Fabrication of a Sandwiched Core Carbon Sphere@Na₃V₂(PO₄)₂O₂F@N-Doped Carbon Cathode for Superior Sodium-Ion Batteries

Cathode materials play a decisive role in the building of high energy density sodium-ion batteries. Among them, Na3V2(PO4)2O2F (NVPOF) is receiving active attention due to its high structural integrity, satisfactory work voltage, and outstanding energy density. However, its practical application is still impeded by the poor rate performance related to its low intrinsic electronic conductivity. Herein, Na3V2(PO4)2O2F grown on the surface of the core carbon sphere (CS@NVPOF) was initially synthesized via a one-step hydrothermal method. Subsequently, by carbonization of polydopamine, the above particles coated with a uniform N-doped carbon layer (CS@NVPOF@NC) were successfully fabricated. This sandwiched structure built by a core carbon sphere and carbon layer can significantly improve the electronic conductivity, boost sodium-ion diffusion kinetics, and mitigate volume change of the electrode upon the cycling process. As a result, the as-prepared electrode exhibits a desirable specific capacity of 119.8 mA h g–1 with an attractive capacity retention of 96% after 100 cycles at a current density of 1 C. Even at a high current density of 10 C, the capacity can retain 103.9 mA h g–1, and the capacity decay per cycle is only 0.032% during 500 cycles, indicating outstanding electrochemical performance. All the analyses demonstrate that CS@NVPOF@NC has enormous potential as an applicable cathode for sodium-ion batteries.