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Olivine-structured LiFePO4 has attracted extensive interest as a promising cathode material for lithium ion batteries due to its appealing advantages, such as relatively high theoretical capacity, environmental benignity, good cycle stability, high safety, and inexpensive cost. However, due to the low intrinsic electronic conductivity and slow lithium ion diffusion, it is difficult to utilize the full theoretical capacity at useful rates. The conventional modification method carbon coating not only impact the volumetric energy density of LiFePO4/C more severely than the gravimetric energy density, but also hindering lithium ion diffusion and resulting in an increase of polarization. To improve the rate capability of LiFePO4 and to overcome the detrimental effects on the tap density and volumetric energy density accompanied by conventional carbon-based additives, the aim of this dissertation is to develop composite modification to synthesis LiFePO4 with graphene incorporated.(1) Ni doped LiFePO4/graphene composite cathode material by two-step reaction. The phase structure, surface morphology and electrochemical performance of LiFe0.95Ni0.05PO4/graphene were characterized by XRD, SEM, TEM, cyclic voltammetry and galvanostatic charge-discharge cycling tests, respectively. The results show that LiFe0.95Ni0.05PO4/graphene composite has excellent electrochemical performance.Its initial discharge capacities are 147.2,128.0 and 89.5mA-h/g at 0.1C,1C and 5C rates, respectively, and no obvious capacity fading is found after 20 cycles.(2) C-LiFePO4/graphene composite was successfully developed by in-situ solvothermal synthesizing with unique structure, exhibiting favorable electrochemical properties. A key to its realization is, besides the preparation of nanorod-like active particles, the use of a graphene matrix and thin carbon coating, which synergically serve as a mixed conducting nano-network, enabling optimal mixed electronic-ionic transport. Such ultra-thin dual-coating structure leads to electronic interparticle connection, but does not block the transport of lithium ions, resulting in fast Li+ mobility and lower charge-transfer resistance. At 0.1,1,10,50 and 100C, the capacities of C-LiFePO4/graphene are 162.8,140.3,109.2,77.5 and 59.0 mAh/g. Both low and high rate performances are excellent. As compared to the total electrode size, the space and proportion occupied by graphene is negligibly small, which greatly reduces the volume of conductive additive, opening up the possibility to increase substantially the energy storage density and rate capability at equal amounts of loading of conductive additives.
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