Study On Synthesis And Modification Of LiFePO₄as Cathode Materials For Lithium Ion Batteries

The continuous exploitation of new energy sources has become an important foundation for sustainable development of human society. With the rapid improvement of sciences and technologies, it has higher requirements on the properties of the batteries. The new-type rechargeable Li-ion battery has become the development trend and the research hot spot now, due to the merits of low costs, high capacities, no pollution and light mass etc.,. In numerous cathode materials of lithium ion battery, the olivine-structured LiFePO4are considered as one of the greatest potential candidates for power battery beacause of its high theoritical capacity, high energy density, excellent cyclic stability, high safty and environmental friendliness. But its low ionic diffusion, poor electronic conductivity and bad tap density are the obstacles to restrain its application. Therefore, this paper aims to improve the material properties via optimizing material synthesis process, carbon coating and metal ion doping.In this paper, structure, morphology and electrical performances of LiFePO4and its derivatives as cathode materials were studied by using various electrochemical methods in combination with modern techniques such as XRD, SEM, EDS, DSC-TGA, TEM, room-temperature constant-current charging and discharging, AC impedance and cyclic voltammetry.LiFePO4is synthesized by the solid state method using FeC2O4·2H2O、NH4H2PO4and Li2CO3as raw materials, and carbon coated by different content of glucose (5-20wt%). Study on the calcined temperature, the content of carbon and calcined time one by one. The optimal material synthesis process:calcined temperature is650℃, the content of carbon is15wt%, and the calcined time is9h. The product LiFePO4/C shows a high purity, good crystallinity, small particle size and homogenous size distribution. And the initial capacity of LiFePO4gets from the optimized synthesis technology discharged at0.1C is129.4mAh/g. After20cycles, the capacity retention rate of it is97.84%.On the basis of optimizing material synthesis process, remain the content of carbon unchanged, LiFePO4/(C+grapnene) which is prepared by adding some graphene and glucose, is studied on its structure, morphology and electrical performances. The optimal capacity of LiFePO4/(C+grapnene) resulted in the content of graphene is3wt%and glucose is12wt%. The initial capacity of LiFePO4/(C+grapnene) at0.1C was146.9mAh/g, showing13.52%higher specific capacity than LiFePO4/C, and the capacity retention remains98.43%after20charge-discharge cycles under the0.1C magnification.Apart from the graphene coating experiment, this paper has also studied on the electrical performances of LiFePO4, which is doped by metal ion Mg、Mn and Ti and carbon coated by glucose. This modification has improved the electrical performances of LiFePO4a lot. The initial discharge capacity of Li0.99Mg0.01Fe0.8Mn0.1Ti0.1PO4/C is154.8mAh/g, which shows19.62%higher than that of LiFePO4/C, and the capacity retention remains95.8%after20charge-discharge cycles under the0.1C magnification.