Application Of New Carbon Nanotube Composites In Lithium Ion Batteries

It is critically important to discover new electrode materials in the development of high performance lithium ion batteries. Due to the high capacity, silicon materials have been widely studied as an ideal candidate. However, they often show an obvious volume change during the charge and discharge processes, so the resulting batteries have a limited stability. To this end, a second phase such carbon nanotube (CNT) that is explored for remarkable mechanical, electronic, and thermal properties has been extensively introduced to enhance the battery stability. Unfortunately, CNTs are randomly dispersed in the composite materials, and the charges have to transport among many boundaries in the CNT network with a low efficiency. Therefore, the improved degree is far away from the expected. This thesis has developed a novel, aligned CNT/Si composite film that shows combined excellent properties such as high tensile strength and electrical conductivity. When it is used as a cathode, the resulting battery shows high specific capacitance and cyclic stability.Highly aligned and spinnable CNT arrays are firstly synthesized by chemical vapor deposition. Aligned CNT sheets are then drawn from the array by a dry spinning process. The aligned CNT/Si composite film is finally prepared by coating silicon onto the aligned CNT sheet by electron beam deposition. The Si content can be accurately controlled by varying the deposition time. It is found that Si has been uniformly deposited on the outer surface of CNTs. Both Raman spectroscopy and transmission electron microscopy verify that the Si shows an amorphous structure.A series of lithium ion batteries are fabricated by using aligned CNT/Si composite film as a cathode, metal lithium as the anode, LB303-lithium hexafluorophosphate solution (concentration higher than99.3%) as the electrolyte, and porous polypropylene as the spacer. The impact of the composition and thickness of the composite film on the battery property has been systematically studied. It is found that a higher Si content and film thicknesses increases the specific capacity. In addition, the aligned structure of CNTs and CNT/Si core-sheath morphology can effectively provide combined advantages from both CNT and Si, and may also overcome the challenge of low capacity of CNTs and low stability of Si. The battery shows a specific capacity of over2000mAh/g with high cyclic stability.