The team, led by Mark Hersam of Northwestern, used a technique called density gradient ultracentrifugation (DGU) to separate metallic and semiconducting SWCNTs. (As-grown SWCNTs are always produced in a mix of both electronic types – typically 33% metallic and 67% semiconducting). The researchers dispersed unsorted tubes in water using two surfactants. Because the surfactant wraps around the tubes in a different way depending on their electronic type, the metallic and semiconducting tubes end up with different buoyant densities and can thus be separated using DGU.

After sorting the tubes into metallic and semiconducting batches, the team processed them into freestanding films by vacuum filtration. The films were subsequently used as the cathodes in lithium-ion half-cell batteries with the lithium metal as the anode. The researchers measured properties such as cell capacity, Coulomb efficiency and battery cycling rates of devices made from each type of tube to determine how easily each one took up lithium. These studies were corroborated with theoretical calculations.

SWCNTs improve battery performance

Hersam and colleagues found that metallic SWCNTs accommodate lithium much more efficiently than their semiconducting counterparts. Another important discovery was that, if made denser, the semiconducting SWCNT films also begin to take up lithium at levels comparable to metallic SWCNTs. This is because lithium is more easily accommodated at the junctions between tubes, says Hersam.

“This work answers some fundamental questions concerning how lithium interacts with SWCNTs, which in turn affects the performance of lithium-ion batteries that contain these carbon nanostructures,” he told nanotechweb.org. “SWCNTs improve battery performance thanks to increased charge and discharge rates as well as having the added advantage of prolonging battery life.”

The team says that it is now exploring other nanomaterials such as graphene as additives or coatings in lithium-ion battery electrodes. “By understanding the attributes and limitations of each class of material, we will be able to rationally design composites that maximize overall battery performance,” said Hersam.

The current work is detailed in ACS Nano DOI: 10.1021/nn405921t.

Further reading

"SMC" breaks energy storage records (Sep 2011)
'Medusa front' spotted in nanobatteries (Dec 2010)
Nanotubular morphology upgrades lithium-ion storage (Aug 2009)
Nanocomposite enhances cycling performance of lithium-ion batteries (Jul 2009)
Porous nanofibres power up rechargeable batteries (Apr 2009)