The Synthesis And Electrochemical Properties Research Of Li₃V₂（PO₄）₃/rGO As Cathode Material For Lithium Ion Battery

Lithium ion battery is considered as one of the most efficient energy storage system for its high energy density and long lifetime. Among the phosphate cathode materials of the lithium ion battery, with a theoretical specific capacity of197mAhg-1(in the potential range of3to4.8V), which is the highest value among those of the phosphate cathode materials, the monoclinic lithium vanadium phosphate (LVP) is a promising cathode material candidate for lithium ion batteries. However, in practice, the specific capacity which one can achieve for LVP is usually lower than197mAhg-1. This is because that the poor electric conductivity of LVP largely limits its power density. In recent studies, due to excellent electronic conductivity, high mechanical/electrochemical stability, graphene can improve the specific capacity and rate capability of the cathode when it is mixed with electrochemical active materials. In this paper, LVP/reduced graphene oxide (rGO) composite was synthesized via a sol-gel method followed by heat treatment. Meanwhile, Li3VO4/rGO was synthesized by the same method. The work was shown as follows:Firstly, the pure Li3V2(PO4)3sample were synthesized via a sol-gel method following heat treatment. By XRD、SEM, the result shows that the sample was single phase and the size of LVP was about400nm. According to the electrochemical tests, at the current density of0.075C (1C=132mAhg-1), in the potential range of2to4.2V, the capacity of first charge/discharge is128.2/110.9mAhg-1, and the coulombic efficiency is only86.5%. The poor electrochemical performances of the LVP is due to its low electronic conductivity, and the rapid capacity fade on cycling could be influenced by vanadium dissolution into electrolyte.In order to improve the electrochemical performances of LVP, LVP/rGO composite with different contents of3wt.%,7wt.%20wt.%and30wt.%were prepared by a sol-gel method following heat treatment. With the increase of rGO contents in the composites, the peaks of XRD for LVP broaden and decrease, and the average size of LVP particles shows a monotonous decrease with the content of rGO increasing. By electrochemical tests, the LVP/rGO composite with7wt.%delivers a specific capacity of141.6mAhg-1at the current density of0.075C (1C=132mAhg-1) in the potential range of2to4.2V, as well as an initial coulombic efficiency of more than100%. Long-term cycling for the composite at the rate of0.075C shows that the composite still delivers a high specific capacity of139.8mAh g-1(98.7%retention) after50cycles. Raman spectroscopy of LVP/rGO (7%) indicates that composite has a high ID/IG value, which proved that rGO has a high degree of disorder in carbon arrangement. XPS suggests that oxygen-containing functional groups are present in the composite with7wt.%rGO. Not only does the rGO sheet of the sample prevent the aggregation of LVP particles during charging and discharging, but also contribute to the reversible lithium ions storage, and the good conductivity of the rGO increases the electron transfer rate during charging and discharging, thus greatly enhances the electrochemical performance of LVP/rGO composites.Additionally, as a new anode material of the lithium ion battery, Li3VO4was synthesized by the same method mentioned above. Although the valence of vanadium is+5, it can be easily synthesized even in the presence of carbon and reducing atmosphere. The result of XRD and SEM shows that Li3VO4/rGO and Li3VO4are both single phase. The morphology of Li3VO4/rGO composite is better than Li3VO4, and the crystallinity of Li3VO4prepared at700℃is better than that at650℃.