【作者】 王丽；

【导师】 毕彩丰；

【作者基本信息】 中国海洋大学 ， 应用化学， 2011， 硕士

【摘要】 锂离子电池具有高的工作电压和能量密度、长循环寿命、无记忆效应等优点成为可移动电源的首选。其中正极材料和负极材料对电池的性能影响很大。石墨烯是由sp²杂化的碳六元环组成的两维（2D）蜂窝状点阵的物质。它具有许多优异的性能比如室温下电阻较低,导电率较高等,因此在储能电池电极材料领域得到了广泛的应用。但是在锂离子电池应用方面,石墨烯也有不足之处,不可逆容量高,库伦效率低。研究表明通过N元素掺杂能够有效地提高石墨烯的电化学性能。本文用一种简单易行的方法制备了氮掺杂的石墨烯片层材料,运用XRD,SEM,XPS和电化学等方法对样品进行表征。XPS结果表明,热处理之后的石墨烯含氮百分比为2%。交流阻抗测试表明,经过氮元素掺杂的石墨烯,在5次循环之后,电荷转移阻抗降低,而且氮掺杂之后带来一些结构缺陷,产生新的储锂位点,提高了其容量和倍率性能。当电流密度为42 mA·g^-1时,氮掺杂石墨烯片层放电比容量达到900 mAh·g^-1,而石墨烯只有600 mAh·g^-1。在高电流密度下2.1 A·g^-1 （2.5 C）,氮掺杂石墨烯片层的比容量大概在250 mAh·g^-1,几乎是石墨烯容量(大约50 mAh·g^-1)的5倍。石墨烯不仅可以应用在储能活性材料方面,还可以作为高导电率的物质与其他材料进行复合,提高其它材料的电子传输性能,从而提高复合材料的电子和离子传输能力。LiFePO₄作为新一代锂离子电池正极材料,其理论比容量为170 mAh·g^-1,电压平台为3.4 V（相对于Li/Li⁺）,具有价格低廉,对环境友好,热稳定好,安全性高,循环性能优越等优点,是现在锂离子电池正极材料中研究热点。但是由于电子电导率(¹0^-9 S·cm^-1)比较低,造成倍率性能比较差。本文采用水热法结合热处理制备LiFePO₄,研究了反应物浓度、热处理温度、热处理气氛对产物性能的影响。为了提高LiFePO₄性能,将氧化石墨与LiFePO₄通过水热法原位复合,然后在高温惰性氛围中热处理得到LiFePO₄和石墨烯的复合物,并结合XRD、SEM、Raman光谱以及电化学测试等分析手段研究材料的晶体结构、表面形貌和电化学性能。实验结果表明合成LiFePO₄最佳条件是FeSO₄·7H₂O浓度为0.75 mol/L,热处理温度为600℃,热处理气氛为H2/Ar混合气。通过水热法制备复合物LiFePO₄/氧化石墨后,在以上热处理条件下制备复合材料LiFePO₄/Graphene。结果发现,加入石墨烯之后并没有影响LiFePO₄的晶体结构。在0.1 C倍率下放电比容量达到160.3 mAh·g^-1,10 C时比容量维持在81.5 mAh·g^-1。所得结果表明加入石墨烯之后能够提高LiFePO₄动力学性能和倍率性能。氮化钒（VN）作为锂电池负极材料,文献报道具有较高的理论比容量,但是由于室温电导率比较低,限制其进一步应用。加入高导电率的石墨烯,构筑混合电子和离子传输网络,是提高性能的有效途径之一。本文运用溶胶凝胶法结合高温热处理方法制备了纯相VN以及不同比例的VN与石墨烯的复合材料。实验结果表明,所有复合物的电化学性能都优于纯相VN。当VN与石墨烯质量比为7: 3时,制备的复合物电化学性能最好。在电流密度为42 mA·g^-1时,含有30%石墨烯的VN放电比容量达到410 mAh·g^-1,而纯相的VN只有110 mAh·g^-1。更多还原

