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Carbon Nanotubes Boost Power of Lithium Battery

A new battery demonstrated a power output 10 times higher, for its size, than what is expected of a conventional rechargeable lithium battery



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Imagine that the same rechargeable battery in your cell phone could power a device that requires 10 times the energy. That possibility may be closer than you think.

A battery created by researchers at Massachusetts Institute of Technology demonstrated an increased capacity for charge by roughly a third and a power output 10 times higher, for its size, than what is expected of a conventional rechargeable lithium battery. The results were published yesterday in Nature Nanotechnology.

The research team, led by Yang Shao-Horn, an associate professor of materials science and mechanical engineering, and Paula Hammond, professor of chemical engineering at MIT, achieved this by creating an entirely new kind of electrode -- in this case, by modifying the positive end of the conventional battery, which is called the cathode.

The collaboration began through graduate student Seung Woo Lee, studying fuel cells, who was advised by both Shao-Horn and Hammond. Lee defended his doctoral dissertation this spring.

Using commercially available carbon nanotubes -- hollow cylinders 50,000 times thinner than a human hair but composed of carbon atoms -- the team fabricated the cathode entirely out of the nanotubes put down in layers.

The large surface area of a nanotube allows it to store more charge than other types of carbon, such as graphite, but previous battery fabrication methods tended to obscure these surfaces.

Using the exposed surfaces allows more charge to be stored -- increasing capacity -- while also letting those charges migrate more easily -- increasing power.

The findings of this research challenge the conventional wisdom about what materials could be used in the cathode of a battery. It also stimulates discussion about what such powerful batteries could be used for.

Small scale experiments so far
Increased power output makes for a great capacitor as well, by efficiently storing charge and delivering that energy precisely when it is needed. Their work, Shao-Horn said, could "lead to a device with performance that bridges batteries and electrochemical capacitors."

So far, the thickest cathode the group has made for these experiments is only 3 micrometers -- 3 one-thousandths of a millimeter. This is tiny when compared to conventional lithium-ion batteries that have electrodes roughly 100 to 200 micrometers thick.

In their present form, Shao-Horn said, their cathode "could be ideal for microelectronic devices."

But these battery-capacitors are also useful in a number of other applications such as emergency power, "energy capture and power assist in cars, trucks and machinery requiring many start-stop cycles," said Shao-Horn. Successfully scaling up this design could dramatically reduce the inefficiencies in future lithium-ion batteries.

However, Shao-Horn preferred to err on the side of caution when peering into the future of this new technology, saying they are only just beginning to understand the underlying chemistry involved.

"Further work is required," said Shao-Horn, "to demonstrate that power and energy performance is maintained with thicker electrodes." A crucial next step of this research is to demonstrate an electrode with a thickness of 50 micrometers -- more than 10 times the size of what they made for their experiments.

The next phase is scaling it up
Doing so would allow the researchers to test whether the electrical properties of the carbon nanotubes can be successfully scaled up to greater and greater thicknesses. Potentially, Shao-Horn said, there is "no limit" on thickness. But in order to do this, Hammond's expertise in biomaterials will be essential.

The layer-by-layer fabrication technique used to make the 3-micrometer-thick carbon nanotube electrode described in the published paper was an extremely time-consuming process. For each layer of nanotubes, a sample had to be dipped into a solution awash with nanotubes.

Then, covered in the solution, the sample had to be left out for 15 to 20 minutes as gravity slowly pulled the nanotubes down through the liquid and onto the sample surface. This procedure had to be repeated about 400 times in order to pile up enough layers to reach a thickness of 3 micrometers.

To bring the layering process up to reasonable, commercially viable speeds, Hammond is appropriating an automatic spray technique she developed for producing layers of polymer materials.

"The spray method is 40 to 100 times faster," she said, taking only seconds to lay down each new layer of nanotubes rather than the 15 to 20 minutes it normally takes. The true test will come once much thicker electrodes are tested.

Other battery research from Shao-Horn's group has been highlighted in other ClimateWire stories.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500

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