Synergistic Effect of Carbon Encapsulation and Iron Doping Based on Metal–Organic Framework Precursor Enhances NaVPO4F Electrochemical Performance for Sodium-Ion Batteries

Sodium fluorovanadium phosphate (NaVPO4F) has long been recognized as a promising cathode material for sodium-ion batteries. However, its low conductivity has limited its practical application due to its poor rate performance and long-term stability. The study introduces a method to enhance the electrochemical performance of NaVPO4F by using the metal–organic framework (MOF) as a precursor. This method involves the incorporation of Fe-doping and MOF-derived carbon encapsulation. The synthesized NaV0.94Fe0.06PO4F/MC samples (NVPF-Fe/MC) exhibit extraordinary qualities as cathode material for sodium-ion batteries: a high capability (123.1 mAh/g at 1C), outstanding ultralong cyclability (remaining 83.0% of its capacity even after 1000 cycles at a high cycling rate of 5 C), and significantly improved rate performance. The NVPF-Fe/MC||HC full cell also exhibits excellent reversible capacity (retaining 87.7% sodium storage capacity after 100 cycles). The mesoporous carbon nanonetwork shortens the ion-electron diffusion pathway, promoting ionic-electronic conductivity and reaction kinetics. Additionally, Fe3+ doping increases the cell volume to enlarge the diffusion channel, while activating part of the V5+/V4+ (4.0 V vs Na+/Na) due to the induced effect of Fe3+, thereby improving the specific capacity and cycling stability.