mix boosts batteries
Technology Research News
A truly good battery should be made of
relatively inexpensive materials, store a significant amount of electricity,
and discharge this energy as quickly as an electrical device needs it.
And in a world that's increasingly contaminated by the residues of technology,
it should be rechargeable and nontoxic.
The common lithium batteries that power portable electronic devices like
laptop computers and cell phones use lithium metal oxide electrodes. Five
years ago, scientists discovered a cheaper, nontoxic lithium electrode
-- lithium iron phosphate. But initial promise turned to disappointment
when the material turned out to be a bad conductor, and so could not discharge
electricity at rates high enough to be useful.
Researchers from the Massachusetts Institute of Technology have now shown
that doping, or mixing, lithium iron phosphate with positive ions of another
metal can drastically boost the material's conductivity. Ions are atoms
that have fewer or more electrons than electrically neutral atoms and
so have a positive or negative charge.
The doping metal increased the conductivity of the lithium iron phosphate
by 100 million times, making it an even better conductor than standard
lithium metal oxide electrodes, according to Yet-Ming Chiang, a professor
of materials science and engineering at the Massachusetts Institute of
The raw materials that go into the compound are only about one-quarter
the cost of those that make up lithium metal oxide electrodes and the
compound is nontoxic, Chiang said. The material gives a battery an extremely
high rate of charge and discharge, "while at the same time being low in
materials cost and very safe," he said.
Lithium iron phosphate batteries could bring on a new class of devices
that would bridge the gap between super capacitors, which deliver short
bursts of high power, but can only store limited amounts of total energy,
and batteries, which have the opposite trade-off, said Chiang.
The material promises to improve batteries for electric and hybrid cars,
backup power for implantable medical devices, and fuel cells, according
Batteries generate electricity when the pair of materials that make up
the bulk of the battery react chemically, with one material giving up
electrons and the other material gaining electrons. Rather than flowing
directly from one material to the other, however, the electric current
leaves the battery through one electrode and returns through another.
Connecting an electronic device between a battery's electrodes, which
act as gatekeepers that determine how quickly the electricity flows, powers
Batteries made with the researchers' new electrode material would deliver
voltage similar to conventional lithium batteries, but the material's
better conductivity allows for much higher power density, or rate of charge
and discharge, said Chiang. A cell containing the new electrode could
be charged or discharged in as little as three minutes, while typical
batteries might require a half-hour or more, he said.
This is important for electric vehicles because they need a high rate
of energy to accelerate and because they need to store electricity quickly
in order to reuse breaking energy, said Chiang. "Battery power density
is required for rapid acceleration and also to accept the regenerative
breaking energy when someone slams on the brakes," he said.
The material could also eventually be used as electrodes for electrochemical
applications like fuel-cells and membranes for separating hydrogen gas,
according to Chiang. "These are other applications that require rapid
electron transport as well as ion transport -- in these cases the ion
is hydrogen rather than lithium as in the battery," he said.
The team synthesized more than 50 different mixtures by adding different
metals and baking the samples at temperatures as high as 850 degrees Celsius
in order to change the crystal structure of the material to improve its
conductivity, said Chiang. The metals included magnesium, aluminum, titanium,
zirconium, niobium, and tungsten. The challenges were getting the additive
to be uniformly distributed in the crystal lattice of the lithium iron
phosphate at the right positions in the lattice to have the necessary
effect on conductivity, he said.
They knew they were on to something when something strange happened. "Lithium
iron phosphate is normally medium gray color, not surprising for an electronic
insulator," Chiang said. When one of the samples came out jet-black, "we
realized that something special had happened," he said. "Highly conductive
materials are usually either metallic in luster -- gold, silver, copper,
aluminum -- or black in color -- carbon, oxide superconductors, magnetic
The material has a nanoporous crystal structure, said Chiang. Nanoporous
materials contain holes nearly as small as atoms. "The nanoporous structure
allows for rapid lithium transport into the electrode without impeding
the electronic conductivity," he said.
The formulation has proven very stable in abuse tests. This is "especially
important for batteries that pack a lot of energy and will be used under
a wide range of temperatures and electrical conditions," Chiang said.
Lithium iron phosphate is also the basic formulation of a mineral found
in the earth's mantle. Both this and the dopants the researchers added
are considered nontoxic compared to nickel-cadmium or lead acid batteries,
said Chiang. "We expect no environmental issues concerned with disposal,"
The new type of lithium iron phosphate "looks like a major breakthrough,"
said John Owen, a reader in electrochemistry at the University of Southampton
in England for. "This discovery will certainly bring forward the arrival
of a new type of lithium battery in its the next year or two," he said.
Once the scope and mechanism of the effect are fully understood, "the
way will be open to use a similar technique to improve many other materials
in the field of energy conversion, [including] fuel cells and solar cells,"
If the result turns out to be reliable, then this is most certainly interesting,
said Josh Thomas a professor of solid-state electrochemistry at Uppsala
University in Sweden, and director of the university's Advanced Battery
Centre. The material "is at the absolute front line... in implementing
new materials for developing better, cleaner, more powerful batteries
for ever larger applications -- ultimately for traction applications in
electric and electric/hybrid vehicles," he said.
The researchers are currently working to understand exactly how the material
conducts so well, said Chiang. "We want to understand the crystal chemistry
and mechanisms of conduction in this material at a deeper level, to know
where the atoms and electrons are and why the conduction is as high as
it is," he said. The researchers are also planning to investigate similar
compounds, he said.
Because batteries based on the new electrode can use existing materials
for the rest of the battery, the material could find its way into products
within two years, Chiang said.
Chiang is co-founder of A123Systems, which has licensed the technology
from MIT and is working to commercialize it, according to Chiang.
Chiang's research colleagues were Sung-Yoon Chung and Jason T. Bloking.
They published the research in the September 22, 2002 issue of Nature
Materials. The research was funded by the Department of Energy (DOE).
Timeline: 2 years
TRN Categories: Energy; Materials Science and Engineering
Story Type: News
Related Elements: Technical paper, "Electronically Conductive
Phopho-Olivines as Lithium Storage Electrodes," Nature Materials, September
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