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 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 and Hubert Gasteiger, professors of 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.