A comparative structural and electrochemical study of monoclinic Li3V2(PO4)3/C and rhombohedral Li2.5Na0.5V(2−2x/3)Nix(PO4)3/C

In order to synthesize pure derivative of rhombohedral Li3V2(PO4)3 (LVP), lithium-ion batteries materials Li2.5Na0.5V(2−2x/3)Nix(PO4)3/C (x = 0.03, 0.06, 0.09) and its control, monoclinic Li3V2(PO4)3/C (LVP/C), were prepared by sol–gel method. The samples were investigated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) spectroscopy, scanning electron microscopy (SEM), Raman spectroscopy, and electrochemical methods. The XRD patterns of Li2.5Na0.5V(2−2x/3)Nix(PO4)3/C are in good agreement with that of rhombohedral LVP, which indicates that the Na+–Ni2+ composite doping can change the structure of monoclinic LVP. All the composite doping samples displayed a single flat plateau at 3.7 V in the charge/discharge voltage profile, which is caused by transformation of multi-phase mechanism to single-phase mechanism. For Li2.5Na0.5V1.98Ni0.03(PO4)3/C, a specific discharge capacity of 108 mAh g−1 was achieved at a 0.5 C charge rate and a 1 C discharge rate, and a 99.0% retention rate of the initial capacity was obtained after 50 cycles.