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nanotimes News in Brief Batteries //

New Nanostructure for Batteries Keeps Going and Going © Based on Material by Stanford University

improving lithium-based batteries: a cleverly de- signed double-walled nanostructure that lasts more than 6,000 cycles, far more than needed by electric vehicles or mobile electronics.

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“This is a very exciting development toward our goal of creating smaller, lighter and longer-lasting batte- ries than are available today,” Cui said. Lithium-ion batteries are widely used to power devices from electric vehicles to portable electronics because they can store a relatively large amount of energy in a relatively lightweight package. The battery works by controlling the flow of lithium ions through a fluid electrolyte between its two terminals, called the an- ode and cathode.

The promise – and peril – of using silicon as the anode in these batteries comes from the way the lithium ions bond with the anode during the char- ging cycle. Up to four lithium ions bind to each of the atoms in a silicon anode – compared to just one for every six carbon atoms in today’s graphite anode – which allows it to store much more charge. Howe- ver, it also swells the anode to as much as four times its initial volume. What’s more, some of the electro- lyte reacts with the silicon, coating it and inhibiting further charging. When lithium flows out of the anode during discharge, the anode shrinks back to its original size and the coating cracks, exposing fresh

The new double-walled silicon nanotube anode is made by a clever four-step process: Polymer nanofi- bers (green) are made, then heated (with, and then without, air) until they are reduced to carbon (black). Silicon (light blue) is coated over the outside of the carbon fibers. Finally, heating in air drives off the carbon and creates the tube as well as the clamping oxide layer (red). © both images:Hui Wu, Stanford, and Yi Cui

silicon to the electrolyte. Within just a few cycles, the strain of expansion and contraction, combined with the electrolyte attack, destroys the anode through a process called “decrepitation.”

Over the past five years, Cui’s group has progres- sively improved the durability of silicon anodes by making them out of nanowires and then hollow silicon nanoparticles. His latest design consists of a double-walled silicon nanotube coated with a thin layer of silicon oxide, a very tough ceramic ma- terial.This strong outer layer keeps the outside wall of the nanotube from expanding, so it stays intact.

team led by materials scientist Yi Cui of Stanford University and SLAC has found a solution for

12-04 :: April/May 2012