【作者】 舒叶；

【导师】 马晓华；

【作者基本信息】 复旦大学 ， 材料学， 2011， 硕士

【摘要】 橄榄石结构的LiFePO4因其较高比容量、对环境友好、循环性能理想、安全性能好、成本低廉等优点而成为最具开发和应用潜力的新一代锂离子电池正极材料。然而,低的锂离子扩散系数和电子电导率,使得LiFePO4在大倍率充放电时比容量迅速下降,而阻碍了其进一步实用化。而传统的改性方法碳包覆不仅会降低正极材料的体积能量密度,而且也会阻碍锂离子的迁移与扩散。因此,为了提高LiFePO4的倍率性能、克服碳包覆给LiFePO4的能量密度所带来的负面影响,本文在引入石墨烯的基础上对于LiFePO4进行复合改性。(一)利用两步法合成了掺杂Ni的磷酸铁锂/石墨烯复合材料,并用XRD、SEM和TEM等手段对LiFePO4石墨烯的物相结构和形貌进行表征,用循环伏安法与恒流循环充放电测试样品的电化学性能。研究结果表明,合成的LiFe0.95Ni0.05PO4/石墨烯复合材料具有优异的电化学性能,该样品在0.1C,1C和5C倍率下的首次放电比容量分别为147.2,128.0和89.5mA-h/g,循环20次后的放电比容量均无明显衰减。(二)通过溶剂热原位合成法制备了C-LiFePO4/石墨烯复合材料,电化学测试表明这种正极材料具有良好的电化学性能,尤其是倍率性能十分理想。这是由于C-LiFePO4呈纳米棒状结构,石墨烯片层和热解碳发挥协同作用,两者共同构成的导电网络为LiFePO4提供了良好的电子传输和离子散的通道,不但提高了LiFePO4的导电性能,也使得锂离子能够在活性材料中快速迁移。C-LiFePO4/石墨烯复合材料在0.1,1,10,50和100C充放电倍率下的可逆比容量分别为162.8,140.3,109.2,77.5和59.0 mAh/g。本实验仅仅加入了很少量的石墨烯,所以也不会影响复合材料的体积能量密度。更多还原

【Abstract】 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.更多还原