| As polyanion cathode materials for lithium ion batteries, lithium iron phosphate(LiFePO4) and lithium vanadium phosphate(Li3V2(PO4)3)are attractive for stable structure, excellent cycle performance and safety. However, their low electronic conductivity impedes their use as electrode materials. Therefore, it is important to improve the conductivity of these two materials. In this paper, LiFePO4/C doped with Sn,Li3V2(PO4)3/C doped with Mn were prepared by high temperature solid-state method, at the same time,2LiFePO4-Li3V2(PO4)3was synthetized, And the effects of LiFePO4-Li3V2(PO4)3doped with Ni or Mg were investigated.It was found that with the doping amount of Sn increasing, crystal structure and surface morphology of lithium iron phosphate material doped with Sn was greatly different, and the particles became more disperse and small. The electrochemical performance test show that the charge and discharge curves of lithium iron phosphate material doped with different amount of Sn had the similar shape. Sn doping had little effect on discharge capacity, but great effect on the charge-discharge efficiency and capacity retention rate. When the doping amount of Sn was0.02, the material was more outstanding than the others, with the retention rate of97.30%and the charge and discharge efficiency of96.13%after50cycles. With the CV test, it was found that the lithium iron phosphate with Sn doping had the smaller polarization than undoped ones. And the impedance of the lithium iron phosphate with doped Sn was also smaller than the undoped ones by AC impedance measurements.The effect of manganese doped in lithium vanadium phosphate material was studied in this paper. Phase Analysis by XRD shows that a small amount of Mn doped in lithium vanadium phosphate material will not affect the crystal structure. The SEM results showed that the incorporation of manganese in lithium vanadium phosphate material can reduce the particle size to a certain extent. When the doping amount of Mn was0.09, the cycling stability of lithium vanadium phosphate had the best performance. In order to achieve the purpose of improving the performance of LiFePO4and Li3V2(PO4)3at the same time, the2LiFePO4-Li3V2(PO4)3was synthesised by nickel and magnesium doped. Sintering temperature had great effect on the electrochemical properties of the material. After sintering the materials12h at700℃, it had higher capacity of148mAh·g-1, also the platform was more stable, performance was better. Comparing with electrochemical performance of composite material doped with Ni or Mg,2LiFePO4-Li3V2(PO4)3for electrochemical performance improvement when the doping amount was higher. For example, when Ni-doped concentration x was0.01and Mg-doped concentration x was0.03, their first discharge capacities were147mAh·g-1and145mAh·g-1,it has the best discharge capacity. |
