Today's most popular rechargeables, lithium ion batteries, are made from negative and positive electrodes separated by an electrolyte through which positively charged lithium ions can flow back and forth. In most such cells, the negative electrode is made of graphite, a form of layered carbon, whereas the positive electrode is made from lithium cobalt oxide or a related material. During use, lithium ions stored in the graphite flow to the lithium-based electrode, where they form chemical bonds with oxygen atoms, a reaction that generates an electric current. When the battery is recharged, the lithium-oxygen bonds break and an electric voltage pushes the ions back into the graphite.
Researchers have long sought to replace the graphite in the negative electrodes with carbon nanotubes, strawlike tubes of carbon. The hope is to create a more porous material with a higher surface area that could hold on to more lithium ions and thus make longer-lived batteries.
But in a paper posted online today in Nature Nanotechnology, the MIT team, led by materials scientist Yang Shao-Horn, took a very different approach: using carbon nanotubes to replace the oxide-based positive electrode. Normally, lithium ions wouldn't bind to plain carbon nanotubes. So Shao-Horn and her colleagues decorated the outer surfaces of their nanotubes with two different types of oxygen-containing chemical groups that gave them opposite charges. They then dipped their electrode starting materials alternatively in solutions containing the oppositely charged nanotubes, binding successive layers of tubes atop one another to build up their nanotube electrodes.
The result was a highly porous carbon nanotube electrode with lots of oxygens exposed on the surface, ready to bind with lithium. Detailed tests showed the new batteries hold five times as much energy as conventional quick-discharging devices called capacitors do, and they deliver that power 10 times as quickly as conventional lithium ion batteries can.
"This is certainly pioneering work," says Ray Baughman, a chemist at the University of Texas, Dallas. Baughman cautions, however, that the MIT team achieved its best results with very thin electrodes. The performance dropped off considerably as the electrodes were made thicker. Because thicker electrodes can store more charges, they allow a battery to hold more energy. So for now, hybrid batteries will be best suited to applications with low overall power demands, such as powering electronic circuitry in smart cards, credit cards with electronic chips that hold more information than magnetic strips do. For the batteries to be useful in hybrid cars or other power-hungry applications, researchers will need to find a way to make thicker electrodes that can still move charges quickly, a project Shao-Horn says she is working on now.
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