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
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
Timeline: >2 years
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.
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