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Li-ion cells with 10x capacity under development

Posted: 21 Nov 2011 ?? ?Print Version ?Bookmark and Share

Keywords:lithium-ion battery? energy capacity? charge rate?

A team of engineers from the Northwestern University has developed a new form of lithium-ion batteries that boasts 10 times the capacity of regular batteries and can be charged 10 times faster. According to the researchers, the boost in energy capacity and charge rate was due to an electrode that they have created.

When the battery reaches the market in the next three to five years, this could enable mobile devices that include smartphones and tablets. Portable CEs that need charging for only 15 minutes that would last for more than a week is not unlikely, they reckoned.

In a paper titled "In-Plane Vacancy-Enabled High-Power Si-Graphene Composite Electrode for Lithium-Ion Batteries," the researchers described how they combined two chemical engineering approaches to tackle two major battery limitations!energy capacity and charge rate. For them, the technology could pave the way for more efficient, smaller batteries for electric cars.

"We have found a way to extend a new lithium-ion battery's charge life by 10 times," said Harold H. Kung, lead author of the paper. "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."

Lithium-ion batteries charge through a chemical reaction in which lithium ions are sent between two ends of the battery, the anode and the cathode. As energy in the battery is used, the lithium ions travel from the anode, through the electrolyte and to the cathode. As the battery is recharged, they travel in the reverse direction.

With current technology, the performance of a lithium-ion battery is limited in two ways. Its energy capacity!how long a battery can maintain its charge!is limited by the charge density, or how many lithium ions can be packed into the anode or cathode. Meanwhile, a battery's charge rate!the speed at which it recharges!is limited by another factor: the speed at which the lithium ions can make their way from the electrolyte into the anode.

In rechargeable batteries, the anode, which is made of layer upon layer of carbon-based graphene sheets, can only accommodate one lithium atom for every six carbon atoms. To increase energy capacity, scientists have previously experimented with replacing the carbon with silicon, as silicon can accommodate much more lithium: four lithium atoms for every silicon atom. However, silicon expands and contracts dramatically in the charging process, causing fragmentation and losing its charge capacity rapidly.

Presently, the speed of a battery's charge rate is hindered by the shape of the graphene sheets. They are extremely thin!just one carbon atom thick!but by comparison, very long. During the charging process, a lithium ion must travel all the way to the outer edges of the graphene sheet before entering and coming to rest between the sheets. And because it takes so long for lithium to travel to the middle of the graphene sheet, a sort of ionic traffic jam occurs around the edges of the material. Kung's research team, which also included Xin Zhao, Cary M. Hayner and Mayfair C. Kung, has combined two methods to combat these problems. First, to stabilize the silicon in order to maintain maximum charge capacity, they sandwiched clusters of silicon between the graphene sheets. This allowed for a greater number of lithium atoms in the electrode while using the flexibility of graphene sheets to accommodate the volume changes of silicon during use.

"Now we almost have the best of both worlds," Kung noted. "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."

Kung's team also used a chemical oxidation process to create miniscule holes (10-20nm) in the graphene sheets so the lithium ions would have a 'shortcut' into the anode and be stored there by reaction with silicon.

This research was all focused on the anode. Next, the researchers will begin studying changes in the cathode that could further increase effectiveness of the batteries. They also will look into developing an electrolyte system that will allow the battery to automatically and reversibly shut off at high temperatures, a safety mechanism that could prove vital in electric car applications.

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