An Investigation On The Synthesis And Electrochemical Performance Of Lithium Iron Phosphate Cathode Material

LiFePO₄ cathode material plays an important role in the lithium-ion battery cathode materials which have been applied in large-scale, for its high stability, high safety performance, low cost and other prominent features. But its low electron and ion conductivity lead to poor performance with large current density. By improving synthesis methods, it can solve the problem of poor conductivity of LiFePO₄ and improve the electrochemical properties of LiFePO₄. The LiFePO₄ materials were studied of surface coating, metal ion doping and multi-composite materials, and characterized by X-ray diffraction（XRD）, scanning electron microscopy（SEM）, transmission electron microscopy（TEM）, cyclic voltammetry（CV） and charge-discharge tests.The solid-state method was used to synthesis LiFePO₄ with divalent iron compounds and high-valence iron compounds, respectively. The crystal structure and morphology characterization showed that when the content of carbon was lower, the effect was better. The results of SEM and TEM showed that there were large numbers of microporous existing on the surface of materials, and the thickness of amorphous carbon layer was about 2-5 nm. According to the result of electrochemical property test, when using Fe PO4 and Fe（OH）3 as raw materials, the electrochemical performance of LiFePO₄ was better. The discharge capacity could maintain 155 m Ah·g-1 and 143.7 m Ah·g-1 after 50 cycles at the rate of 0.2 C, the capacity retention was over 98%.LiFePO₄ was prepared of small particle size by sol-gel and polyol soft chemical synthesis method. It used different solvent, in order to explore the effects on the electrochemical property of LiFePO₄. It used N,N-dimethylformamide as solvent to synthesis LiFePO₄ by sol-gel method, the morphology of material was regular, and the particle size was uniform. The discharge capacity was 128.5 m Ah·g-1 at the rate of 1 C, and the capacity was increased at high current density. LiFePO₄ was successfully prepared by polyol method using DEG as solvent, refluxing 6 h at 130 ℃ on oil bath. The material particle size was less than 100 nm, and the initial discharge capacity was 144.1 m Ah·g-1 at the rate of 0.2 C. The polyol synthesis method has good prospect for short time, low temperature and other advantages.The ion doped LiFePO₄ and（1-x）LiFePO₄·x Li Mn PO4,（1-x）LiFePO₄·x Li3V2（PO4）3 compound were prepared by sol-gel method to improve the electrochemical characterization. Proposed to use H2O2 to dissolve V2O5 powder, achieving a highly solubility of V2O5 in the precursor. The result showed that doping vanadium ions could develop the capacity of LiFePO₄ by improving electron conductivity and migration rate of Li+. The discharge capacity of LiFePO₄ with doping amount of Fe:V=0.97:0.03 was 117.2 m Ah·g-1 at the rate of 5 C, and remained 110 m Ah·g-1 after 30 cycles. When the ratio of Fe and Mn was 0.8:0.2, the initial discharge capacity of（1-x）LiFePO₄·x Li3V2（PO4）3 compound was 137.1 m Ah·g-1 of 1 C, even maintained 120 m Ah·g-1 at the rate of 10 C. Compared with ion doping, composite material had increased discharge platform and high voltage so as to enhance the overall energy density of the material.