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Energy forest

2:56pm, January 8, 2008

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 a battery with anodes made of 100 nanometer-wide silicon wires. During the first charge cycle, the nanowires swell,

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