Iron Power: Eking more juice from batteries

Many of the rechargeable batteries that power cell phones, laptop computers, medical implants, and hybrid cars contain some of the same electrode technology that was used in Thomas Edison’s day. Now, chemists have come up with a modern alternative that could potentially multiply the capacity of such batteries.

A battery typically consists of two electrodes—a positive cathode and a negative anode—immersed in a substance through which charged atoms, or ions, can flow. The new technology holds promise for replacing the energy-limiting nickel-based cathode in nickel-metal hydride batteries, says chemist Stuart Licht of the University of Massachusetts in Boston, who led the research. Those are the most popular rechargeable batteries for portable electronic gadgets.

Whether powering a gadget or a car, nickel ions in the cathodes of such batteries each capture one electron arriving from the anode. By contrast, the new cathode employs salts of an unusual ionic form of iron—hexavalent or superoxidized iron—that readily accept three electrons per ion, Licht explains. The more electrons the cathode can accept, the more electricity the battery can supply.

Using superoxidized iron in a cathode isn’t new. Licht and his coworkers achieved that milestone in 1999 (SN: 8/28/99, p. 141). However, the researchers relied on iron-salt crystals the size of fine sand grains, and the prototype batteries made with these materials weren’t rechargeable.

Licht and Ran Tel-Vered of the Technion Israel Institute of Technology in Haifa have turned to electrochemical-processing techniques to make grains roughly a billionth the volume of previous ones. With these smaller grains, the scientists created a cathode so thin that incoming electrons can find their way to virtually all of the cathode’s ions. Because of that same nanoscale thinness, the electrode can be charged and discharged 200 times, the scientists report in the March 10 issue of Chemical Communications.

Although that’s less than half the minimum number of recharging cycles expected from a commercial battery, the new work shows that there’s no fundamental barrier to designing rechargeable batteries with superoxidized iron, Licht says.

“Iron has possibilities,” comments Stanford R. Ovshinsky of the company Energy Conversion Devices in Rochester Hills, Mich. Still, the new technology is “far from being a useful battery,” notes Ovshinsky, who invented the metal hydride battery about 20 years ago.

“What [Licht] has done is extremely encouraging,” says Digby D. Macdonald of Pennsylvania State University in State College. While the new cathode technology is still at an early stage, he adds, “it’s certainly one of the most promising cathodes around.”