Improved structural stability and electrochemical performance of Na3V2 (PO4)3 cathode material by Cr doping

Cr-doped sodium vanadium phosphate (NVP) in the form of Na3V2-xCrx(PO4)(3) (x = 0, 0.02, 0.04, 0.08, 0.10) is synthesized via a facile sol-gel route as cathode materials for sodium ion batteries. The structure and morphology of these materials are systematically characterized by x-ray diffraction (XRD), Fourier-infrared spectra (FT-IR), and scanning electron microscope (SEM). XRD analysis reveals that with the increasing amount of Cr, the crystallographic parameters show a descending trend. Electrochemical tests show that the cycle stability and the specific capacity of the sodium ion batteries can be significantly improved by doping Cr into NVP. Among all the Cr-doped cathode materials, Na3V1.92Cr0.08(PO4)(3) achieves the highest capacity of 112.2 mAh g(-1) and the capacity retention is 97.2 % after 50 cycles. Electrochemical impedance spectroscopy measurements demonstrate that Cr doping is an effective method to reduce the contact resistance of interparticles by suppressing irreversible phase transformation at low sodium contents.