Abstract:

Li₃V₂(PO₄)₃ and its triple-cation-doped counterpart Li_2.85Na_0.15V_1.9Al_0.1(PO₄)_2.9F_0.1 were prepared by a conventional sol-gel method. The effect of Na-Al-F co-doping on the physicochemical properties, especially the electrochemical performance of Li₃V₂(PO₄)₃, were investigated by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), Raman spectroscopy, scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS), galvanostatic charge/discharge, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). It was found that combined with surface coating from residual carbon, this triple-cation co-doping stabilizes the crystalline structure of Li₃V₂(PO₄)₃, suppresses secondary particle agglomeration, and improves the electric conductivity. Moreover, reversible deinsertion/insertion of the third lithium ion at deeper charge/discharge is enabled by such doping. As a consequence, the practical electrochemical performance of Li₃V₂(PO₄)₃ is significantly improved. The specific capacity of the doped material at a low rate of 1/9C is 172 mAh·g^-1 and it maintains 115 mAh·g^-1 at a rate of 14C, while the specific capacities of the undoped sample at 1/9C and 6C are only 141 and 98 mAh·g^-1, respectively. After 300 cycles at 1C rate, the doped material has a capacity of 118 mAh·g^-1, which is 32.6% higher than that of the undoped counterpart. It is also noteworthy that the multiplateau discharge curve of Li₃V₂(PO₄)₃ transforms to a slope-like curve, indicating the possibility of a different lithium intercalation mechanism after this co-doping.

Key words: Lithium vanadium phosphate, Doping, Sol-gel, Cathode material, Lithiumion battery, E nergy storage