Material may help batteries hold a recharge
(This story appeared on page 324 of the
Nov. 22, 1997, Science News.)
By Sid Perkins
Science News
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 fits the bill.
The new material, called a manganese oxyiodide, 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 of
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 possible 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 oxyiodide, 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 produced using 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,
senior scientist at 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.
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Copyright 1997 by Science Service.
All rights reserved.
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