| Summary: | Sodium–ion batteries (SIBs) are essential for large–scale energy storage attributed to the high abundance of sodium. Polyanion Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> (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) Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>/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<sup>−1</sup> at 100 mA g<sup>−1</sup> and delivers an outstanding high–rate capability of 75.3 mAh g<sup>−1</sup> at 6000 mA g<sup>−1</sup>. Moreover, it shows that the NVP@C cathode is able to display a volumetric energy density of 54 Wh L<sup>−1</sup> at 6000 mA g<sup>−1</sup> (31 Wh L<sup>−1</sup> for NVP bulk), as well as excellent cycling performance of 65.4 mAh g<sup>−1</sup> after 1000 cycles at 2000 mA g<sup>−1</sup>. Furthermore, the NVP@C exhibits remarkable reversible capabilities of 81.9 mAh g<sup>−1</sup> at a current density of 100 mA g<sup>−1</sup> and 60.2 mAh g<sup>−1</sup> at 1000 mA g<sup>−1</sup> 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.
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