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Hierarchical Material Architecture Design for Better Energy Storage

Abstract

Human civilization has been driven by energy consumption. The rapid increase in energy consumption for past decades is leading to 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. At the same time, the search for strategies that can reduce fossil-fuel consumption and decrease CO2 emission has become an essential endeavor. 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 toward 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 cost effectiveness. However, the performance of current batteries and ECs has been constrained by electrode materials and other factors. The objective of this dissertation is to develop better energy storage materials through rational architecture design of the electrode materials and electrode architectures. To achieve this goal, this dissertation work focuses on the fabrication of multifunctional architectures by integrating active materials with highly conductive scaffolds, creating a new family of high-performance energy storage materials with desired properties.

Different types of energy storage architectures were investigated to demonstrate such design concept. First, Nb2O5 nanocrystals of low dimensions were synthesized, and Nb2O5/CNTs nanocomposite electrode architecture was designed and fabricated by a physically mixing method. Compared with pure Nb2O5 materials, such architecture could create more accessible outer surface, which is very important in fast lithium storage. Synergistic effects between Nb2O5 nanocrystals and CNTs resulted in an optimal composition with the highest storage performance. Second, for better electrode conductivity and stability, an in-situ intimate growth of Nb2O5 nanocrystals on the CNT framework was also developed. Compared with physically mixed composites, such an electrode architecture showed superior cycling stability, while remaining the excellent rate performance and high specific capacitance. Third, various architectures were designed and fabricated by directly coating thin film LiMn2O4 on conductive indium-tin-oxide (ITO) glass, and by conformably coating nanocrystals on pre-formed CNT papers. Last, electrode architecture obtained by integrating nanocrystals and highly conductive graphene sheets was also designed and fabricated through an aerosol-assisted process using Li4Ti5O12 nanocrystals as a model system. The pomegranate-like architecture provides an electrode with outstanding rate capability, as well as good electrode stability. 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 active 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.

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