Making Renewable Power
More Viable for the Grid
“Air-breathing” battery can store electricity for months, for about a fifth the cost of current technologies.
Rob Matheson | MIT News Office
More Viable for the Grid
“Air-breathing” battery can store electricity for months, for about a fifth the cost of current technologies.
Rob Matheson | MIT News Office
October 11, 2017 -- Wind and solar power are increasingly popular
sources for renewable energy. But intermittency issues keep them from
connecting widely to the U.S.
Now MIT researchers have developed
an “air-breathing” battery that could store electricity for very long durations
for about one-fifth the cost of current technologies, with minimal location
restraints and zero emissions. The battery could be used to make sporadic
renewable power a more reliable source of electricity for the grid.
For its anode, the rechargeable
flow battery uses cheap, abundant sulfur dissolved in water. An aerated liquid
salt solution in the cathode continuously takes in and releases oxygen that
balances charge as ions shuttle between the electrodes. Oxygen flowing into the
cathode causes the anode to discharge electrons to an external circuit. Oxygen
flowing out sends electrons back to the anode, recharging the battery.
“This battery literally inhales and
exhales air, but it doesn’t exhale carbon dioxide, like humans — it exhales
oxygen,” says Yet-Ming Chiang, the Kyocera Professor of Materials Science and
Engineering at MIT and co-author of a paper describing the battery. The
research appears today in the journal Joule.
The battery’s total chemical cost —
the combined price of the cathode, anode, and electrolyte materials — is about
1/30th the cost of competing batteries, such as lithium-ion batteries.
Scaled-up systems could be used to store electricity from wind or solar power,
for multiple days to entire seasons, for about $20 to $30 per kilowatt hour.
Co-authors with Chiang on the paper
are: first author Zheng Li, who was a postdoc at MIT during the research and is
now a professor at Virginia Tech; Fikile R. Brushett, the Raymond A. and Helen
E. St. Laurent Career Development Professor of Chemical Engineering; research
scientist Liang Su; graduate students Menghsuan Pan and Kai Xiang; and
undergraduate students Andres Badel, Joseph M. Valle, and Stephanie L. Eiler.
Finding the right balance
Development of the battery began in
2012, when Chiang joined the Department of Energy’s Joint Center
A major issue with batteries over
the past several decades, Chiang says, has been a focus on synthesizing materials
that offer greater energy density but are very expensive. The most widely used
materials in lithium-ion batteries for cellphones, for instance, have a cost of
about $100 for each kilowatt hour of energy stored.
“This meant maybe we weren’t
focusing on the right thing, with an ever-increasing chemical cost in pursuit
of high energy-density,” Chiang says. He brought the issue to other MIT
researchers. “We said, ‘If we want energy storage at the terawatt scale, we
have to use truly abundant materials.’”
The researchers first decided the
anode needed to be sulfur, a widely available byproduct of natural gas and
petroleum refining that’s very energy dense, having the lowest cost per stored
charge next to water and air. The challenge then was finding an inexpensive
liquid cathode material that remained stable while producing a meaningful
charge. That seemed improbable — until a serendipitous discovery in the lab.
On a short list of candidates was a
compound called potassium permanganate. If used as a cathode material, that
compound is “reduced” — a reaction that draws ions from the anode to the
cathode, discharging electricity. However, the reduction of the permanganate is
normally impossible to reverse, meaning the battery wouldn’t be rechargeable.
Still, Li tried. As expected, the
reversal failed. However, the battery was, in fact, recharging, due to an
unexpected oxygen reaction in the cathode, which was running entirely on air.
“I said, ‘Wait, you figured out a rechargeable chemistry using sulfur that does
not require a cathode compound?’ That was the ah-ha moment,” Chiang says.
Using that concept, the team of
researchers created a type of flow battery, where electrolytes are continuously
pumped through electrodes and travel through a reaction cell to create charge
or discharge. The battery consists of a liquid anode (anolyte) of polysulfide
that contains lithium or sodium ions, and a liquid cathode (catholyte) that
consists of an oxygenated dissolved salt, separated by a membrane.
Upon discharging, the anolyte
releases electrons into an external circuit and the lithium or sodium ions
travel to the cathode. At the same time, to maintain electroneutrality, the
catholyte draws in oxygen, creating negatively charged hydroxide ions. When
charging, the process is simply reversed. Oxygen is expelled from the
catholyte, increasing hydrogen ions, which donate electrons back to the anolyte
through the external circuit.
“What this does is create a charge
balance by taking oxygen in and out of the system,” Chiang says.
Because the battery uses
ultra-low-cost materials, its chemical cost is one of the lowest — if not the
lowest — of any rechargeable battery to enable cost-effective long-duration
discharge. Its energy density is slightly lower than today’s lithium-ion batteries.
“It’s a creative and interesting
new concept that could potentially be an ultra-low-cost solution for grid
storage,” says Venkat Viswanathan, an assistant professor of mechanical
engineering at Carnegie
Mellon University
Lithium-sulfur and lithium-air
batteries — where sulfur or oxygen are used in the cathode — exist today. But
the key innovation of the MIT research, Viswanathan says, is combining the two
concepts to create a lower-cost battery with comparable efficiency and energy
density. The design could inspire new work in the field, he adds: “It’s
something that immediately captures your imagination.”
Making renewables more
reliable
The prototype is currently about
the size of a coffee cup. But flow batteries are highly scalable, Chiang says,
and cells can be combined into larger systems.
As the battery can discharge over
months, the best use may be for storing electricity from notoriously
unpredictable wind and solar power sources. “The intermittency for solar is daily,
but for wind it’s longer-scale intermittency and not so predictable. When it’s
not so predictable you need more reserve — the capability to discharge a
battery over a longer period of time — because you don’t know when the wind is
going to come back next,” Chiang says. Seasonal storage is important too, he
adds, especially with increasing distance north of the equator, where the
amount of sunlight varies more widely from summer to winter.
Chiang says this could be the first
technology to compete, in cost and energy density, with pumped hydroelectric
storage systems, which provide most of the energy storage for renewables around
the world but are very restricted by location.
“The energy density of a flow
battery like this is more than 500 times higher than pumped hydroelectric
storage. It’s also so much more compact, so that you can imagine putting it
anywhere you have renewable generation,” Chiang says.
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