Carbon Nanotubes

Carbon nanotubes (CNT) are being added to the active material to improve both electrical and mechanical properties.

single wall carbon nanotubes have a diameter of around 0.5–2.0 nanometres
- ~100,000 times smaller than the width of a human hair
Current density of 4 × 10⁹ A cm⁻²
- 3 orders of magnitude greater than copper [1]
thermal conductivity > 5000 W m⁻¹ K⁻¹
- an order of magnitude greater than copper [1]

strength to weight ratio of carbon nanotubes

Takakura et al [2] show the strenght-to-weight ratio of Carbon Nanotubes vs other common materials that would be considered.

So we can see the electrical, thermal and mechanical benefits of CNT’s. Now we need to understand their application in battery cells.

The conclusions of Xiong et al [3] might be out of date, but they look at the mechanism of lithium ion intercalation and diffusion in CNTs, and the influence of different structures and morphologies when used in the anode of a Lithium battery.

Elham Shahpouri et al [4] came to the following general trends:

The influence of CNT-based additives reveals largely at high rates. To put it another way, they raise the C-rate threshold.
Adding CNT mostly raises the electron conductivity in the evaluated electrode materials; however, in some cases, it can also improve the ion conductivity. For the latter, it would improve the inherent properties.
Carboxylation of CNT enhances its effectiveness as an additive for the electrodes.
Using CNT alongside with other conductive media was usually more effective.
The following conclusions are drawn for different cathode materials:
CNTs were superior additives for LFS, LMS, LVP, and LMO electrodes. They moderately improved the properties of LFP, NCA, LCO, NCM811, and NCM523.
For LFS, increasing CNT up to 5% increased C-rate threshold up to >2C.
Regarding LFP and NCM, modifying the morphology played the main role in the properties. CNTs were effective; however, it is better to use them with other conductive additives (e.g. Super-P, Graphene, etc.). Graphene was more effective than CNTs.
Regarding LCO, adding 3% CNT showed the best properties. Adding Super-P can destroy the CNT advantage.
For LMO, adding CNT had superior effects. 1% CNT was sufficient; however, 5%CNT can increase C-rate threshold to 2C.

From our brief review of papers we see a huge range of applications of carbon nanotubes in the battery cell. The carbon nanotubes themselves are not just one structure, but a range with single and multi-wall, different chirality and doping. There are also numerous ways in which these can be incorporated into the battery materials. They can be an enabler for other materials such as silicon based anodes:

The synergistic effect of silicon for high capacity and CNTs for structural integrity and conductivity culminates in Si-based anodes that exhibit improved energy density, cycle stability, and rate performance.
The Utilization of Carbon Nanotubes in Advanced Silicon – Carbon Anode Materials for Lithium-Ion Batteries [5]

Overall carbon nanotubes can improve the performance of anodes and cathodes, but they are not a simple “just sprinkle in some CNT’s and all is better”.

This post has been built based on the support and sponsorship from: Thermo Fisher Scientific, Eatron Technologies, About:Energy and Quarto Technical Services.