A catalyst developed by researchers at MIT makes rechargeable lithium-air batteries significantly more efficient--a step toward making these high-energy-density batteries practical for use in electric vehicles and elsewhere.
The catalyst consists of nanoparticles of a gold and platinum alloy; in testing it was able to return 77 percent of the energy used to charge the battery as electricity when discharged. That's up from the previously published record of about 70 percent, the researchers say. The work, which was reported online this week in the Journal of the American Chemical Society, suggests a new approach to lithium-air battery catalysts that could lead to the even higher efficiencies of 85 to 90 percent needed for commercial batteries.
Lithium-air batteries, which generate electricity by reacting lithium metal and oxygen from the air, are attractive for their potential to store vast amounts of energy. They could be a practical way to store more than three times as much energy, by weight, as today's lithium-ion batteries, extending the range of electric vehicles, for example.
But prototype lithium-air batteries are plagued with problems. In addition to being very inefficient, they typically only last a few dozen charge and discharge cycles. They are also sluggish--only releasing their energy slowly--and prone to contamination by carbon dioxide and water. And the lithium metal used for one of the electrodes is dangerously reactive and eventually grows dendrites, which can lead to short circuits.
By improving the battery's efficiency, the new catalyst research, led by Yang Shao-Horn, a professor of materials science and engineering and mechanical engineering, in collaboration with Kimberly Hamad-Schifferli, a professor of mechanical engineering and biological engineering, addresses one of their most serious problems. The catalysts could also help make such batteries longer lived.
When lithium-air batteries are discharged, the lithium metal reacts with oxygen to form lithium oxide and release electrons. When charged, oxygen is released and lithium metal reforms. The new catalysts promote these reactions, and so reduce the amount of energy wasted as the cells are charged and discharged. The gold atoms in the catalyst facilitate the combination of lithium and oxygen; the platinum helps the opposite reaction, freeing the oxygen.
In some ways the findings fly in the face of previous assumptions. Platinum, known for being one of the best catalysts for promoting the combination of hydrogen and oxygen in fuel cells, was one of the first materials tried for catalyzing lithium and oxygen in lithium-air batteries. But experiments showed that it actually did a poor job, so platinum was dropped.
The MIT researchers found that platinum is useful in lithium-air batteries, but for the opposite reaction--freeing oxygen from lithium oxide during charging. "Everyone knew that platinum was inactive for discharging the battery, but we showed that platinum was one of the best catalysts for charging," Shao-Horn says.
On the other hand, gold is typically considered a poor catalyst because it is inert, Shao-Horn says. Indeed, the MIT researchers had first used gold as a sort of control in experiments to measure reactions involving a poor catalyst. To their surprise, they found that gold does a good job of catalyzing the combination of lithium and oxygen--much better than platinum. (Toyota researchers had shown this previously, and issued a patent a few months before Shao-Horn's group saw the effect.) Furthermore, the researchers found that both catalysts became more effective when they were combined as nanoparticles. "Together they work synergistically," Shao-Horn says.
In addition to improving efficiency, promoting these reactions could also potentially increase the number of times that lithium-air batteries can be recharged, by minimizing the accumulation of lithium oxide, which otherwise clogs up the battery. As they continue to develop lithium-air batteries, the MIT researchers will explore this possibility; they will study the gold platinum catalysts in more detail to understand how they work; and develop new catalysts with different combinations of materials.
The MIT researchers are also working to cut the cost of the catalyst by using less platinum and gold. One option is to coat nanoparticles made of cheaper materials with thin layers of these precious metals. Other researchers have demonstrated that inexpensive manganese oxide catalysts can be effective for lithium-air batteries, says Jean-Marie Tarascon, a professor at the Universite de Picardie Jules Verne in France. He says this material recently has been shown to produce even higher efficiencies than Shao-Horn's catalysts.
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