Better batteries with nano-cables

28 January 2010

Nano-sized cables made with titanium dioxide (TiO₂)-coated carbon nanotubes could hold the key to developing new high-capacity batteries, report chemists in Germany and China.

Lithium-ion batteries are in great demand for applications from laptops to hybrid cars - but the list of requirements is long. They need to be lightweight, cheap and environmentally friendly, but also store enormous charge.

High resolution transmission electron microscopy image of the titanium coated nanotubes cables © Chemistry of Materials

As lithium-ion batteries are charged, large amounts of lithium ions are held in the anode, which is typically made from graphite. When the battery is used, these ions migrate to the cathode, sending electrons through the circuit. However, graphite has a fairly low storage capacity and release rate, so finding alternatives is key to making batteries that last longer and produce more power.

Carbon nanotubes and TiO₂ have both been investigated for use as electrodes, but have been deemed impractical until now. 'Titanium dioxide on its own is totally unsuitable for electrodes,' says Joachim Maier of the Max Planck Institute for Solid State Research in Stuttgart, Germany, who collaborated on the research.

'Although it can hold lithium ions effectively, they are slow to diffuse through the structure - and it can take years to fill a millimetre-thick crystal. However, if the TiO₂ is only 10nm thick, it is filled in milliseconds,' he says.

With this in mind, Maier worked with colleagues at the Beijing National Laboratory for Molecular Sciences in China to coat carbon nanotubes with a nanoporous layer of TiO₂. The result is a crystalline solid made up from 'coaxial cables' that are perfect for trapping lithium ions. The nanotubes form a highly conductive core and act as fast-track pathways for electron transfer in the structure, making the electrodes highly conductive.

Coaxial nanocables with electronically conducting core and nanoporous sheath. The team also formed conducting 3D networks using the nanocables and carbon black (right) © Chemistry of Materials

'Because the two compounds have interfacial contact, they form a symbiotic relationship that boosts their storage ability even further,' Maier told Chemistry World. When combined, the storage capacity of TiO₂ is four times higher than usual and the nanotubes hold three times as many ions.

Unlike some other compounds that can fracture when repeatedly charged and discharged, the nanocables appeared reliable, showing almost no capacity loss after one hundred cycles.

Since the material is simple to produce and far cheaper than electrodes that are based on rare metals, the team are hoping that it can be more widely applied - perhaps for other energy storage devices such as supercapacitors.

'Not much attention has been paid for applying hybrid materials like this to lithium batteries,' says Vasant Kumar, who works on next generation batteries based on novel Li chemistry at the University of Cambridge, UK. 'I think this work could potentially open up many new opportunities that make use of the synergy between different materials.'

Lewis Brindley

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Nanotube Superbatteries

Friday, January 09, 2009

Dense films of carbon nanotubes store large amounts of energy.

By Katherine Bourzac

Pure power: Pure thin films of carbon nanotubes can store and carry large amounts of electrical charge, making them promising electrode materials. This scanning-electron-microscope image shows a film made up of 30 layers of the nanotubes on a silicone substrate. Credit: Journal of the American Chemical Society

Researchers at MIT have made pure, dense, thin films of carbon nanotubes that show promise as electrodes for higher-capacity batteries and supercapacitors. Dispensing with the additives previously used to hold such films together improved their electrical properties, including the ability to carry and store a large amount of charge.

Carbon nanotubes can carry and store more charge than other forms of carbon, in part because their nanoscale structure gives them a very large surface area. But conventional methods for making them into films leave significant gaps between individual nanotubes or require binding materials to hold them together. Both approaches reduce the films' conductivity--the ability to convey charge--and capacitance--the ability to store it.

The MIT group, led by chemical-engineering professor Paula Hammond and mechanical-engineering professor Yang Shao-Horn, made the new nanotube films using a technique called layer-by-layer assembly. First, the group creates water solutions of two kinds of nanotubes: one type has positively charged molecules bound to them, and the other has negatively charged molecules. The researchers then alternately dip a very thin substrate, such as a silicon wafer, into the two solutions. Because of the differences in their charge, the nanotubes are attracted to each other and hold together without the help of any glues. And nanotubes of similar charge repel each other while in solution, so they form thin, uniform layers with no clumping.

The resulting films can then be detached from the substrate and baked in a cloud of hydrogen to burn off the charged molecules, leaving behind a pure mat of carbon nanotubes. The films are about 70 percent nanotubes; the rest is empty space, pores that could be used to store lithium or liquid electrolytes in future battery electrodes. The films "can store a lot of energy and discharge it rapidly," says Hammond. The capacitance of the MIT films--that is, their ability to store electrical charge--is one of the highest ever measured for carbon-nanotube films, says Shao-Horn. This means that they could serve as electrodes for batteries and supercapacitors that charge quickly, have a high power output, and have a long life.

The MIT group is not the first to use the layering technique to create nanotube films. But previously, researchers using the method layered a positively charged polymer with negatively charged nanotubes, resulting in films that were only half nanotubes. No polymer can equal the electrical conductivity of carbon nanotubes, so these films' electrical properties weren't as impressive as those of Hammond and Shao-Horn. Others have made films by growing the nanotubes from the substrate up, but the resulting forest of vertically aligned nanotubes is insufficiently dense.

"I see particular importance of these findings for supercapacitors, because all-nanotube materials can potentially store a greater amount of charge," says Nicholas Kotov, a professor of chemical engineering and materials science at the University of Michigan.

In addition to their high capacitance, the nanotube films have other advantages as electrode materials, says Shao-Horn. Conventional high-energy-density electrodes are made of carbon powder held together with a binder. But particles of the binder in the surface of the electrode reduce its active area and make it difficult to modify. With carbon nanotubes, says Shao-Horn, "you have systematic control of surface chemistry." Adding charged molecules to the electrodes' surface, for example, could increase their capacitance and energy density.

"Many researchers are pursuing thin films of carbon nanotubes for diverse applications in electronics, energy storage, and other areas," says John Rogers, a professor of materials science and engineering at the University of Illinois at Champaign-Urbana. The MIT group is primarily focused on developing the films for electrochemical applications like batteries, but the layering technique is versatile. By varying the pH of the nanotube solutions and the number of layers in the films, it's possible to tailor the films' electrical properties. This is "an attractive feature of this approach," says Rogers. The technique could be used to make nanotube films for flexible electronics, for example. Kotov also sees other potential uses of the nanotube films. When immersed in liquid, the films swell. "This will be useful, because it changes both the conductivity and capacity of the material, which opens up a lot of prospects for sensing applications and smart coatings," says Kotov.

The layer-by-layer method is time consuming, however. Typical electrodes are 10 to 100 micrometers thick; those that the MIT group has made so far are only about 1 micrometer thick. But Hammond, a pioneer in layer-by-layer assembly of polymers, has developed a layer-by-layer spraying technique that should be adaptable to nanotubes. "This reduces the time it takes by an order of magnitude, which will be necessary for commercial development," says Shao-Horn.

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