A comparative structural and electrochemical study of monoclinic Li_3V_2(PO_4)_3/C and rhombohedral Li_(2.5)Na_(0.5)V_((2-2x/3))Ni_x(PO_4)_3/C

摘要

In order to synthesize pure derivative of rhombohedral Li_3V_2(PO_4)_3 (LVP), lithium-ion batteries materials Li_(2.5)Na_(0.5)V_((2-2x/3))Ni_x(PO_4)_3/C (x = 0.03, 0.06, 0.09) and its control, monoclinic Li_3V_2(PO_4)_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 Li_(2.5)Na_(0.5)V_((2-2x/3))Ni_x(PO_4)_3/C are in good agreement with that of rhombohedral LVP, which indicates that the Na~+-Ni~(2+) 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 Li_(2.5)Na_(0.5)V_(2-2x/3)Ni_x(PO_4)_3/C, a specific discharge capacity of 108mAhg~(-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.