Synthesis and Characterisation of Layered Metal Oxides/Graphene Composite for High Rate Lithium-ion Battery Cathodes

除了用作简单的导电添加剂外，石墨烯还被用于构建先进的电极结构。例如，Yoon等人报道了构建垂直排列的还原氧化石墨烯以获得更好的离子和电子导电性；Wang等人证明将石墨烯折叠成连续电极可以减少传输障碍，Modarres等人证明氧化石墨烯辅助TiO2合成可以产生紧密堆积的纳米线，提供四倍高的体积容量。因此，探索石墨烯在阴极侧的适用性，特别是其在先进电极结构构建方面的潜力是很有吸引力的。由于Ni， Mn和Co之间的协同作用，LiNi1/3Mn1/3Co1/3O2 （MNC111）可以在高度稀薄状态下提供相对较高的比容量，结构和热稳定性。在商用NMC阴极中，通常使用微米大小的由聚集的纳米颗粒组成的二次颗粒，因为它们允许更容易的混合/涂覆电极和更小的表面积，这有助于稳定颗粒。然而，这些二次粒子具有较长的Li+扩散路径和较差的粒子间电导率。此外，结构完整性不像纳米颗粒那样刚性，即在长期循环过程中容易发生晶间开裂，特别是在截止电压较高的情况下。纳米尺寸的颗粒可以提供高速率性能，但较大的表面积会导致更多的阴极电解质界面（CEI）的形成，并可能导致锰从阴极上更快地溶解。此外，纳米颗粒的低密度不可避免地会降低电极的体积容量，使其不适合商业应用。为了解决这一难题，人们提出了纳米颗粒的分层电极结构。例如，Oh等人合成的二次Li1.2Ni0.2Mn0.6O2粒子，典型直径为10mu m，由向外排列的拉长的初级粒子组成，由于减少了畴边界，从表面到核心的离子和电子传输效率都很高。此外，聚集的二次颗粒可以减少暴露在电解质中的表面积，从而限制CEI的形成量。同时解决了二次颗粒速率性能有限、一次颗粒出丝密度低和大面积CEI形成等问题。另外，Modarres等人合成了氧化石墨烯包裹的TiO2纳米线，该纳米线在材料形成过程中经历了自组装过程。这是因为按顺序排列的圆柱体的最大堆积密度为0.91，大于球体的0.74。制备的密集排列的纳米线可以用作独立电极，导致体积容量增加四倍。带着这些灵感，我的研究重点是将这两种意识形态应用于层状金属氧化物，以提高速率性能，并在石墨烯的帮助下增加锂离子电池阴极的体积容量。具体来说，具有不同镍含量的石墨烯包裹的NMC纳米棒将被合成并排列成海胆状结构，以方便离子/电子传输，或密集排列的自组织结构，以获得高容量。此外，通过共沉淀法合成的LiNi0.5Mn0.5O2颗粒将转化为纳米线，并以类似的方式密集堆积，旨在同时实现高速率性能和高体积容量。

Apart from being used as a simple conductive additive, graphene is also being used to construct advanced electrode structures. For example, Yoon et al. reported constructing vertically aligned rGO for better ionic and electronic conductivity; Wang et al. showed folding graphene into continuous electrode could reduce transport barriers and Modarres et al. demonstrated GO assisted TiO2 synthesis could result in closely-packed nanowires, delivering four times higher volumetric capacity. Therefore, it is attractive to explore on the applicability of graphene on the cathode side, especially on its potential for advanced electrode structure constructions. LiNi1/3Mn1/3Co1/3O2 (MNC111)can provide a relatively higher specific capacity, structural and thermal stabilities at highly delithiated states due to the synergistic effects between Ni, Mn and Co. In commercial NMC cathodes, micrometre-sized secondary particles consist of aggregated nanoparticles are typically used because they allow for easier mixing/coating of the electrode and lower surface area, which helps stabilising particles. However, these secondary particles suffer from a longer Li+ diffusion path and poor inter-particle electric conductivities. In addition to that, the structural integrity is not as rigid as nanoparticles, namely inter-granular cracking is likely to happen over long term cycling, especially when cut-off voltage is high. Nano sized particles could deliver high rate performance, but the large surface area causes more cathode electrolyte interface (CEI) formation and may lead to faster dissolution of Mn from the cathode. Moreover, the low tap density of nanoparticles will inevitably reduce the volumetric capacity of the electrode, rendering it unsuitable for commercial applications. Hierarchical electrode structuring of nanoparticles has been proposed to solve this dilemma. For example, Oh et al. synthesised secondary Li1.2Ni0.2Mn0.6O2 particles with a typical diameter of 10mu m consists of elongated primary particles arranged pointing outwards, which is efficient in terms of both ion and electron transport from the surface to the core because of the reduced amount of domain boundaries. In addition to that, the agglomerated secondary particles could reduce surface area exposed to electrolyte, thus limiting the amount of CEI formation. As a result, the problems of limited rate performance of secondary particles, low tap density of primary particles and large area CEI formation could be solved simultaneously.Alternatively, Modarres et al. synthesised rGO wrapped TiO2 nanowires that undergo a self-assembly process during material formation. This is because the maximum packing density of cylinders arranged in order is 0.91, which is larger than that of spheres of 0.74. The as-prepared densely packed nanowires can be used as free standing electrodes, leading to a fourfold increment in volumetric capacity. With these inspirations in mind, my research focuses on applying the two ideologies on layered metal oxides to enhance the rate performance and to increase the volumetric capacity of LIB cathodes with the help of graphene. To be specific, graphene wrapped NMC nanorods with varying Ni content will be synthesised and arranged in either urchin-like structure for facile ion/electron transport or densely packed self-organised structures for high volumetric capacity. Besides, LiNi0.5Mn0.5O2 particles synthesised from co-precipitation will be converted into nanowires and packed densely in a similar manner, aiming at achieving high rate performance and high volumetric capacity at the same time.