Carbon Nanotubes: Power Density Boosters in Electrochemical Storage Devices

Key Takeaways

• A carbon nanotube (CNT) is an allotrope of graphene used in electrochemical storage devices to improve power density.

• The properties of the CNT changes with the chirality vector. The two variants of CNTs available are single-walled CNTs ❲SWCNT❳ and multi-walled CNTs ❲MWCNT❳.

• When used as an electrode additive, carbon nanotubes enhance the power density, energy density, voltage profile, reversible capacity, and cyclability of batteries.       

A  carbon nanotube

As the portable electronics market explodes, product manufacturers are diverting their research and development teams towards obtaining energy storage devices with high energy density, power density, and cycle life. The reliability—and therefore, demand—of portable, wireless gadgets increase by extending battery life and making energy storage maintenance-free. 

Electrochemical storage devices such as batteries, fuel cells, and supercapacitors have undergone many improvements over the past two decades. Carbon nanotubes (CNTs) are a promising candidate in improving the performance of batteries, especially the Li-Ion Battery (LIB). Carbon nanotubes enhance the power density of LIBs, and remove the operational limits of the cells. CNTs have introduced state-of-the-art electrode technology into electrochemical storage cells.

 CNTs improve the performance of Li-Ion batteries, shown above.

Carbon Nanotubes as Power Density Boosters

CNTs have a proven track record of enriching battery power density. The specific electrochemical and mechanical properties of CNTs make them applicable as battery electrodes. When used as free-standing electrodes or current collectors, they have a profound impact on upgrading the specific energy density of the battery. Usually, CNTs are employed as the anode and cathode outer film, otherwise called an electrode additive.

CNTs redefine the electrode properties in batteries with low charge-transfer resistance. Typically, the electrodes in the battery will exhibit internal resistance and contact resistance with the current collectors. When a thin film of CNT is deposited on electrodes, it drops the equivalent series resistance of the electrode and catalysts the charge movement. The surface area of the electrode increases when it is coated with CNT. The magnification in the surface area leads to improvements in the energy density and ion conductivity of the battery.  Thus CNT composite electrodes amplify the energy density, power density, and cycle life of the cell, with the given volume and mass. 

The CNT layers are accountable for the sharp improvement in the stability and durability of the electrodes in batteries. The open structure and helicity of CNTs boost the electrical conductivity and capacity of the battery. The molecular structure and the properties of the CNTs are responsible for high-performance electrodes in present-day batteries. 

Assessing the Structure of CNTs

When graphene sheets are rolled up, they form CNTs. The chirality vector ❲angle of the rolling❳ of the graphene sheets is significant in determining the properties of the CNT. The armchair and zigzag CNTs are two variants with entirely different physical properties, obtained by changing the angle of hexagonal carbon-atoms in graphene. Armchair CNTs get transformed into zigzag CNTs with a slight change in the chirality vector of the graphene sheets. The chirality vector possesses the power of converting the CNT into metal or semiconductor.

CNTs exhibit strong molecular interactions due to the presence of sp2bond. It possesses a natural tendency to bind together due to the van der Waals forces. According to the constructional difference in the tubular formation, CNTs are classified into two types:

1. Single-walled CNT ❲SWCNT❳- SWCNT is the 1-D material formed by rolling the graphene into a hollow cylindrical structure. The diameter of the SWCNT is typically less than 2nm, and the length can go up to 1.5cm. The electrical conductivity of SWCNTs is in the order of 106  S/m. 

2. Multi-walled CNT ❲MWCNT❳-SWCNTs concentrically placed and interlinked to each other forms MWCNTs. The diameter of MWCNTs can go above 100nm, and the length can range from micrometers to millimeters. The electrical conductivity of SWCNTs is in the order of 105  S/m.

Multi-walled carbon nanotube.

Both SWCNTs and MWCNTs are suitable to use as additives in battery electrodes.  It is reported that the introduction of solvents, such as propylene carbonate, to the CNT additives can improve the cycling behavior of LIBs. The purity of the CNT is a crucial parameter in determining the voltage profile and reversible capacity of batteries.

Properties of CNTs

The electrical, mechanical, and thermal properties of CNTs are dependent on the direction of the graphene sheet rolling. We have already learned that the electrical conductivity of a CNT is impressively high. Other properties of CNTs that are beneficial for its application in battery systems include:

• High mechanical tensile strength- The tensile strength of a CNT is about 400 times that of steel. The chemical structure of graphene is such that its transformation to CNT can give tensile strength in the range of 50-200 GPa. The durability and stability of the CNT is the spin-offs of its longitudinal strength.

• Thermal conductivity- Due to the high thermal conductivity of CNTs, the heat dissipation of CNT additives within the electrode composite is remarkable. CNT additives supplement the safety of electrodes when compared to traditional carbon additives. 

• Aspect ratio- The aspect ratio of CNTs can reach over the value of 10,000. This property enables lower weight doping levels and improves the electrical percolation threshold when used as an electrode additive. The thin fibers or springs of CNT drawn from graphene can be considered another benefit of this property.

• Chemically stable- CNTs remain almost inert to any chemical reaction. This supports the thin film formation over battery electrodes. The inertness of CNTs make them corrosion-free.

• Stretchability-The honeycombed structure of CNTs can be compressed into bellows (resembling that from an accordion), which can be stretched inwards and outwards. With the given space, CNTs provide a provision to accommodate more layers of material. The flexibility of the CNT material offsets the cracking and rupturing of battery electrodes.

• Lightweight-CNTs are lightweight—approximately 6 times less dense than steel. Despite being lightweight, CNTs demonstrate enormous strength. The lighter, thinner version of batteries with higher capacity can be established by employing CNTs as the electrode additives.

Right now, we are witnessing the revolution set by LIBs in portable, wearable electronics and electric vehicles. CNT conductive additives seem to be the trendsetter in enhancing the electrical conductivity of anode, as well as cathode electrodes in LIBs. As we’ve discussed, Carbon nanotubes have numerous benefits when used as a  power density booster, especially in LIBs.

Utilizing CNTs in your designs can improve your power distribution. But analyzing properly, ensuring stability and reliability, and confidently modeling the power capacities of your circuit are more important than ever when working on improving your designs. This way, you’ll want to be sure you’re up to date with your power integrity and circuit modeling capacities in your design tools.