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

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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.