A redesign points the way for air-breathing
rechargeable batteries that pack in much more energy than today’s
lithium-ion cells
Lithium, the lightest metal, when paired with
oxygen from the atmosphere, makes for a tantalizing partnership
in a lightweight battery. In theory, this “lithium-air
battery” offers several times the energy per kilogram of
today’s lithium-ion batteries. But so far, in laboratory tests
the lithium gets distracted by dead-end chemical reactions that
prevent the batteries from recharging. A one-shot battery is
fine for a wristwatch, but not for a next-generation electric
vehicle.
Last month, researchers in Rome and Seoul, South Korea,
reported
designing a lithium-air battery that did not have the recharging
problem, achieving approximately 100 charge and discharge cycles
with little capacity loss. And this week, a research team led by
chemist Peter Bruce of the University of St. Andrews, in
Scotland, reports in the journal
Science a differently
designed lithium-air battery that reached 100 cycles, losing
only 5 percent of its capacity in the process. Bruce says that
these recent results show that lithium-air batteries can sustain
“a truly reversible reaction.” The group measured the mix of
chemicals involved, showing that the lithium was really being
recycled through a reversible substance, lithium peroxide (Li
2O
2),
during each charge and discharge.
Lithium-air batteries work, at least in theory, by exposing a
lithium anode to an electrolyte that grabs its positively
charged lithium ions and drives them toward the cathode, made of
a different, porous material that allows oxygen from the air to
form the crucial lithium peroxide.
But until these latest developments, there wasn’t a good way
to reverse the process. “There’s no electrolyte that currently
works well,” explains M. Stanley Whittingham, an expert in
materials science at Binghamton University, part of the State
University of New York. But researchers have made some headway
in understanding why that is. Linda Nazar, an energy-storage
researcher at the University of Waterloo, in Ontario, Canada,
recently reported that carbon, from which most cathodes are
made, reacts with lithium, costing the battery some of its
ability to recharge. And in some experimental batteries,
researchers found that lithium sometimes reacted with the
electrolytes.
With that in mind, Bruce and his colleagues replaced the
carbon cathode with one made from nanoporous gold and replaced
the organic carbonates and ethers used as electrolytes in
earlier lithium-air batteries with lithium perchlorate (LiClO
4)
solvent and dimethyl sulfoxide. When they discharged their
battery and measured the resulting chemicals, they found mostly
lithium peroxide, a chemical that is easy to use for reversing
the reaction—or recharging the battery. “Up to this point, the
electrode materials haven’t been able to deliver pure lithium
peroxide,” Bruce says. Why it works is a bit of a mystery. The
St. Andrews scientists knew gold had been tried in some previous
batteries, but they still don’t particularly understand, in
detail, what it is about this combination that delivers this,
Bruce says.
IBM and some carmakers are
exploring
lithium-air technology, but it is by no means the front-runner
for car batteries, says Whittingham. Such batteries are on the
bleeding edge of technology, and currently
funding
for their research is primarily provided by big risk takers,
such as the U.S. Department of Energy’s Advanced Research
Projects Agency (ARPA-E). Other ARPA-E–funded projects, such as
Pellion,
are exploring magnesium as an alternative to lithium for
high-density batteries. Magnesium carries two electrons versus
lithium’s one, so scientists think it may prove more efficient.
One problem is that in order to provide the same energy as,
say, gasoline, lithium-air batteries require more space—although
not much more weight—than the fossil fuel. Whittingham predicts
that the technology will be used for large, stationary
energy-storage applications rather than for portable batteries.
Another problem is that lithium-air batteries are chemically
delicate: The lithium gets diverted into dead-end reactions with
water or carbon dioxide in the air, so the electrodes in any
future battery may need to be sealed or filtered so that they
interact only with dry air and the electrolyte, an added expense
and complication.
At any rate, the material used by Bruce’s group is handy for
experiments but probably too hazardous, expensive, and heavy for
commercial use. But University of Cambridge chemist Clare Grey
says that identifying stable electrodes and electrolytes will
help research on lithium-air batteries, even if there are a
large number of practical things that still need to be sorted
out before commercial versions hit the road as part of electric
vehicles.
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