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磷酸铁锂正极材料的合成及其聚合物包覆改性研究
【作者】 瞿毅；
【导师】 唐新村；
【作者基本信息】 中南大学 ， 高分子化学与物理， 2011， 硕士
【摘要】 锂离子电池由于具有高电压、高能量密度、循环性能好、环保、无记忆效应而受到人们的青睐。橄榄石磷酸铁锂由于其突出的循环性能优良,价格低廉、原材料来源广泛而被认为是最有开发前景和潜力的锂离子正极材料。但是较低的导电性和振实密度阻碍了其商业化开发进程。研究工作者在这两方面进行了大量的工作来提高其导电性和振实密度。本文针对上述提出的问题,开展了如下工作：(1)采用溶剂热法分别以乙醇、乙二醇以及丙三醇/水为溶剂合成了橄榄石结构的磷酸亚铁锂(LiFeP04)。运用XRD、SEM、FTIR等手段,对产物晶体结构、颗粒形貌和表面微观结构进行表征,详细探讨了溶剂热合成LiFePO4时不同溶剂对产物形貌和结构的影响。运用恒流充放电测试和循环伏安方法对产物的电化学性能进行研究。结果表明：以乙二醇为溶剂合成的LiFePO4呈均匀片状结构,具有尺寸小,厚度薄的特点,这种结构缩短了锂离子的扩散距离,有利于电化学性能的提高,其倍率放电比容量达到161.7mAh·g-1,1C倍率放电时容量仍保持132.6mAh·g-1,0.1C倍率50次循环后容量衰减仅为1.98%。(2)通过简单易行的低温溶剂热法合成了由纳米片自组装而成、具有分级结构的球形磷酸铁锂颗粒。制备得到的球形产物直径大小在8um左右,由20nm左右厚的纳米薄片按特定的方式紧密组装而成。这种分级结构的球形磷酸铁锂的振实密度能达到1.6g·cm-3,同时由于一次颗粒尺寸较小,有利于锂离子的嵌入和脱出,从而倍率容量能得到有效改善。在制备过程中尿素和柠檬酸的加入对纳米片状的形成和分级结构的组装有着重要意义。通过时间单因素实验的结果,我们提出了一个合理的分级结构形成组装机理图。通过SEM、TEM、XRD、BET比表面积测试和拉曼光谱对其结构和性能进行了进一步表征。充放电结果显示制得的材料具有较高的可循环容量(10C放电倍率下达到93.6 mAh·g-1)和优异的循环性能。(3)采用原位氧化聚合法将吡咯直接原位聚合包覆到磷酸铁锂颗粒表面。在原位氧化聚合过程中,加入SDBS以提高聚吡咯包覆的均匀性改善包覆效果,提高材料的整体导电性。LiFePO4、PPy-LiFePO4复合材料、PPy/SDBS-LiFePO4都采用XRD、SEM、TG进行表征。采用恒流充放电和交流阻抗对样品电化学性能进行测试。结果表明,在SDBS加入的情况下,吡咯包覆的磷酸铁锂复合材料表现出良好的动力学性能,并在O.1C倍率放电下容量达到142.7 mAh·g-1。交流阻抗显示,SDBS加入后,吡咯的包覆能更有效地降低电荷转移电阻,这是由于SDBS可以使聚吡咯的包覆更均匀,同时还起到了掺杂提高导电性的作用。
【Abstract】 Lithium ion batteries are favorable because of the properties of high voltage, high energy density, long cycling life, little local action, non-memory effect and pollution-free. Olivine LiFePO4 is acknowledged to be the most promising cathode material in HEV for its high theoretical capacity, brillianit cyele stability, cost effective raw materials and non-toxicity. But the low eleetrical conduetivity and low tap-density obstructs its’proeess of commereialization. To cope with this problem, researchers try to improve the eleetrochemical properties and enhance the tap-density of LiFePO4 powders. Aiming at solving the problem mentioned above, we started works as the follows:(1) The olivine lithium ion phosphate (LiFePO4) was prepared via solvothermal reaction using ethanol, ethylene glycol(EG), and glycerol-water as solvents respectively. The crystalline structure, particle morphology, and surface microstructure were characterized by high-energy synchrotron XRD, SEM and FTIR spectroscopy. The effects of different solvents on the morphologies and structurtes were investigated in detail. The electrochemical properties were also investigated by charge/discharge test, cyclic voltammetry (CV). The results showed that LiFePO4 nanosheet obtained by using EG as solvent has smaller size, thin features, such a structure reduced the lithium ion diffusion distance and favors the improvement of electrochemical performance. It can deliver a initial discharge capacities of 161.5mAh·g-1 at 0.1 C,132.6mAh·g-1 at 1C and the cycling capacity retention rate reaches 98.02% over 50 cycles at 0.1C.(2) LiFePO4 sphere with hierarchical microstructure self-assembled by nanoplates has been successfully synthesized via a low-temperature solvothermal reaction followed by high-temperature treatment. These resulting sphere show a uniform size distribution of～8um and are hierarchically constructed with two-dimensional nanoplates with～20nm thicknesses, while these tiny plates are densely aggregated in an ordered fashion. The hierarchical structure gives a relatively high tap density of 1.6g.cm-3, and simultaneously, the primary nanoplates could provide a huge electrochemically available surface for enhancing the rate capability of the lithium insertion/deinsertion reaction.The presence of urea and citric play an important role in the formation of nanoplates and construction of hierarchically self-assembled microsphere. A reasonable formation mechanism is proposed on the basis of the result of time-dependent experiments. The materials’ physical properties were further characterized by SEM, TEM, XRD, BET and Raman spectroscopy. The charge-discharge test showed that the hierarchical microspheres as a cathode-active material demonstrated high reversible capacity (93.6mAh·g-1 at 10C) and excellent cycle stability.(3) A simple chemical oxidative polymerization of pyrrole directly onto the surface of LiFePO4 particles was applied to the synthesis of polypyrrole-LiFePO4 (PPy-LiFePO4) powder. The LiFePO4 sample without carbon coating was synthesized by a solvothermal method. The dodecyl benzenesulfonic acid, sodium salt (SDBS) was used as additive during PPy polymerization for improving the homogeneous distribution of the coating and conductivity of PPy-LiFePO4. Physical properties of resulting LiFePO4, PPy-LiFePO4 and PPy/SDBS-LiFePO4 were characterized by XRD, SEM, TG. The electrochemical behavior of the samples was examined against lithium counter electrode by galvanostatic charge/discharge measurements. Carbon-free LiFePO4 coated with PPy/SDBS hybrid film exhibited good electrode kinetics and a stable discharge capacity of 142.7mAh·g-1 at 0.1C. Impedance measurements showed that the PPy/SDBS coating decreased the charge-transfer resistance of the corresponding LiFePO4 cathode material very effectively, which was attributed to a homogeneous coating and doped effction.