Novel Flexible Energy Storage Devices Based On Aligned Carbon Nanotubes

The advent of flexible electronics has attracted much attention. Compared with conventional electronic devices, flexible electronic products exhibit many advantages such as being flexible, lightweight, wearable and implantable. To this end, increasing interstes are attracted to develop matchable powering systems such as flexible supercapacitors and lithium-ion batteries, and it is a key to enhance their electrochemical performances. Due to the unique one-dimensional structure, high specific surface area and remarkable chemical and physical properties, carbon nanotubes (CNTs) have been widely studied as electrode materials for supercapacitors and lithium-ion batteries. However, the randomly dispersed structure of CNTs largely decreases their electrochemical properties as charges have to transport among many bundaries among CNTs with low efficiencies. In this dissertation, a class of aligned CNT films have been discovered as promising electrode materials, and novel flexible supercapacitors and lithium-ion batteries are developed with high performances.Through chemical vapor deposition method with ethylene as carbon and iron film as catalyst, spinnable CNT arrays were grown in a tube furnace, and aligned CNT films were dry-spun from the array. The aligned CNT films demonstrated high flexibility and electrical conductivities (e.g.,570 S/cm).Conducting polymers such as polyaniline and polypyrrole were electrochemically deposited onto the CNT films to produce flexible CNT/polymer composite films. The CNTs remained highly aligned in the composite films, which enables remarkable mechanical and electrical properties. The aligned CNT/polymer composite films had been then used to fabricate supercapacitors with polyvinyl alcohol/phosphoric acid as gel electrolyte. The resulting flexible supercapacitor showed specific capacitances up to 233 F/g, and they also remained almost unchanged after 1000 charge-discharge cycles. In addtion, the supercapacitor can be made into transport devices that are promising for the future electronics.A novel wire-shaped lithium-ion battery was also produced by depositing silicon onto the aligned CNT film, followed by twisting into a fiber electrode and assembly into the battery. The aligned structure and core-sheath architecture of the CNT/silicon nanotubes provide the battery with high flexibility and electrochemical performances.For instance, the battery showed a high specific capacity of 1460 mAh/g after bending for 100 cycles, and the capacity had been maintained by 80% after 20 charge-discharge cycles. The high electrochemical performance in the battery may be explained by the aligned and core-sheath structure of CNT/silicon nanotubes that can effectively accommodate the volume expansion of silicon and enhance the charge transport.