Effects of bismuth doping on the physical and electrochemical properties of Na3V2-xBix (PO4)3/C (0 ≤ x ≤ 0.07) composite cathode materials

Na3V2-xBix(PO4)3/C (0 ≤ x ≤ 0.07) samples were prepared by a facile sol-gel method. Dependence of their physical and electrochemical properties on the amount of Bi3+ doping are investigated by X-ray diffraction (XRD) analysis, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma optical emission spectrometer (ICP-OES), impedance spectroscopy (EIS), cyclic voltammetry (CV), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (HRTEM). It is found that Bi3+ is substituted on the vanadium site without damage to the structure of Na3V2(PO4)3 compound. The Na3V1.97Bi0.03(PO4)3/C cathode materials demonstrate superior rate capability and delivered specific capacity of 116.3 and 77.4 mAh∙g−1 at charge-discharge of 0.1 C and 20 C (1 C =117.6 mA∙g−1), respectively. Circulating at a high current rate of 20 C, the discharge capacity of 67.3 mAh∙g−1 can be still reached, and the capacity can remain at 86.68% over 100 cycles. Na3V2-xBix(PO4)3/C composites possess an excellent electrochemical characteristic, which mainly ascribes to its nano-sized particles, stability in structure, and enhancement in ionic and electronic conductivity arose from Bi3+ doping.