New Carbon Nanotube Electrode Material for Li-ion Batteries Tackles the Power Performance Gap Between Electrochemical Capacitors and Batteries

Researchers at MIT have developed a new carbon nanotube electrode material for a Li-ion battery based on redox reactions of functional groups on the surfaces of the nanotubes. The electrode, which is several micrometers thick, can store lithium up to a reversible gravimetric capacity of ~200 mAh g^-1_electrode while also delivering 100 kW kg_electrode^-1 of power and providing lifetimes in excess of thousands of cycles, both of which are comparable to electrochemical capacitor electrodes.

A paper on the work, led by Associate Professor of Mechanical Engineering and Materials Science and Engineering Yang Shao-Horn, in collaboration with Bayer Chair Professor of Chemical Engineering Paula Hammond, was published online in the journal Nature Nanotechnology 20 June. The lead authors are chemical engineering student Seung Woo Lee PhD ’10 and postdoctoral researcher Naoaki Yabuuchi.

Layer-by-layer techniques are used to assemble an electrode that consists of additive-free, densely packed and functionalized multiwalled carbon nanotubes. A device using the nanotube electrode as the positive electrode and lithium titanium oxide as a negative electrode had a gravimetric energy ~5 times higher than conventional electrochemical capacitors and power delivery ~10 times higher than conventional lithium-ion batteries.

—Lee et al.

The performance can be attributed to good conduction of ions and electrons in the electrode, and efficient lithium storage on the surface of the nanotubes, the researchers said.

While such electrodes might initially find applications in small portable devices, with further research they might also lead to improved batteries for larger applications, such as in vehicles, the team suggests.

The layer-by-layer fabrication method involves alternately dipping a base material in solutions containing carbon nanotubes that have been treated with simple organic compounds that give them either a positive or negative net charge. When these layers are alternated on a surface, they bond tightly together because of the complementary charges, making a stable and durable film.

The carbon nanotubes self-assemble into a tightly bound structure that is porous at the nanometer scale. In addition, the carbon nanotubes have many oxygen groups on their surfaces, which can store a large number of lithium ions; this enables carbon nanotubes for the first time to serve as the positive electrode in lithium batteries, instead of just the negative electrode.

This electrostatic self-assembly process is important, Hammond says, because ordinarily carbon nanotubes on a surface tend to clump together in bundles, leaving fewer exposed surfaces to undergo reactions. By incorporating organic molecules on the nanotubes, they assemble in a way that “has a high degree of porosity while having a great number of nanotubes present,” she says.

The electrodes the team produced had thicknesses up to a few microns, and the improvements in energy delivery only were seen at high-power output levels. In future work, the team aims to produce thicker electrodes and extend the improved performance to low-power outputs as well, they say.

In its present form, the material might have applications for small, portable electronic devices, says Shao-Horn, but if the reported high-power capability were demonstrated in a much thicker form—with thicknesses of hundreds of microns—it might eventually be suitable for other applications such as hybrid cars.

While the electrode material was produced by alternately dipping a substrate into two different solutions—a relatively time-consuming process—Hammond suggests that the process could be modified by instead spraying the alternate layers onto a moving ribbon of material, a technique now being developed in her lab.

This could eventually open the possibility of a continuous manufacturing process that could be scaled up to high volumes for commercial production, and could also be used to produce thicker electrodes with a greater power capacity.

Funding for the work was provided by the Dupont-MIT Alliance; the US Office of Naval Research; and the MRSEC Program of the National Science Foundation.

Source: Green Car Congress