Fe-Doped\nNaVPO<sub>4</sub>F/C Compounds for Sodium-Ion\nBattery Cathodes: Electrochemical Performance and Analysis of Kinetic\nProperties

NASICON-type NaVPO<sub>4</sub>F (NVPF)\nhas received much\nattention\nfrom researchers due to its open three-dimensional framework and high\ntheoretical capacity. However, the limited internal electronic conductivity\nand energy density hinder the widespread commercialization of NVPF.\nHerein, a series of NaV<sub>1–<i>x</i></sub>Fe<sub><i>x</i></sub>PO<sub>4</sub>F/C (<i>x</i> = 0,\n0.04, 0.06, and 0.08) was constructed via a sol–gel method\nby introducing a partial amount of Fe<sup>3+</sup> into NVPF. The\nintroduction of Fe<sup>3+</sup> doping effectively enhanced the structural\nstability of the material, thereby facilitating rapid Na<sup>+</sup> intercalation/deintercalation and leading to improved electrochemical\nproperties. The results show that NVPF–Fe<sub>0.06</sub> has\nexcellent electrochemical performance, with a discharge specific capacity\nof 133.8 mAh g<sup>–1</sup> at 0.2 C and a capacity retention\nof 94.5% after 100 charge/discharge cycles. Even after 1000 cycles\nat 5 C, it still has a high capacity retention of 86.2%. Additionally,\nthe de-embedding mechanism of the NVPF–Fe<sub>0.06</sub> electrode\nmaterial is revealed by <i>ex situ</i> X-ray diffraction\nand <i>in situ</i> electrochemical impedance spectroscopy\nresults. More importantly, the specific discharge capacity of assembled\n HC//NVPF–Fe<sub>0.06</sub> full cell reached 112.1 mAh g<sup>–1</sup> at 5 C. This work explores a new avenue to facilitate\nthe commercialization of polyanionic cathode materials.