Human civilization relies on an abundant and sustainable supply of energy. Rapidly increasing energy consumption in past decades has resulted in a fossil-fuel shortage and ecological deterioration. Facing these challenges, humankind has been diligently seeking clean, safe and renewable energy sources, such as solar, wind, waves and tides, to offset the diminishing availability or to take place of fossil fuels. At the same time, the search for strategies to reduce fossil-fuel consumption and decrease CO2 emission, such as to replace tradition vehicles by electrical vehicles (EVs), is demanded. However, the energy harvested from renewable sources must be stored prior to its connection to electric grids or delivery to customers, and EVs need sufficient on-board power sources. These essential needs have made energy storage a critical component in the creation of sustainable society.
Among all energy storage technologies, electrochemical energy storage within batteries or electrochemical capacitors (ECs) is the most promising approach, since as-stored chemical energy can be effectively delivered as electrical energy with high energy density and power density, high efficiency, long service life and effective cost. However, the performance of current batteries and ECs are constrained by poor material properties, though great effort has been made to improve materials during the past few years. The objective of this dissertation is to address the limitation of current energy storage materials by rational architecture design according to the well-recognized principles and criteria. To achieve this goal, the research strategy is to design and fabricate multifunctional architectures by integrating distinct material structures and properties to address the limitation of traditional materials and create a new family of high-performance energy storage materials with desired properties.
Different types of energy storage architectures were investigated and compared with conventional structures to demonstrate such design concepts. First, hierarchically porous carbon particles with graphitized structures were designed and synthesized by an efficient aerosol-spray process. By comparison with commercially available activated carbon and CNTs, it was found that hierarchical pore architecture is important for providing high surface area and fast ion transport, which leads to high capacitance and high power EDLC materials. Secondly, MnO2/mesoporous carbon nanocomposites were designed. MnO2 layers with different thicknesses were deposited on mesoporous carbon scaffolds with hierarchical pore structure and the charge storage performance of the composites was correlated to MnO2 layer thickness. It was determined that a suitable thickness is critical to ensure good electronic conductivity, sufficient electrolyte diffusion and high capacitance. Thirdly, interpenetrating oxide nanowire/CNT network structures were designed and fabricated by an in situ hydrothermal reaction. The composition, CNT length, pore structure, V2O5 structure, electrode thickness and architecture are critical factors. Synergistic effects obtained between V2O5 nanowires and CNTs resulted in an optimal composition with the highest storage performance. Long CNTs led to robust flexible electrodes, while a hierarchical V2O5 structure enabled storage of both lithium and sodium ions at high rates. Thus, electrode architectures can be engineered to achieve high-rate, thick electrodes for bulk energy storage. Last, various architectures obtained through integrating nanocrystals and CNTs were designed and fabricated using ultrafine TiO2 nanocrystals as a model system. Electrodes were fabricated by directly coating thin film TiO2 on conductive Indium-Tin-Oxide (ITO) glass, by conformably coating nanocrystals on pre-formed CNT papers, or by solvation-induced assembly between nanocrystals and CNTs. It was demonstrated that thick electrodes with high charge capacity, high rate performance and cycling stability rely on functional architecture that simultaneously provides high electronic conductivity, easy ion diffusion, abundant surface actives sites and robust structure and interfaces.
The general conclusion derived from these studies is that the energy storage performance of electrode materials can be significantly improved by constructing rational architectures that provide effective ion diffusion, good electronic conductivity, fast electrode reaction, robust structure and a stable interface, which normally cannot be obtained with conventional materials. This strategy also can be extended to other devices, such as batteries and fuel cells, providing a general design platform for high performance energy materials. Further exploration in this research direction will ultimately lead to high energy, high power, and long life energy storage devices for many applications, including portable electronics, EVs and grid-scale energy storage.