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2011年1月4日星期二

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Charging a cell phone or laptop could take minutes instead of hours, thanks to a new nanomaterial.

The material is made up of what the researchers call nanoscoops — “cones” of carbon and aluminum with “scoops” of silicon on top. These nanoscoops would replace the graphite anode used in today’s lithium-ion batteries, which are the most common type of batteries found in consumer electronics.

So far, the researchers from Rensselaer Polytechnic Institute (RPI) in New York have made a coin cell Li-ion battery (similar in size to those used in watches) with a nanoscoop anode and performed standard battery tests on the prototype. The result: The new battery charged 40 to 60 times faster than a typical Li-ion battery. (The battery was run down to zero and then charged again — which is referred to as a charge cycle — 100 times and showed little sign of degradation.)

The researchers expect the battery technology to be commercial in a few years.

Downside of li-ion batteries

The problem with today’s lithium-ion batteries is that they must be charged and discharged slowly to prevent the anode from falling apart. (Batteries include two electrodes: the positively charged cathode and the negatively charged anode.)

Inside the battery, lithium ions move in and out of the anode, causing the negatively charged electrode to swell and shrink. This change in volume puts stress on the anode and, over time, this stress accumulates to the point where the anode stops working. (This is why a year-old iPhone battery, for example, must be charged more often than a new battery.)

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“If you charge and discharge very slowly then the stress buildup is not that serious a problem, but if you try to charge and discharge too quicky then the stress buildup is so instantaneous that it becomes a problem … slowly the battery starts to get worse and worse, and then at some point it will fail completely,” said Nikhil Koratkar, co-author of the study and a professor at RPI.

To address the stress, the researchers developed a new anode made from nanomaterials.

“The advantage of nanoscale is that the structures are very flexible,” Koratkar said. “They can withstand a change in volume, and they tend not to fracture and fail.”

But nanomaterials have been tried in the past, and while they show some improvement they don’t have the quick charge-discharge time exhibited by the nanoscoops. For instance, a quick discharge would be useful in electric cars to accelerate after stopping at a light.

Instead of just one nanomaterial, the team created a layered structure of carbon nanorods coated with a thin layer of aluminum and topped with a scoop of silicon. Each layer acts as a sort of “stress absorber” for the next layer, reducing the overall stress felt by each individual material and resulting in an improved anode.

“You sort of create this increase in strain where you’re going from zero strain on the substrate to a small amount on the carbon, to more strain on the aluminum. [And] then you’ve got maximum strain on the top,” Koratkar said. “That gradient in strain means that the structure is less likely to peel off or break off the substrate.”

Next steps

The next step will be to scale up the nanoscoops.

The amount of charge stored in a Li-ion battery is directly related to the mass of the electrode. And because these nanoscoops are so light, they need a lot of room to reach the necessary mass (approximately one gram for a typical cell phone). Because there are size constraints with consumer electronics, there isn’t an option of making the anode larger horizontally