K+ and Mg2+ co-doped bipolar Na3V2(PO4)3: An ultrafast electrode for symmetric sodium ion full cell

Na3V2(PO4)(3) (NVP) with a NASICON type structure is a promising electrode material to develop symmetric Na ion full cells with operating voltage of similar to 1.8 V. However, poor electronic conductivity and structural degradation during electrochemical cycles adversely affects its electrochemical performance. In this study we report about the superior electrochemical performance of novel NVP@C/RGO composite electrode material in which NVP is codoped with K+ and Mg2+ . K-ions that selectively replace the immobile Na-ions in NVP unit cells facilitate Na-ion diffusion by increasing the unit cell volume and improve its structural stability. Whereas Mg-ions replace the Vions which give rise to improved electronic conductivity by formation of mixed oxidation state of V (V3+ and V4+). We demonstrate that such bulk modification of NVP based electrodes in addition to surface modification by carbon and RGO enhances its rate capability and cycling stability. Further, we demonstrate a symmetric full cell using doped-NVP@C/RGO composite electrodes which delivers a specific capacity of 52.1 mAhg(-1) with excellent capacity retention of 95% even after 500 cycles at a rate as high as 10C with power density of 937.8 W kg(-1).