Batteries that Breathe

Source: IEEE Spectrum, Neil Savage  (2/1/11)

"Using oxygen as a cathode could give lithium batteries 10x the energy."


Oxygen cathode could give lithium batteries It's been a big year for electric vehicles (EVs), but their batteries still have a limited range without a recharge. For cars running on today's lithium-ion batteries to match the range of a tank of gasoline, it'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 a lithium-air battery could hold 5x–10x as much energy as a lithium-ion battery.

"No other battery has that kind of energy density, so far as we know," says Savannah River National Laboratory (SRNL) Principal Scientist Ming Au.

In such a battery, the anode is made of lithium. The cathode is oxygen, drawn from 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 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 EVs, they must be able to handle thousands of such cycles.

The SRNL group is in the midst of a two-year, US$1M project on lithium-air batteries, but it could be many years until a rechargeable lithium-air battery reaches the market.