Applications Of Multiwalled Carbon Nanotubes In High Energy Batteries Based On CVD Direct Grown Technique

Nickel-metal hydride(Ni-MH) battery with high energy density, long life, no memory effect, and no pollution play a significant role in the field of new energy source. Ni-MH battery electrodes usually are prepared by wet coating process. However, this technique deals with a long preparation process, and the active material is easy to loss during charge and discharge cycle, resulting in shorter cycle life and lower capacity. The aim of this work is to improve the Ni-MH battery discharge capacity, cycle life, and to reduce the battery internal resistance.Carbon nanotubes(CNTs) possess excellent mechanical, electrical, and structural properties with high specific surface areas, and can be applied to various fields. At present, there are mainly two types of methods for the fabrication of CNTs: the chemical vapor deposition method with nano membrane as catalyst and the method of attaching CNTs on the substrate with other bonding materials. However, there is a lack of natural bonding between CNTs and the substrate for above two production processes, resulting in a large contact resistance, poor conductive performance and adhesion strength. The focus of this paper is to obtain the strong adhesion between CNTs and Ni foam, and to improve the performance of Ni-MH battery.In this work, we select Ni foam as the substrate to grow CNTs directly by the CVD technique, without extra catalyst layer. The CNT-Ni foam is employed to fabricate the Ni-MH battery current collector, the positive, and negative electrodes by the dry powder roller pressing process, then the Ni-MH battery is assembled followed by the experimental investigations on the charge-discharge and electrochemical properties.Results show that CNTs can root into the Ni foam base, and interwove through the active material particles as electronic conducting bridges, increasing the mechanical strength of the electrode, improving the charge transfer ability, shortening the charge transfer path, and lowering the charge transfer resistance. In addition, CNTs can tightly fix the active material to the Ni foam substrate, slow down the loss of active material, and increase the battery internal resistance, resulting the extension of the cycle life. The battery with directly grown CNTs possesses the maximum discharge specific capacity of 360.9mAh/g, 18% higher than the battery without CNT growth, with the charge transfer resistance drop of about 37.5%. Meanwhile, the charging-discharging cycle life has been greatly improved by CNT growth and addition of PTFE, declining 10.8% only after 220 cycles. Therefore, growth of CNTs directly on Ni foam substrate by thermal CVD can greatly improve the Ni-MH battery charge-discharge properties.