Synthesis And Electrochemical Performance Of Li₃VO₄/rGO And LiMnPO₄/C Materials For Lithium-ion Battery

Li3VO4 has recently been studied as a candidate of anode materials for lithium ion battery due to its safe potential (0.5-1.0 V vs. Li+/Li) and high theoretical capacity (394 mAh g-1). However, its low electronic conductivity, resulting in poor electrochemical performance, prevents it from being widely used. In order to improve the electrochemical performance, L13VO4 needs to be modified. Graphene, which has excellent electrical conductivity and large specific surface area, can be used to improve the electrochemical performance of Li3VO4. Compared to commercialized LiFePO4, LiMnPO4, another olivine cathode material, has the advantages of high energy density, rich manganese resources, low cost and non-toxic. However, the practical use of this material still faces some barriers, such as the low electronic conductivity. Based on this, in order to improve the electrochemical performances of Li3VO4 and LiMnPO4 materials, LisVO4/rGO and LiMnPO4/C composite materials were synthesized by the sol-gel method combined with the high temperature solid-state method in this paper. The main work was shown as follows:(1) Li3VO4/rGO composites with different contents of 5.9,11.0,23.8% rGO were prepared by the sol-gel method. They were characterized by XRD, SEM, TEM, XPS, Raman, CV and electrochemical tests. The results showed that graphene morphology did not affect the structure of Li3VO4, but can tune the particle size of LisVO4. With the increase of rGO content, the first charge/discharge capacity increased gradually, and the first coulomb efficiency decreased. Too low or too high rGO content was not conducive to the electrochemical properties of Li3VO4/rGO composites. Herein, when the rGO content of the composite was 11.0%, at 0.05 C (1 C= 394 mAh g-1 for Li3VO4) in the voltage range of 0.005-3.0 V, the first charge capacity reached 579.8 mAh g-1 which was much higher than the theoretical capacity of Li3VO4. Long-term cycling showed that the composite with 11.0%rGO still delivered a high specific capacity of 489.0 mAh g-1 (84.3% retention) at 0.05 C after 50 cycles. Compared with the pure Li3VO4, Li3VO4/rGO composites exhibited superior electrochemical performance.(2) LiMnPO4/C was prepared by the sol-gel method using citric acid as carbon source. It was investigated for the effect of calcination temperature on the structure and electrochemical performance of LiMnPO4/C. As a result, when the calcination temperature is 700℃, the LiMnPO4/C has a good crystallinity of LiMnPO4. At a current density of 0.06 C (1C=170 mAh g-1 for LiMnPO4) in the voltage of 2.2-4.5 V, the LiMnPO4/C composite delivered initial charge and discharge capacity of 146.5 and 120.2 mAh g-1, respectively, and maintained 96.7 and 93.8 mAh g-1 after 50 cycles.