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A 'revolution' in batteries

Nature Nanotechnology / Stanford

Photomicrographs show silicon nanowires before and
after charging (left and right, respectively).


If you've ever rushed to save your files before your laptop battery gave out, or scrambled to recharge your iPod, or wished out loud for the resurrection of the electric car ... relief is in sight.

Yet another battery breakthrough is on its way to market, taking its place alongside improved hybrid-electric vehicles, the promise of ultracapacitor systems and even better AA power cells. Next-generation batteries could well last several times as long as current power packs, thanks to nanotechnology.

"This idea will have a really high impact on battery technology," said Stanford chemist Yi Cui, who is the lead researcher behind a study appearing in this month's issue of Nature Nanotechnology. "This is really revolutionary."

The key innovation involves using silicon nanowires instead of the usual carbon to store energy in a lithium-ion battery's anode.

Silicon has more than 10 times as much charge capacity as carbon. If commercial batteries could live up to that performance level, you could theoretically be running your laptop for 20 to 40 hours straight rather than the typical two to four hours. An electric car could go 400 miles on a charge rather than 40 miles.

Of course, the reality is more complex than the theory. But more about that later. The first question is whether this technology is actually for real. If silicon is that good at storing electrical energy, why isn't it being used already?

That's where nanotechnology makes the difference: For years, engineers have been trying to harness silicon electrodes for battery applications. But the problem with silicon is that its volume bulks up by a factor of four when you add the lithium - and then shrinks by the same factor when power is extracted. That quickly pulverizes an electrode made of silicon film or particles, rendering the battery useless.

Cui and his colleagues took a different approach: They grew nanowires of silicon directly on a stainless-steel plate. Each wire was about 90 nanometers wide, or a thousandth of the width of the typical human hair. When the filaments were filled with lithium-ion power, they thickened up and lengthened into curls, like tiny spongeworms - but they retained their resiliency through dozens of power cycles.

"This idea really made these silicon materials possible to be used in battery technology," Cui said.

Challenges still lie ahead: First of all, Cui's team focused on retooling the anode, which is just one of the electrodes in a battery. To get the full tenfold improvement, Cui told me, "you would need to improve also the other electrode ... but with one electrode improvement, you can improve a lot already." For example, you could make the anode smaller, leaving more space for a bigger cathode.

Cui's team also found that there was a one-time capacity drain after the first charge. But that's no biggie. The nanowires' storage capacity was still about eight times higher than carbon, Cui said. "This won't prevent this technology from going forward," he said.

On the plus side, silicon-nanowire batteries wouldn't have to look like the battery bricks that are typically used in laptops or cell phones. "It's a fundamentally different structure from the current technology," Cui said. And that could result in batteries that are better-shaped to conform to the available space.

Cui said a patent application has been filed for the technology, and he's considering starting up a company to commercialize the concept. So when might silicon-nanowire batteries hit the market? "I'm thinking in the next three to five years," Cui said.

Some companies are already knocking on the lab door. Cui acknowledged that Tesla Motors, the company working on an all-electric sports car, is just one of the outfits expressing interest. "There are lots," Cui told me, "but it's better not to mention their names now."

To learn more about Cui's work, check out this interview at GM-Volt.com and this story in The Stanford Daily. In addition to Cui, the authors of the Nature Nanotechnology paper include Candace Chan, Halin Peng and Robert Huggins of Stanford University, Gao Liu of Lawrence Berkeley National Laboratory, and Kevin McIlwrath and Xiao Feng Zhang of Hitachi High Technologies.