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 ~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 co-doped 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 V-ions 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. [Display omitted] •Symmetric Sodium ion full cell.•K and Mg co-doped Na3V2(PO4)3.•Ultra fast charging and discharging.•Long cycle stability.