【Abstract】 The lithium ion batteries have many advantages such as high voltage and energy density, long cycle life and no memory effect. It has become the prefeered mobile power. The cathodes and anodes have great effect on perfoemance of lithium ion batteries.Graphene possesses a two-dimensional （2D） hexagonal structure with carbon atoms connected by sp2 bonding. It has plenty of fascinating properties such as very low resistivity, high mobility of charge carries at room temperature, promising a wide range of potential applications in energy storage. However, graphene suffers from large irreversible capacity, low initial coulombic efficiency when investigated as anode material of lithium-ion batteries. Electrical measurements show that N-doped graphene can effectively modulate the electrical properties of graphene. In this paper, nitrogen-doped graphene nanosheets （N-GNS） were prepared by a simple method, XRD, SEM, XPS and electrochemical techniques are employed to characterize the prepared product .Result of XPS shows that the doping level of nitrogen in the graphene was 2%. The charge-transfer resistance of N-GNS electrode is found to be smaller than graphene after 5 cycles through AC impedance. The graphene after N doping offer more structural defects as active sites to store Li, thus it is expected to improve the capacity and rate capability. The N-GNS delivered a more reversible capacity of around 900 mAh·g^-1 at a current density of 42 mA·g^-1, while it was only 600 mAh·g^-1 for graphene. Highly stable capacities of over 250 mAh·g^-1 could still be obtained at higher current densities of 2.1 A·g^-1 （2.5 C）, which is approximately 5 times higher than the value of GNS electrode (around 50 mAh·g^-1).Graphene can not only used in active material for energy storage, but also can composite with other materials because of its high electric conductivity. It can improve the electron transport performance of materials, thus improve the electrons and ions transport performance of composite materials.LiFePO₄ is emerging as a promising cathode material for lithium-ion batteries because of low cost and environmental compatibility. In addition, LiFePO₄ has a large theoretical capacity of 170 mAh·g^-1, a flat discharge potential of 3.4 V versus Li/Li+,the good cycle stability, and the excellent thermal stability. However, it is difficult to attain the full capacity because of the lower electronic conductivity (¹0^-9 S·cm^-1) which leads to poor rate capability. In this paper, LiFePO₄ was prepared via hydrothermal process followed by heat treatment. The effects of synthesis conditions such as the concentration of reactants,pyrolysis temperature,sintering atmosphere on the physic-electrochemical properties of the materials prepared were investigated. To improve the performance of LiFePO₄, mixed （electron and ion） conducting LiFePO₄/Graphene composites have been prepared through a facile hydrothermal route followed by heat treatment. The structure, morphology, and electrochemical performance of materials were also studied by means of XRD,SEM,AC impedance and electrochemical techniques.The optimum conditions obtained from this work is that the concentration of FeSO4·7H2O was 0.75 mol/L and the product was heated at 600℃under H2/Ar atmosphere. LiFePO₄/Graphene composites was prepared through a facile hydrothermal route followed by heat treatment. It was found that introduction of graphene had no effect on the structure of LiFePO₄. An effective three-dimensional conducting network was formed by bridging graphene nanosheets, which can facilitate electron transport effectively. The LiFePO₄/Graphene composites exhibited a discharge capacity of 160.3 mAh·g^-1 at 0.1 C and 81.5 mAh·g^-1 at 10 C, respectively. All the results show that addition of graphene improve the kinetics and rate performance of LiFePO₄.It was reported that VN has high theoretical capacity as a anode material for lithium-ion batteries. However, it suffers from low conductivity at room temperature which limits it for further application. The addition of graphene is one of the useful methods to improve the performance of VN. In this paper, we presented a facile sol-gel process followed by heat treatment under ammonia atmosphere for preparation of a series of VN/Graphene hybrid composites.The results show that the electrochemical properties of all composites are better than pure VN. VN / Graphene （70: 30 in wt.%） reached the highest value of 410 mAh·g^-1 at a current density of 42 mA·g^-1, while it was only 110 mAh·g^-1 for pristine VN.更多还原