Rocking ions improve lithium battery.

Though often considered the next generation of portable power, lithium-based batteries have so far failed to live up to their potential. Over time, lithium electrodes become unstable and can cause reactions that make the battery unsafe.

Now solid-state chemists think they have gotten around those shortcomings by developing a rechargeable, "rocking-chair" battery. To charge and discharge the battery lithium ions "rock" back and forth between the battery's two electrodes, says Dominique Guyomard of the University of Nantes, France.

He and Jean-Marie Tarascon at Bellcore in Red Bank, N.J., use lithium manganese oxide as the positive electrode and coke or graphite as the negative electrode.

When first developing the battery for Bellcore, they discovered that when they initially charged the battery, it lost 25 percent of its capacity because some lithium ions got stuck at the carbon and didn't cycle back and forth. So they added enough excess lithium ions to the system to compensate for those that got stuck and learned how to add this lithium in such a way that the electrodes remained stable, Guyomard says.

In addition, they came up with a new electrolyte composition that does not oxidize at higher temperatures, so the battery lasts longer, says Guyomard.

When fashioned into prototype AA batteries, this new technology performs better than current nickel-cadmium rechargeable batteries. The rocking-chair battery has the same capacity, but it packs three times the output voltage, asserts Guyomard. It averages 3.7 volts compared to the nickel-cadmium's 1.2 volts. Lithium, manganese, and carbon are cheap and easier to recycle and dispose of than cadmium-based batteries. In addition, unlike the nickel-cadmium battery the rocking-chair battery can be short-circuited and totally discharged without being destroyed, he notes.

At Bellcore, other scientists are seeking to modify this technology to make a thin-film rocking-chair battery that would be part of a computer chip. This rechargeable battery would kick in during power failures to keep the chip from losing its data, says Frough K. Shokoohi of Bellcore.