Insight into the preparation process of Fe-doped Na3V2(PO4)3@C from the aspects of particle morphology design, crystal structure modulation and carbon graphitization regulation.

The peak-loading shift function of sodium ion batteries in large-grid energy storage station confront great challenge owing to the poor rate performance of cathodes. NASICON type Na3V2(PO4)3 with stable three-dimensional structure and fast ion diffusion channels has been regarded as one of the potential candidates and was widely studied. Nevertheless, a multilevel integrated strategy to boost the performance of Na3V2(PO4)3 from crystal structure modulation, coated carbon graphitization regulation and particle morphology design is rarely reported and deserves much attention. In this study, organic ferric was used to prepare Fe-doped Na3V2(PO4)3/C cathode on account of the low cost, environmentally friendliness and the catalysis function of Fe on carbon graphitization. The DFT calculation depicts that the most stable site for Fe atom is V site and moderate substitution of Fe at V site would reduce the band gap energy from 2.19 to 1.76 eV and improve electron transfer, which is crucial for the intrinsic poor conductivity of Na3V2(PO4)3. The experimental results show that Fe element can be introduced into the bulk structure successfully, modulating relevant structural parameters. In addition, the coated carbon layer graphitization degree is also regulated due to the catalysis function of Fe. And the decomposition of organic ferric would infuse the formation of porous structure, which can promote electrolyte permeation and shorten electron/ion diffusion. Finally, the optimized Na3V1.85Fe0.15(PO4)3@C could deliver a high capacity of 103.69 mA h g-1 with 91.45% retained after 1200 cycles at 1.0 C and 94.45 mA h g-1 at 20 C. And the excellent performance is comprehensively elucidated via ex-situ XRD and pseudocapacitance characterization. The multi-function contribution of Fe-doping may provide new clue for designing porous electrode materials and a new sight on Fe doping carbon-coated material.