| Lithium-ion rechargeable batteries are the most promising power system that can offer a higher operative voltage and energy density being used in large scale energy storage, electric vehicles and hybrid electric vehicles. Among the components in lithium-ion batteries, the cathode material has attracted much attention because it has a significant impact on battery capacity, cycle life, safety and cost structure. LiFePO4is of particular interest for use in large size batteries, due to its intrinsic structural and chemical stability that leads to safe and long cycle life batteries. Besides, LiFePO4is composed of low cost and environmentally friend, which is an important merit for large scale applications. The main obstacles for practical applications of LiFePO4is its poor rate capability, which can be attributed to slow kinetics of lithium-ion diffusion coefficient and the poor electronic conductivity. At the same time, the graphene has been attracted a great deal of attention due to its excellent properties, and also yield unusually brilliant results in lithium ion batteries, due to its great electronic conductivity.In this paper, we prepared successfully the LiFePO4by a facile hydrothermal method that used the as-prepared Li3PO4as Li source and P source. The influence of the reactive conditions and were researched, and the electrochemical performance was discussed. On this basis, both the porous microsphere and3D-assemble nano-composite LiFePO4/graphene were prepared and studied. The mainly research achievements are as follows:1. The LiFePO4is prepared by a facile hydrothermal method that used the as-prepared Li3PO4as Li source and P source. The influence of different reactive concentrations has been studied for the morphology of Li3PO4. And using Li3PO4as raw material, the LiFePO4is prepared. The morphology and the electrochemical performance of LiFePO4that prepared by different Li3PO4and reactive concentrations is compared. It is seen that the porous microsphere structure with relatively high tap density or nanosized LiFePO4with outstanding rate capability can be synthesized at the proper reactive condition. The growth mechanism is speculated by observing the precursor before hydrothermal treatment. The specific capacity of nanosized LiFePO4is161.5mAh/g at0.1C and138.6mAh/g at2C, respectively, retaining about85.9%.2. Based on LiFePO4prepared with Li3PO4, LiFePO4/graphene porous composite is synthesized by hydrothermal method. First, Li3PO4/GO is prepared by precipitating with graphene oxide (GO). The morphology is Li3PO4microsphere wrapped by GO nanosheets, and there is a little effect on the morphology is Li3PO4in spite of adding GO. Using as-prepared Li3PO4/GO, LiFePO4/graphene is prepared by hydrothermal method. It is found that graphene coat on the surface of LiFePO4and do not affect the porous structure of LiFePO4. By measuring the electrochemical performance, it is shown that the specific capacity is enhanced from112.4mAh/g increased to148.1mAh/g at the rate of0.1C and the retention rate of specific capacity is from57.5%to72.2%at the rate of10C after compounding with rGO. After100cycles at1C, the capacity of LiFePO4/graphene drops to99%. It is explained by EIS measurement that the improved electrochemical performance is only attribute to the electronic conductivity, when graphene is added.3. Using the3D-assembly capability of graphene oxide under the hydrothermal condition, LiFePO4/graphene nanocomposite is prepared by hydrothermal method. By mix GO and the precursor of LiFePO4, the3D-assembly of GO can still happen and LiFePO4can also be prepared without impurity after hydrothermal treatment. The grain size of LiFePO4compounded with graphene is decreased to20nm, and these nanoparticles are able to uniformly load onto the surface of graphene. Compared to pristine LiFePO4, the electrochemical properties of LiFePO4/graphene are greatly improved, especially the rate capability and cyclic performance. Because of this structure, the electrochemical properties of LiFePO4/graphene are distinctly enhanced compared to LiFePO4. The specific capacity of LiFePO4/graphene is169.4mAh/g at0.1C and135.5mAh/g at10C, retaining nearly80%of the initial capacity. While it is only161.4mAh/g and108.5mAh/g for LiFePO4, which is equal to67%. After600cycles at10C, the capacity of LiFePO4/graphene drops to only92%, while LiFePO4falls to88%. The CV process in LiFePO4/graphene is short in spite of a rate of10C, which occupies only4%of the total capacity. This means that the cell made from LiFePO4/graphene can be fully charged faster and requires little CV. The reason for improvement is further demonstrated by the EIS results that show LiFePO4/graphene has a faster lithium ion diffusion rate of smaller sized particles and a higher electronic conductivity of composite. |
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