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Title: Investigation on shape-controlled synthesis of nanoscale composites and their applications
Authors: Lin, Mei
Degree: Ph.D.
Issue Date: 2015
Abstract: The decreasing availability of fossil fuels and global warming have attracted significant attention, requiring the society to move toward the development of sustainable and renewable resources. Meanwhile, the demand of energy storage systems has increased rapidly because of their essential roles played in electric vehicles or hybrid electric vehicles. The two major electrical energy storage systems are batteries and electrochemical capacitors (ECs). However, we need to further improve their performance substantially to meet the higher requirements for the next generation energy storage systems, by developing new materials with novel structures and advancing our understanding of the electrochemical interfaces at the nanoscale. Novel architectured LiFePO₄ (LFP) that consisted of ordered LFP nanocubes was prepared through a facile hydrothermal method using polyethylene glycol (PEG) as a surfactant. The micro/nanostructured LFP with various morphologies ranging from cube cluster to rugby-like structure were synthesized via controlling the pH values of the precursor. A reasonable assembly process elucidating the formation of the hierarchical structure is also provided based on the experimental results. After a combination of carbon (C) coating and reduced graphene oxide (RGO) wrapping, the obtained LFP/C/RGO composites exhibit enhanced electrochemical performance. Among as-synthesized cube-cluster-like, dumbbell-like, rod-like, and rugby-like composites, the rugby-like LFP/C/RGO reveals the best electrochemical properties with the discharge specific capacity of ~150 mA h g⁻¹ after 100 cycles and a high reversible specific capacity of 152 mA h g⁻¹ at 0.1 C (1 C=170 mA h g⁻¹ ). The prepared LFP/C/RGO composite can be a promising cathode material for high energy, low cost, and environmentally friendly lithium-ion batteries. By replacing the surfactant, corn cob-like LFP cathode material was synthesized through hydrothermal method using block copolymer (PEG-PPG-PEG) as the surfactant. The influence of pH value and reaction time on the morphology of LFP has been systematically investigated. The presence of copolymer plays an important role in the construction of the hierarchical microstructures. The electrochemical performance of the obtained LFP were evaluated by carbonation of PANI coating. Corn cob-like LFP/C exhibits the best electrochemical performance, discharge specific capacities of 120 mAh g⁻¹ after 100 cycles with capacity retention ratios of 80% at 0.1 C. Moreover, this work provides the possibility for further investigation into the shape-dependent electrochemical performance of other materials by optimizing the experimental parameters during hydrothermal synthesis. A LiMn₂O₄cathode material with nanoparticle cluster morphology was fabricated by using honeycomb ε-MnO2 nanospheres as precursor. The prepared LiMn₂O₄ powders were characterized with XRD, SEM and TEM in terms of structures and performance for lithium ion batteries. The results demonstrate that the fabricated product has a porous structure that architectured with single-crystalline spinel nanoparticles with diameters ranging between 20-50 nm. The galvanostatic charge/discharge tests show that the synthesized product delivers a reversible discharge capacity of 110 mA h g⁻¹ at 1 C rate and a discharge capacity of 115 mA h g⁻¹ at rate of 1 C after 500 cycles, revealing excellent rate capability and cyclic stability. The improved performance of LiMn2O4 nanoparticle clusters can be attributed to its special structure. The porous LiMn2O4 nanoparticle clusters are consisting of single-crystalline nanoparticles, meanwhile some parts are cross-linked. This special porous structure provides the short Li ion diffusion lengths and more reaction sites for lithium insertion/extraction, the void space from the pores accommodates the volume changes during charge/discharge.
A well-organized ε-MnO2 hollow spheres/reduced graphene oxide (ε-MnO2HS/RGO) composites have been successfully constructed via a facile fabrication process under room temperature. The ε-MnO₂ hollow spheres with the diameter of ~500 nm are grown in situ and homogeneously on both surfaces of graphene oxide (GO) sheets in an aqueous suspension. The obtained ε-MnO2HS/RGO composite and the formation mechanism have been systematically investigated, delivering a superior specific capacitance and good cycling capability. The galvanostatic charge/discharge curves show a specific capacitance of 301 F g-1 at 0.5 A g-1 (based on total mass of active materials). The hollow structures of ε-MnO2 and the crumpled RGO sheets enhance the electroactive surface area and improve electrical conductivity, which further facilitate the charge transport. Therefore, the ε-MnO₂HS/RGO composite exhibits a high capacitance of 220 F g⁻¹ at 2 A g⁻¹ (86 % retention) even after 1000 cycles, suggesting excellent cyclic performance. The prominent electrochemical performance might be attributed to the combination of the pseudo-capacitance of ε-MnO2 nanospheres with the hollow structure and good electrical conductivity of RGO sheets. This work explores a new concept in designing metal oxides/RGO composites with hollow structures applied as electrode. Hollow micro/nanostructures, exhibiting many superior physical and chemical properties, has attracted great interest in many current and emerging areas of technology. Success in each has inspired multiple variations to drive the rapid evolution of the field. For example, the void space in a hollow has been widely used to encapsulate and control release of sensitive materials. However, the fabrication of large-area self-healing hollow micro/nanostructures remains a challenge. Here we report a spray-drying approach based on a superhydrophobic substrate to synthesize a stretchable, self-healing MnO₂ hollow micro-structure. We present the real-time SEM imaging of the stretching and self-healing process. The detailed structural analysis reveals that MnO₂ hollow spheres possess sponge-like inner wall and alternate jointed hull, endowing its stretchable and self-healing characteristics. Such stretchable and self-healing hollow structure opens up a wide horizon for diverse practical usages of microcapsules.
Subjects: Energy storage.
Nanostructured materials.
Hong Kong Polytechnic University -- Dissertations
Pages: xvii , 146 pages : illustrations (some color)
Appears in Collections:Thesis

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