Synthesis Of Positive Materials Of Modified Lithium Iron Phosphate And Their Electrochemical Performance

Demands on batteries is sharply increasing in the recent years due to energy and environmental issues, and the rapid development of modern science and technology. Lithium-ion batteries (LIB) are one of the favorable portable power source because of their high energy density, long cycling life, good safety, and environmental friendliness. Intensive research is being focused on further improving the performance-price ratio of lithium-ion batteries. LiFePO4 is one of the most promising cathode materials for rechargeable lithium-ion batteries due to its abundant supply, environmentally benign, appreciable theoretical capacity, and good thermal stability; therefore, LiFePO4 shows great potential competition in application in electric vehicles. However, the poor high rate performance due to the low electronic conductivity should be improved before practice application.Olivine-type LiFePO4 materials were synthesized by solid-state reaction at high temperature; modified LiFePO4 were thereafter prepared by carbon coating and metal-ion doping techniques to improve the electrochemical performance of LiFePO4. X-Ray diffraction (XRD), scanning electron microscopy (SEM), charge-discharge cycling, cyclic voltammetry (CV), electrochemical impedance spectroscope (EIS), and fourier transform infrared spectroscopy (FTIR) techniques were employed to investigate the relationship among synthesis condition, surface physicochemical performance, and electrochemical performance.The main results and conclusions in the present research are summarized as follows:(1) Li1.05Fe(PO4)1-x(GeO3)x/C (x=0,0.02,0.05,0.08) composites were prepared using solid-state reaction at 700℃. The results show that when x equals 0.05 the obtained Li1.05Fe(P04)0.95(Ge03)0.05/C exhibits the higher discharging voltage plateau than that of the undoped sample, and shows a increased capacity of 10 mAh·g-1 at 2 C rate. Furthermore, the Ge doped sample shows more excellent cyclic performance and smaller charge-transfer impedance compared to the undoped sample.(2) Li1+xFePO4Fx/C (x=0,0.02,0.05,0.08) composites were prepared by solid-state reaction at 700℃. When x equals 0.05 the obtained Li1.05FePO4F0.05/C composite exhibits an initial capacity of 115.3 mAh g-1 at 2 C rate; by contrast, the sample LiFePO4/C shows the capacity of 96.1 mAh·g-1. The Li-ion diffusion coefficient of the F doped composite was calculated to be 7.91×10-11 cm2·s-1. SEM observation reveals that the products of Li1.05FePO4F0.05/C consist of microballs which are composed of smaller primary particle size compared to the undoped LiFePO4/C.(3) Li1+xFe1-yMnyPO4Fx/C composites were prepared by two-step solid-state reaction. The experimental results show that the micro-Li1.05Fe0.90Mn0.10PO4F0.05/C composite at 2 C rate exhibits capacities of 130.9 and 126.1 mAh g-1 at the 1st and the 30th cycle, respectively. The doped sample shows slow degenerate rate of discharging capacity at various rates; i.e., such a sample processes excellent cyclic performance.