Batteries that Breathe

With the launch of the Nissan Leaf and Chevy Volt, it's been a big year for electric vehicles, but their batteries still have a fairly limited range without a recharge. For a car running on today's lithium-ion batteries to match the range provided by a tank of gasoline, you'd need a lot more batteries, which would weigh down the car and take up too much space.

But what if you could take away one of the electrodes in a battery and replace it with air? Researchers estimate that a lithium-air battery could hold 5 to 10 times as much energy as a lithium-ion battery of the same weight and double the amount for the same volume. In theory, the energy density could be comparable to that of gasoline.

"No other battery has that kind of energy density, so far as we know," says Ming Au, principal scientist at Savannah River National Laboratory (SRNL), in Aiken, S.C. Au was one of several scientists who reported new research into rechargeable lithium-air batteries during the fall meeting of the Materials Research Society, in Boston.

In such a battery, the anode is made of lithium. The cathode is oxygen, drawn from the surrounding air. As the lithium oxidizes, it releases energy. Pumping electricity into the device reverses the process, expelling the oxygen and leaving pure lithium.

"You can certainly make a lithium-air battery for one-time usage," says Au. In fact, such lightweight batteries are commonly sold to power hearing aids. "But to make this battery rechargeable is difficult," he says.

Rechargeable lithium-air batteries face several challenges. For one, lithium reacts violently with water, so the battery's electrolyte cannot contain any, and water vapor must be separated from incoming air. Turning the lithium oxide—the product of discharging the battery—back to lithium is difficult and only partially possible even when assisted by special catalysts: The oxide builds up and retards the process, limiting the number of charge-discharge cycles to a mere handful. Before lithium-air batteries can find use in hybrid and electric cars, they must be able to handle thousands of such cycles.

As for the time it takes to discharge and recharge the battery, "that process is very sluggish," says Yang Shao-Horn, associate professor in the Electrochemical Energy Lab at MIT. But she recently reported that she could increase that round-trip efficiency to 77 percent by incorporating nanoparticles of gold and platinum into the cathode end. Gold speeds the combination of oxygen with lithium, and platinum catalyzes their separation.

The SRNL group, meanwhile, is in the midst of a two-year, US $1 million project on lithium-air batteries. So far, they've demonstrated a coin-size battery with a capacity of 600 milliampere-hours per gram of material. That's a leap from traditional lithium-ion batteries, with capacities of 100 to 150 mAh/g. But lithium-ion batteries have about 1000 charge/discharge cycles, and Au's device tops out at about 50.

It could be many years until a rechargeable lithium-air battery reaches the market. Au points out that lithium-ion batteries were first described in 1976 but weren't for sale until 1997. "You have to have some big investment from the government or some corporation," he says. And that hasn't arrived yet.