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November 16, 2011

Current battery technology is limited in its capacity and charging time. Lithium-ion batteries have become increasingly common in laptops, smartphones and electric vehicles, but researchers working at Northwestern University recently announced they had tweaked the battery's composition to generate a dramatically more efficient storage system.

Engineers at the Evanston, Illinois-based university said they had moved closer to developing a battery that is significantly more powerful than models currently used. Their research is so promising, in fact, that they contend they could soon release a battery that can remain charged for more than seven days and can be recharged in fewer than 20 minutes.

The engineers recently published their findings thus far in the journal Advanced Energy Materials. The scientists affirmed they were able to create an electrode for lithium-ion batteries that augments their storage capacity tenfold. What's more, batteries embedded with the specifically engineered electrode can charge 10 times faster than current technology allows, the Northwestern engineers asserted.

To create the new electrode, the researchers employed two chemical engineering approaches in an effort to perfect an approach to the most vexing problems plaguing lithium-ion batteries, their limited capacity and slow charge rate.

Lithium-ion batteries charge through a chemical process in which lithium ions are sent between the anode and the cathode. When a battery is used to power a smartphone, for example, the lithium ions make their way from the anode to the cathode, passing through the electrolyte. When a battery is charged, on the other hand, the process is reversed.

Engineers have endeavored to increase capacity in the past, but they have been thwarted by the ostensibly limited number of ions that can be placed in the anode or cathode. Moreover, the speed at which lithium-ion batteries can recharge has been limited by how quickly ions can journey through the electrolyte into the anode. The scientists used two techniques to circumvent these problems, according to Northwestern News.

Harold H. Kung, a Northwestern University chemical and biological engineering professor who led the researchers, said the team used silicon to increase capacity. While silicon has been used in the past - with rather mixed results - to augment storage potential, Kung and his colleagues placed clusters of silicon in between graphene sheets, which make up the anode. Silicon can accommodate more lithium ions, and by embedding it between the graphene layers,
the scientists were able to adjust for the changes in volume silicon undergoes during the charging process.

The team then used a chemical oxidation process to generate holes in the graphene sheets measuring between 10 and 20 nanometers. The 'in-plane defects' that they carved into the graphene sheets essentially serve as a shortcut that lithium ions can use to travel into the anode, where they are then stored as they react with the silicon. This helped to slash recharging times. Popular Science reports the trick allows lithium ions to bypass traveling around the edges of the anode, which is where the minuscule traffic jams were occurring.

"We have found a way to extend a new lithium-ion battery’s charge life by 10 times," Kung said. "We have much higher energy density because of the silicon, and the sandwiching reduces the capacity loss caused by the silicon expanding and contracting. Even if the silicon clusters break up, the silicon won’t be lost. Even after 150 charges, which would be one year or more of operation, the battery is still five times more effective than lithium-ion batteries on the market today."

Now that the team has conquered the anode, they have shifted their attention to the cathode, as they hope to wring additional capacity and improved performance from lithium-ion batteries. Kung asserted that the improved lithium-ion battery could be implemented in consumer products within three to five years.



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