A tenfold increase in lithium-ion (Li-ion) battery operating time would be great news for users of mobile phones, laptop computers and other portable devices.
Stanford University scientists now believe that such a performance increase could soon be possible, thanks to an improved silicon nanowire they've developed. The technology produces 10 times the amount of electricity of existing lithium-ion, says Yi Cui, assistant professor of materials science and engineering at Stanford, the research team's leader. "It's not a small improvement," he notes. "It's a revolutionary development."
A Li-ion battery's electrical storage capacity is limited by how much lithium can be held in the battery's anode, which is typically made of carbon. Silicon has a much higher capacity than carbon, but also has a drawback. Silicon placed in a battery swells as it absorbs positively charged lithium atoms during charging, then shrinks during use as the lithium is drawn out of the silicon. This expand/shrink cycle typically causes the silicon (often in the form of particles or a thin film) to pulverize, degrading the battery's performance.
The enhanced technology circumvents this problem through the use of nanotechnology. The lithium is stored in a forest of tiny silicon nanowires, each with a diameter one-thousandth the thickness of a sheet of paper. The nanowires inflate four times their normal size as they soak up lithium. But, unlike other silicon shapes, they do not fracture.
Research on silicon in batteries began three decades ago. "Then people kind of gave up on it because the capacity wasn't high enough and the cycle life wasn't good enough," says research team member Candace Chan, a Stanford graduate chemistry student. The technology's physical size was also a detriment. "It was just too big," she notes.
Then, silicon nanowires arrived. "We just kind of put them together," Chan says. For the experiments, Chan grew the nanowires on a stainless steel substrate, which providing a solid electrical connection. "It was a fantastic moment when Candace told me it was working," Cui says.
Cui says a patent application has been filed. He is thinking about forming a company or, perhaps, an agreement with a battery manufacturer. Manufacturing the nanowire batteries would require "one or two different steps, but the process can certainly be scaled up," he adds. "It's a well understood process
Cui believes that the new technology's greatly expanded storage capacity could make Li-ion batteries attractive to electric car manufacturers as well as mobile device vendors. He suggests that the batteries could also be used in homes or offices to store electricity generated by rooftop solar panels. "Given the mature infrastructure behind silicon, this new technology can be pushed to real life quickly," Cui says.
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