One-Step Synthesis of Three-Dimensional Na₃V₂(PO₄)₃/Carbon Frameworks as Promising Sodium-Ion Battery Cathode

Sodium-ion batteries (SIBs) are essential for large-scale energy storage attributed to the high abundance of sodium. Polyanion Na₃V₂(PO₄)₃ (NVP) is a dominant cathode candidate for SIBs because of its high-voltage and sodium superionic conductor (NASICON) framework. However, the electrochemical performance of NVP is hindered by the inherently poor electronic conductivity, especially for extreme fast charging and long-duration cycling. Herein, we develop a facile one-step in-situ polycondensation method to synthesize the three-dimensional (3D) Na₃V₂(PO₄)₃/holey-carbon frameworks (NVP@C) by using melamine as carbon source. In this architecture, NVP crystals intergrown with the 3D holey-carbon frameworks provide rapid transport pathways for ion/electron transmission to increase the ultrahigh rate ability and cycle capability. Consequently, the NVP@C cathode possesses a high reversible capacity of 113.9 mAh g^-1 at 100 mA g^-1 and delivers an outstanding high-rate capability of 75.3 mAh g^-1 at 6000 mA g^-1. Moreover, it shows that the NVP@C cathode is able to display a volumetric energy density of 54 Wh L^-1 at 6000 mA g^-1 (31 Wh L^-1 for NVP bulk), as well as excellent cycling performance of 65.4 mAh g^-1 after 1000 cycles at 2000 mA g^-1. Furthermore, the NVP@C exhibits remarkable reversible capabilities of 81.9 mAh g^-1 at a current density of 100 mA g^-1 and 60.2 mAh g^-1 at 1000 mA g^-1 even at a low temperature of -15 °C. The structure of porous carbon frameworks combined with single crystal materials by in-situ polycondensation offers general guidelines for the design of sodium, lithium and potassium energy storage materials.