Double-carbon coated Na3V2(PO4)3 as a superior cathode material for Na-ion batteries

Na super ionic conductor (NASICON)-type Na₃V₂(PO₄)₃ (NVP) has been considered as a potential positive electrode material for sodium ion batteries (NIBs) owing to its high theoretical specific capacity. Nonetheless, the practical application of NVP is hindered by the intrinsically poor electronic conductivity. Herein, polydopamine-derived nitrogen-doped carbon-covered Na₃V₂(PO₄)₃/C composites (NVP/C/NC) have been prepared through a self-polymerization of dopamine on the NVP/C surface and subsequent calcination at high temperature. The as-synthesized NVP/C/NC composite cathode exhibits a high initial reversible capacity (109.2 mAh/g at 0.2C), superior rate performance (87.2 mAh/g at a rate up to 20C), and excellent cycling capability (91.2% of the initial capacity is kept after 500 cycles at 2C) in NIBs. Furthermore, compared with NVP/C electrode, the NVP/C/NC electrode presents low resistance and high sodium ions diffusion coefficient. The good performance can be ascribed to the nitrogen-doped carbon layer in improving the electronic conductivity, shortening diffusion length of Na⁺ ions and electrons, and relieving the volume changes of electrode materials. These preliminary results suggest that the as-obtained NVP/C/NC composite is a novel promising electrode material for low-cost sodium energy storage.