CARBON NANOTUBES IN ENERGY STORAGE AND CONVERSION APPLICATIONS

Portable and renewable energy storage technology has gained considerable attention in recent years. Lithium-ion battery (LIB) and Super capacitor (SC) are two main technologies that has been greatly studied and improvised to meet the growing challenges of energy sector. Rapid development of microelectronics and continuous miniaturization of the devices require novel LIBs and SCs with high energy densities and large power delivery capabilities. CNTs being one of the most reliable and sought after nanomaterial has been greatly utilized as electrode materials to improve the storage capacity and efficiency of LIBs and SCs.

Lithium ion Batteries

Lithium ion batteries (LIBs) have emerged as an interesting novel energy storage device for diverse applications due to its superior energy density compared to other battery technologies. Application of LIBs ranges from portable electronic devices to electric vehicles (as viable alternatives to combustion engines). Among the many rechargeable battery technologies, LIBs are low cost, safe, and have minimal side reactions while offering the best energy, voltage, capacity, and tap density. For all these reasons, extensive research has been concentrated toward the design and development of high performance electrode materials. Fig 1 shows the operating principle of LIBs. The electrical energy produced by LIBs is a result of two processes, namely charging and discharging. Li ions are transported from the cathode to the anode by a non- aqueous electrolyte during charging. The difference in lithium chemical potential of the two electrodes causes this process to occur [Sandeep et al.,2018]. When discharged, Li-ion is inserted from the electrolyte electrochemically reducing the cathode and simultaneously oxidising the anode. Thus, an electric current flows throughout an external circuitry to run an electronic device. Specific energy (Wh/kg) and Power density (Wh/L) are two parameters that express the performance of an LIB. Higher the specific energy, higher the energy content of the battery. This is enhanced by availability of large number of charge carriers per unit volume of the electrode to ensure high specific charge (Ah/kg) and high and low redox potentials at the cathode and at anode, respectively, to ensure high cell voltage. CNTs, which have a 1D tubular structure, with its enriched chirality, large surface area and high electrical and thermal conductivity, are of excellent use in electrochemical energy storage devices like LIBs. They also ensure reversible Li-intercalation and extraction without destroying the material structure, large contact area with electrolyte, and increased Li-ion insertion/removal rates through short transport pathways compared to other conventional materials. CNTs have excellent electrochemical properties. Compared to the conventionally used graphite electrodes, CNTs can store more number of Li ions in the spaces between hexagonal carbon rings. Also the diffusion rate of Li into CNT enriched electrodes is lesser compared to the graphite electrode. This results in lower power density. CNTs eliminate the use of binders on electrodes owing to its ability to grow on various current collectors and greater adhesion properties. The well-formed connection between CNTs and current collectors and high mechanical flexibility and stability, significantly increases the specific capacity and stability of binder-free electrodes. Both SWCNTs and MWCNTs can be used as LIB anodes, either by simply depositing them onto a current collector or by directly growing them onto a catalyst-pre-modified current collector. LIB anodes using SWCNT is found to have the highest reversible capacity of lithium insertion amongst carbon based materials. Apart from acting as 1D electrode materials and binders, 3D porous structured, CNT/ graphene hybrids such as CNT/GO (graphene oxide) presents good performance as LIB electrodes [Yi Li et al.,2019]. These hybrid materials are also used as both active and current collectors. CNTs/graphene hybrids can also be used as the conductive substrates to load metal oxides with high theoretical Capacity.