5+ with in-situ carbonization of chitosan quaternary ammonium promoting sodium storage property and safety of Na3V2(PO4)3

Na3V2(PO4)3 (NVP) has been limited by the poor ionic and electronic conductivity. Besides, the sole redox pair of V3+/V4+ releases a relatively low capacity. Currently, a simultaneous modification strategy of Fe2+ substitution and chitosan quaternary ammonium (CHACC) in-situ carbonization is proposed to optimize NVP for the first time. Fe2+ substitution can generate a classic P-type doping effect to supply abundant hole carries, effectively enhancing the electronic conductivity. Noteworthily, a new redox reaction of V4+/V5+ is activated after Fe-doping, which releases extra reversible capacities at a high voltage platform of 3.9 V, obviously increasing the active capacity and energy density. Moreover, the reaction mechanism of reversible V4+/V5+ has been deeply explored by ex-situ XPS. Furthermore, Cl− in the CHACC infiltrates into the NVP phase to replace part of (PO4)3−, which can provide a wider migration path and more dedicated space for Na(2) type Na+, improving the ionic conductivity. Moreover, the N element in the CHACC results in vast of active sites and defects in the carbon substrate, which induces the graphitization of hydrogel carbon materials due to the distortion of lattice caused by defects. Comprehensively, the modified NVP/C-CHACC-Fe0.07 shows superior electrochemical and kinetic properties. It reveals a capacity of 121.1 mAh g−1 at 0.1 C. At 30 C and 120 C, it shows capacities of 81.2 mAh g−1 and 70.5 mAh g−1 with retention rates of 72.4 % and 71.2 % after 4000 and 7000 cycles, respectively. Innovatively, temperature monitoring system analyzes the temperature change and safety of the NVP/C-CHACC-Fe0.07 cell, demonstrating that the variate is only 0.8 °C during each cycle. And the temperature remains stable in the long-term operation, suggesting its excellent stability and safety.