Material may help batteries hold a recharge.

Of what do electronics-laden consumers dream? A cheap, compact battery that lasts a long time, holds up well even after repeated rechargings, and is environmentally friendly.

Now, researchers from the University of Texas at Austin have come up with a substance that may serve as a cathode for a battery that fills the bill. The new material, called a manganese oxylodide, may solve a variety of problems that plague rechargeable batteries, the scientists report In the Nov. 20 Nature.

Lithium-ion batteries, packing lots of power in a little space and offering long operational lifetimes, are the current choice for rechargeable power sources in portable electronics. A major drawback of these batteries is that the cathodes contain cobalt, an element that is both toxic and expensive. They also tend to lose their ability to hold a charge after repeated rechargings.

Many researchers have tried various manganese oxides as cathode materials, because they are cheaper and less toxic than cobalt, but have found that changes in the crystalline structures of these substances often cause problems, says Arumugam Manthiram, a materials scientist at the university. Each cycle of discharging and recharging alters the volume of a cathode, distorting its crystal structure and interfering with its ability to hold a charge.

Manthiram's manganese oxylodide, however, has a nearly amorphous, structure that's less susceptible to the stresses of charging. Tests show that, the substance can be recharged fully, even after 40 cycles of charging and discharging. Indeed, the material shows a slight increase in its ability to hold a charge after repeated rechargings. Manthiram says the amorphous nature of manganese oxyiodide may allow atoms to rearrange themselves each time the battery is recharged.

The amorphous structure of manganese oxyiodide results in large part from the presence of iodine atoms, which are larger than the other atoms in the material. The large atoms create spaces that the small lithium ions can move through as the battery discharges and recharges, says Manthiram.

Manganese oxyiodide is made in a low-temperature, solution-based process that offers advantages over the high-temperature processes used to produce other cathode materials, Manthiram asserts.

"This material seems to solve some of the structural Instability Problems associated with manganese oxide electrodes, and it provides an unusually high recharge capacity at low current rates," says Michael Thackeray of Argonne (Ill.) National Laboratory However, the wide voltage range of the lithium cell, which falls from 4.3 to 1.5 during discharge, may be a limitation in commercial applications, he adds.