| Lithium ion battery cathode material LiFePO4, which has a lot of advantage such as extensive raw material source, low cost, environment friendly, high thermal stability and safety, make its great market prospects in a variety of fields such as mobile power, particularly electric vehicles, new energy vehicles which required large power. However, low tap density, low electrical conductivity are the main obstacles for the commercial application of LiFePO4. Powder tap density is closely related to particle morphology, particle size and its distribution. The spheroidizing of cathode material particles is the best way to improve tap density and volumetric capacity. FePO4is an ideal raw material for synthesizing LiFePO4due to its structure stability, low toxicity, low cost, environmental friendliness and high safety. This paper aimed at synthesizing spherical FePO4, which can be used as the precursor of spherical LiFePO4/C to improve the electrochemical properties.In chapter3, the Fe5(PO4)4(OH)3·2H2O crystals with different morphologies were synthesized by hydrothermal method. The SEM results showed that the product was porous Fe5(PO4)4(OH)3-2H2O microsphere crystal when FeCl3·6H2O was used as iron source, H3PO4as phosphorus source, PVP as dispersant, and the molar ratio of H3PO4and FeCl3·6H2O equaled2:1. FePO4crystals obtained under heat treatment in muffle furnace at200℃for4h and then at650℃for5h also are microspheres with size ranging from2to5μm. FePO4microspheres with a particle size distribution of2～7μm can be synthesized when the NH4H2PO4was used as the phosphorus source, and the molar ratio of NH4H2PO4and FeCl3·6H2O was1:1. Quasi olive shaped FePO4with a narrow size distribution can be obtained when pH value of reaction solution was less than2, Na3PO4was used as the phosphorus source, and molar ratio of Na3PO4and FeCl3·6H2O was1:1. However, quazi spherical Fe5(PO4)4(OH)3·2H2O crystal instead of olive shaped products can be obtained when the pH value ranged from2to6. The corresponding growth mechanism of Fe5(PO4)4(OH)3·2H2O with different morphologies was discussed.In chapter4, porous FePO4·3H2O microspheres were synthesized through controlled crystallization method using FeCl3, H3PO4and NH3·H2O as raw materials. The XRD results showed that the porous microspherical FePO4·3H2O transformed from amorphous state to α-quartz crystalline phase after heating at650℃for10h. The SEM results showed that the particle sizes of the obtained porous FePO4·3H2O microsphere ranged from10to28μm. The growth mechanism of these porous FePO4·3H2O microsphere was discussed.In chapter5, the LiFePO4/C composites were prepared by solid-phase carbon thermal reduction method using porous FePO4·3H2O microspheres, LiO·H2O and glucose as raw material. The SEM results showed that the prepared LiFePO4/C composite material using porous microspherical FePO4·3H2O as the reagent was also porous microspheres with size ranging from10to28μm.The electrochemical properties of LiFePO4/C composites resulted from the FePO4precusors, which were prepared under different reaction solution concentration, ropping speed of the reaction solution and reaction time, were investigated. The results showed that porous microspherical LiFePO4/C composite material exhibited the best electrochemical properties. The initial discharge capacity was about151.8and141.4mAh/g at0.1C and0.5C, respectively. Moreover, the discharge capacity remained140.4mAh/g after50cycles at0.5C. The capacity retention rate could reach99.3%. The porous microspherical LiFePO4/C composite showed initial discharge specific capacity of125.9,116.8,101.6and69.8mAh-g-1at high current rates of0.5,1,2and5C, respectively. It had good cycle performance with the fading rate of0.9,3.7,3.1and5%after10cycles. While the rate returned to0.5C, the capacity of porous microspherical LiFePO4/C composite also are restored. The kinetic mechanism of the LiFePO4/C composite cathode material was also discussed. |
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