Mega-cell: Researchers build better batteries

20 January 2008
Posted by Chris Smith.

US scientists have come up with a way to make lithium batteries last ten times longer, which means a laptop could last all day on just one charge.

BatteriesStanford's Yi Cui and his colleagues made the breakthrough by using a forest of tiny silicon nanowires, which act as the positive electrode (known as the anode) inside the battery.  The present generation of batteries use graphite electrodes, which can only hold a certain amount of lithium, and this limits battery capacity.  But silicon has a much higher charge capacity, so using it should enable scientists to build better batteries, but there's a catch.  When silicon soaks up lithium inside the battery it swells up by 400% and then shrinks again when the lithium is later released.  This cycle occurs every time the battery is charged and discharged, and very quickly the silicon becomes "pulverised" into tiny fragments which lose contact with their parent electrode and cause the battery capacity to fall.

To get around the problem Cui and his team have used a technique called VLS, short for vapour-liquid-solid, to grow clusters of short silicon wires each about one ten-thousandth of a millimetre in diameter.  When the new electrodes were tested as part of a lithium ion cell the increased surface area helped to boost the amount of charge that the battery could hold, the cells worked with over 90% efficiency, and the electrodes were also capable of handling currents five times greater than those tolerated by existing graphite electrodes.  Moreover, when the team used X-rays to study the crystal structure of the silicon nanowires when they were charged and discharged, they found that the wires swelled and shrank without signs of pulverisation.

"We are working on scaling up and evaluating the cost of our technology," Cui said. Cui has filed a patent on the technology, which is published in the journal Nature Nanotechnology. He expects the battery to be commercialised and available within "several years".

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