Na3V2(PO4)3@C core–shell nanocomposites for rechargeable sodium-ion batteries

Na3V2(PO4)3 (NVP) is an attractive cathode material for sodium ion batteries due to its high theoretical energy density and stable three-dimensional (3D) NASICON structure. In this paper, a NVP@C core–shell nanocomposite has been synthesized through a hydrothermal assisted sol–gel method. Ascorbic acid and polyethylene glycol 400 (PEG-400) were synergistically used to control the particle growth and provide the surface coating of conductive carbon. The as-prepared nanocomposite was composed of a nanosized Na3V2(PO4)3 core with a typical size of ∼40 nm and a uniformly amorphous carbon shell with the thickness of a few nanometers. The electrode performance of the NVP@C core–shell nanocomposite as cathode for sodium ion batteries is investigated and compared with that of bare NVP and NVP/C. Among the samples examined, the NVP@C nanocomposite showed the best cycle life and rate capability. It rendered an initial capacity of 104.3 mA h g−1 at 0.5 C and 94.9 mA h g−1 at 5 C with a remarkable capacity retention of 96.1% after 700 cycles. Moreover, a full cell using the as-prepared nanocomposite as both the cathode and the anode active material has been successfully built, showing a reversible capacity of 90.9 mA h g−1 at 2 C with an output voltage of about 1.7 V and a specific energy density of about 154.5 W h kg−1. The enhanced electrode performance is attributed to the combination of particle downsizing and carbon coating, which can favor the migration of both electrons and ions.