Microwave (MW) Assisted Electro-Active Tempo Functionalized Single Walled Carbon Nanotube (SWNT) Electrodes for Li-Ion Batteries

The ever-increasing demand for high-performing, light weight, economical, and safe power storage for high–tech portable devices and electric vehicles leads to augmented research efforts in the field of organic or organic/hybrid materials for energy storage devices.

With the aim of performing a balance between battery (charge producing system) and capacitor (charge storing system), Carbon Nanotubes (CNTs) can work as a template that can be functionalized with stable electro-active redox group. Since both capacitors and batteries can provide limited usability, an electrode material that hosts both properties will be more rewarding for the development of battery technology. Owing to the electrical and physical properties, CNTs are challenging material for the energy storage devices.  There are some noticeable works about producing Carbon Nanotube thin electrodes to fabricate paper battery [1-3]

In the light of these investigations, rapid microwave (MW)  assisted functionalization of Single Walled Carbon Nanotubes (SWNTs) with electro-active TEMPO group was successfully performed with one step nitrene insertion reaction without using any harshly oxidative methods which are uncontrollable and destructive for  SWNT. Their role of Functionalized SWNT samples (f-SWNTs) were characterized as a host for Li storage and their performance as electro-active cathode material were investigated.

f-SWNTs were easily dispersed in solvents and they can form stable dispersions during 1 month where as non-functionalized SWNTs coagulate immediately. This can be considered as an indication of a successful surface functionalization. Depending on the degree of functionalization of SWNTs (without losing its electrical properties) their dispersibility in H₂O and NMP (N-Methyl-2-pyrrolidone) and free standing paper electrode forming possibilities increase (Fig.1). 

Output Open Circuit Voltage (O.C.V) of cells with SWNT and functionalized SWNT (f-SWNT) samples changes between 2.5-3.0V vs. Li metal. The Cyclic Voltammetry study of the TEMPO functionalized f- SWNT 1 sample vs. Li metal is seen in Fig. 2.

The constructed 2016-size button cell of f-SWNT 1 displayed an initial specific capacity of 441 mAh/g which decreased to 127 mAh/g in the second cycle of charge- discharge (Fig. 3). This large first cycle irreversible capacity loss could be attributed to various causes such as the irreversible reduction of oxygenated functional groups that may be originally present on the CNT surface as well as the attached nitroxide radical functional group to its amine oxide form. This electrochemical reduction was known to be an irreversible process in non-aqueous carbonate electrolyte systems and may be responsible for the reduction in the charge capacity. In the fifth cycle it dropped to 54 mA/g and after tenth and fifteenth cycles this value was reduced to 35 and 28 mAh/g, repectively. The reversibility of nitroxide radical group of TEMPO under basic conditions may provide an opportunity for battery systems which is under investigation.

The f-SWNT 1 was subjected to charge and discharge cycles between 3 and 4V vs. Li (Figure 4). The charge curve plateau between 3.6 and 3.7 V can be ascribed to irreversible electrochemical oxidation of nitroxide radicals to oxoammonium species. The charging capacity suddenly faded from 140 mAh/g to 20 mAh/g in 15 charge discharge cycles.  

References

1. Pushparaj V. L, Manikoth S. M., Kumar A., Murugesan S., Ci L., Vajtai R., Linhardt R. J., Nalamasu O., Ajayan P. M.,  Proceedings of the National Academy of Science USA 104, 13574-13577, 2007.Retrieved 2010-08-08.

2. Hu, L. C., J.; Yang, Y.; La Mantia, F.; Jeong, S.; Cui, Y. Proc. Natl. Acad. Sci.U.S.A. 2009, 106, 21490–21494.

3. Ng, S. H. W., J.; Guo, Z. P.; Chen, J.; Wang, G. X.; Liu, H. K. Electrochim. Acta 2005, 51, 23–28.