Macroscopic Architectures Of Super-aligned Carbon Nanotube Composites: Fabrication, Properties, And Applications

Carbon nanotubes(CNTs) are novel one-dimensional nanostructures with outstanding electrical, mechanical and thermal properties, and broad range of applications. In practical applications, micro-sized CNTs and composite particles are usually required to be assembled into macroscopic architectures.Owing to the unique strong interaction among tubes, super-aligned CNTs(SACNTs) are capable of being converted into CNT yarns and films with hundreds of meters long. These yarns and films have been used in various applicaitons. In this dissertation, we are going to expand the types of the macrostructures of SACNT composites, and to study deeply about the fabrication, properties and potential applications of these composites. Here, we are going to discuss them in details:1) The buckypaper-like randomly oriented composite films can be achieved through the ultrasonication and co-deposition(USCD) method, which means that the SACNT and composite paritcles can spontaneously co-deposit to the bottom as soon as the ultrasonication stops. In these composite films, thecontent of SACNTs can be as low as 1 wt%. A continuous SACNT network can be formed in the composite films. All the composite particles are entrapped in the SACNT network, and as a result, the small amounts of SACNTs are sufficient to serve as both the conducting path and the mechanical framework throughout the composite films. These structural features give the films more practical advantanges, such as no need for binder, uniform dispersion, high conductivity and flexibility. SACNT/LiCoO2 composite films are prepared through the USCD method, and demonstrate excellent electrochemical performances with cycling stability, high rate capacity(134 mA/g at 2C) and higher energy density than tranditional electrodes for lithium ion batteries(LIBs).In addition, the USCD method is also appealling because of its suitablity for a wide range of composites, simplicity and controllability in processing, and great potential for large-scale production. The mechanism of the USCD method was also studied, and it is found that one of the necesities lies in the strong interaction and the ability of instant network formation of CNTs during ultrasonication. Many kinds of CNTs have been tried in our experiments, and SACNT is the only one that meets the requirements above and exhibits the USCD behavior.2) The three-dimensional SACNT sponges are built through ultrasonication and self-assembly process. These sponges are quite light-weight(with density of 1.3~40 mg/cm3), highly porous(porosity>99%), conductive and self-supporting. Additionally, the preparation process of these SACNT sponges is simple and completely controllable. Unlike the common preparation of ordinary CNT aerogels with polymer binder, these SACNT sponges are binder-free. In another word, the strong interaction among SACNTs can work as the “binder” thatconnects the SACNTs together into a sponge.A thin layer(<2 nm)of carbon coating could convert these non-elastic sponges into excellent “elastomer”. Besides, a simple oxidation process at 550 oC in air can also transform the hydrophobic SACNT sponges into completely hydrophilic ones.These SACNT and modified sponges are quite appealling in extensive applications, such as energy, biological engineering, adsorption, catalysis, template for other materials, and so on. In particular, the sponges with a carbon layer can be used as anode material for LIBs, and they demonstrate impressively unique electrochemical performances, such as extremely high rate capacity(a capacity retention of 58% at 50 C, compared to 0.1C), and the continuous growing capacity along with super-long cycling(such as 1500 cycles) under high rate.