Study On Preparation And Modification Of LiFePo₄ With High Performance-to-price Ratio As Cathode Material For Lithium-ion Battery

LiFePO4 is considered as one of the most promising cathode materials for lithium-ion batteries, due to its advantages such as abundant raw materials, low cost, environmental friendliness, high theoretical capacity, better cycle performance and excellent thermal stability. However, its low electron conductivity and slow Li-ion diffusion result in poorer high rate capability and limits its application in lithium batteries. Therefore, improving the electron conductivity and Li-ion diffusion rate, and reducing the cost of LiFePO4 are the aim of our study. In this dissertation, LiFePO4/C and doped-LiFePO4/C were prepared by solid state reaction convenient to large-scale production and investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) and electrochemical tests, and the effects of clacination temperature, reaction time, iron source, carbon source, and the quantity of dope Mn(II) on the physical and electrochemical performance of LiFePO4/C or doped-LiFePO4/C composite cathode material for lithium-ion batteries were studied. The preparation of LiFePO4/C and doped-LiFePO4/C was optimized and exemplified for large-scale production. The cost and performance of lithium iron phosphate are the key of application as cathode material for lithium-ion batteries. In this paper, LiFePO4/C, a composite cathode material for lithium-ion battery, was prepared by a solid-state synthesis method using low-cost FeSO4·7H2O as iron source. The structure, morphology and electrochemical performance of the prepared LiFePO4/C were investigated by XRD, SEM and electrochemical tests. The results showed that the influences of calcination temperature, reaction time and carbon source on the performance of the LiFePO4/C composite cathode material are prominent. Sucrose as carbon precursor, the LiFePO4/C synthesized by calcination of raw materials at 700℃for 15 h had olivine structure and best electrochemical performance. Expensive FeC2O4?2H2O as iron source, the effects of the preparation conditions on the physical and electrochemical performance of the LiFePO4/C were studied, and the optimum conditions was obtained. The differences of the LiFePO4/C synthesized by FeSO4?7H2O and LiFePO4/C by FeC2O4?2H2O in structure, morphology and electrochemical performance were evaluated. The results showed that the capacities in the voltage range from 2.3 to 4.2V of LiFePO4/C synthesized by FeSO4?7H2O were 150, 140, 130 mAh/g at the current rates of 0.1C, 0.5C and 1C, respectively, which closed to the capacities of LiFePO₄/C synthesized by FeC2O₄·2H2O at the corresponding current rates. But the cycle performance and the capacity of the former at 5C were better than that of the latter, the capacity of the former still remained 105.2 mAh/g after 30 cycles at 5C, while the capacity of the latter was just only 95.7 mAh/g.A series of Mn-doped lithium iron phosphate LiFe₁-xMn_xPO₄ (x=0.2, 0.4, 0.6, 0.8) were prepared by high temperature solid state reaction and the effects of the quantity of doped Mn on the performance of the LiFe₁-xMn_xPO₄ were studied. The result indicated that the Mn content of the sample had less impact on morphology of the samples, and all LiFe₁-xMn_xPO₄ samples exhibited two discharge voltage plateaus 3.5 and 4.0V, and the voltage difference of the redox peaks of LiFe₁-xMn_xPO₄ is smaller than that of LiFePO₄, indicating the better electrochemical performance, and LiMn0.2Fe0.8PO₄ exhibited the best electrochemical performance among all the LiFe₁-xMn_xPO₄ samples. Additionally, LiMn0.2Fe0.8PO₄/C was synthesized by solid state reaction in optimum conditions and its electrochemical performance was studied. The results demonstrated that LiMn0.2Fe0.8PO₄/C composite exhibited higher discharge capacity, better rate capability and longer cycle life.