Energy forest

Science News,  Jan 12, 2008  by Davide Castelvecchi

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Thickets of microscopic silicon wires can dramatically boost the storage capacity of batteries, at least in the lab.

Lithium-ion batteries power most modern portable gadgets. During use, lithium ions detach from carbon sheets in an anode and migrate to a cathode. During recharge, the lithium ions make the opposite trip.

A carbon anode can accommodate about one lithium ion for every six carbon atoms. On the other hand, scientists have long known that silicon-based anodes could store more lithium--increasing the battery's capacity--by binding about four lithium ions per silicon atom. However, lithium absorption disrupts the diamondlike crystalline structure of silicon, says Yi Cui, a materials scientist at Stanford University. The material swells to four times its original volume and tends to pulverize.

Cui and collaborators have now created abattery with anodes made of 100 nanometer-wide silicon wires. During the first charge cycle, the nanowires swell, but do not break apart. Their thickening is permanent, enabling the nanowires to keep absorbing and releasing large amounts of lithium. "Using nanowires, we are solving this problem of volume expansion," says Cui.

He says his team's batteries can hold up to 10 times more lithium in their anodes than carbon-based batteries. With no changes on the cathode side, that means a twofold improvement in energy storage.

The team's results, published online in December, appear in the January Nature Nanotechnology.

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