Nanotubes pack power

By Chhavi Sachdev, Technology Research News

While there may be no way to keep batteries going forever, researchers at the University of North Carolina have shown they can extend battery life by replacing a graphite electrode in the common rechargeable battery with a related carbon form: the nanotube.

Most portable electronic devices, including cell phones and laptops, draw their power from lightweight, rechargeable lithium-ion batteries. Each battery has a graphite electrode and a metal oxide electrode. The charge stored by the battery is released when lithium ions move from one electrode to the other.

The researchers found a way to increase the amount of charge a battery can hold and, consequently, its lifespan, by replacing the carbon electrode with single-walled carbon nanotubes. Nanotubes are microscopic, rolled-up sheets of carbon atoms.

"When processed right, single-wall carbon nanotubes have twice the storage capacity [of] the graphite electrode," said Otto Zhou, an associate professor of physics and materials science at the University of North Carolina at Chapel Hill.

Naturally formed nanotubes are cylindrical and closed at either end. The researchers tested the storage capacities of both open-ended and closed-end single-walled carbon nanotubes.

The closed-end nanotubes were 1.4 nanometers in diameter and 10,000 nanometers long. The open-ended nanotubes were shortened to either 500 or 4,000 nanometers. A nanometer is a millionth of a millimeter; a red blood cell measures about 5,000 nanometers across.

Closed-end nanotubes were able to hold the same amount of charge as graphite electrodes, but the shorter, open-ended nanotubes held twice as much. "Open-end single-walled nanotubes can store, reversibly, twice the amount of lithium than the regular graphite electrode and closed-end single-walled nanotubes," said Zhou.

A possible reason for the higher capacity of the shorter nanotubes is that lithium ion uptake can occur more quickly through the open ends, according to Zhou. Additionally, the sidewalls of the shortened nanotubes could have defects that allow the ion to diffuse into the inner spaces more freely, he said.

Nanotubes have more theoretical potential to store charge than graphite because, while graphite electrodes allow a battery to store one charged lithium ion for every six carbon atoms, nanotubes allow one charged lithium ion to be stored for every three carbons, according to Zhou. "This is shown convincingly by electrochemistry and nuclear magnetic resonance measurements," he said.

This is a very high quality piece of work, said Michael Heben, a senior scientist at the National Renewable Energy Laboratory. Building on previous efforts to use single-walled carbon nanotubes as anodes in lithium batteries, Zhou and his colleagues have demonstrated lithium capacity beyond what is normally thought to be the limit, Heben said.

The work, "underscores the potential for single-walled carbon nanotubes in battery applications [and] adds significantly to a small but growing body of work," Heben said.

It will be at least two years before carbon nanotubes can be used in lithium-ion batteries, Zhou said. "We show the potential but there are many practical issues that need to be solved," he said.

Zhou's research colleagues were Hideo Shimoda, Bo Gao, Xiao-Ping Tang, Alfred Kleinhammes, Leslie Fleming, and Yue Wu. They published the research in the January 7, 2001 issue of the journal Physical Review Letters. The work was funded by the Office of Naval Research and the National Science Foundation (NSF).

Timeline:  >2 years
Funding:  Government
TRN Categories:  Materials Science and Engineering
Story Type:   News
Related Elements:  Technical paper, "Lithium Intercalation into Opened Single-Walled Carbon Nanotubes: Storage Capacities and Electronic Properties," in Physical Review Letters, January 7, 2001.


February 27, 2002

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Nanotubes pack power


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