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 (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.