Made from inexpensive, abundant materials, an aluminum-sulfur battery could provide low-cost backup storage for renewable energy sources.
By David L. Chandler at MIT News Office
August 24, 2022 -- As the world builds out ever larger installations of
wind and solar power systems, the need is growing fast for economical,
large-scale backup systems to provide power when the sun is down and the air is
calm. Today’s lithium-ion batteries are still too expensive for most such
applications, and other options such as pumped hydro require specific
topography that’s not always available.
Now, researchers at MIT and elsewhere have developed a new kind of
battery, made entirely from abundant and inexpensive materials, that could help
to fill that gap.
The new battery architecture, which uses aluminum and sulfur as its two
electrode materials, with a molten salt electrolyte in between, is described
today in the journal Nature, in a
paper by MIT Professor Donald Sadoway, along with 15 others at MIT and
in China, Canada, Kentucky, and Tennessee.
“I wanted to invent something that was better, much better, than
lithium-ion batteries for small-scale stationary storage, and ultimately for
automotive [uses],” explains Sadoway, who is the John F. Elliott Professor
Emeritus of Materials Chemistry.
In addition to being expensive, lithium-ion batteries contain a flammable
electrolyte, making them less than ideal for transportation. So, Sadoway
started studying the periodic table, looking for cheap, Earth-abundant metals
that might be able to substitute for lithium. The commercially dominant metal,
iron, doesn’t have the right electrochemical properties for an efficient
battery, he says. But the second-most-abundant metal in the marketplace — and
actually the most abundant metal on Earth — is aluminum. “So, I said, well,
let’s just make that a bookend. It’s gonna be aluminum,” he says.
Then came deciding what to pair the aluminum with for the other
electrode, and what kind of electrolyte to put in between to carry ions back
and forth during charging and discharging. The cheapest of all the non-metals
is sulfur, so that became the second electrode material. As for the
electrolyte, “we were not going to use the volatile, flammable organic liquids”
that have sometimes led to dangerous fires in cars and other applications of
lithium-ion batteries, Sadoway says. They tried some polymers but ended up
looking at a variety of molten salts that have relatively low melting points —
close to the boiling point of water, as opposed to nearly 1,000 degrees
Fahrenheit for many salts. “Once you get down to near body temperature, it
becomes practical” to make batteries that don’t require special insulation and
anticorrosion measures, he says.
The three ingredients they ended up with are cheap and readily available
— aluminum, no different from the foil at the supermarket; sulfur, which is
often a waste product from processes such as petroleum refining; and widely
available salts. “The ingredients are cheap, and the thing is safe — it cannot
burn,” Sadoway says.
In their experiments, the team showed that the battery cells could endure
hundreds of cycles at exceptionally high charging rates, with a projected cost
per cell of about one-sixth that of comparable lithium-ion cells. They showed
that the charging rate was highly dependent on the working temperature, with
110 degrees Celsius (230 degrees Fahrenheit) showing 25 times faster rates than
25 C (77 F).
Surprisingly, the molten salt the team chose as an electrolyte simply
because of its low melting point turned out to have a fortuitous advantage. One
of the biggest problems in battery reliability is the formation of dendrites,
which are narrow spikes of metal that build up on one electrode and eventually
grow across to contact the other electrode, causing a short-circuit and
hampering efficiency. But this particular salt, it happens, is very good at
preventing that malfunction.
The chloro-aluminate salt they chose “essentially retired these runaway
dendrites, while also allowing for very rapid charging,” Sadoway says. “We did
experiments at very high charging rates, charging in less than a minute, and we
never lost cells due to dendrite shorting.”
“It’s funny,” he says, because the whole focus was on finding a salt with
the lowest melting point, but the catenated chloro-aluminates they ended up
with turned out to be resistant to the shorting problem. “If we had started off
with trying to prevent dendritic shorting, I’m not sure I would’ve known how to
pursue that,” Sadoway says. “I guess it was serendipity for us.”
What’s more, the battery requires no external heat source to maintain its
operating temperature. The heat is naturally produced electrochemically by the
charging and discharging of the battery. “As you charge, you generate heat, and
that keeps the salt from freezing. And then, when you discharge, it also
generates heat,” Sadoway says. In a typical installation used for load-leveling
at a solar generation facility, for example, “you’d store electricity when the
sun is shining, and then you’d draw electricity after dark, and you’d do this
every day. And that charge-idle-discharge-idle is enough to generate enough
heat to keep the thing at temperature.”
This new battery formulation, he says, would be ideal for installations
of about the size needed to power a single home or small to medium business,
producing on the order of a few tens of kilowatt-hours of storage capacity.
For larger installations, up to utility scale of tens to hundreds of
megawatt hours, other technologies might be more effective, including the
liquid metal batteries Sadoway and his students developed several years ago and
which formed the basis for a spinoff company called Ambri, which hopes to
deliver its first products within the next year. For that invention, Sadoway
was recently awarded this year’s European Inventor Award.
The smaller scale of the aluminum-sulfur batteries would also make them
practical for uses such as electric vehicle charging stations, Sadoway says. He
points out that when electric vehicles become common enough on the roads that
several cars want to charge up at once, as happens today with gasoline fuel
pumps, “if you try to do that with batteries and you want rapid charging, the
amperages are just so high that we don’t have that amount of amperage in the
line that feeds the facility.” So having a battery system such as this to store
power and then release it quickly when needed could eliminate the need for
installing expensive new power lines to serve these chargers.
The new technology is already the basis for a new spinoff company called
Avanti, which has licensed the patents to the system, co-founded by Sadoway and
Luis Ortiz ’96 ScD ’00, who was also a co-founder of Ambri. “The first order of
business for the company is to demonstrate that it works at scale,” Sadoway
says, and then subject it to a series of stress tests, including running
through hundreds of charging cycles.
Would a battery based on sulfur run the risk of producing the foul odors
associated with some forms of sulfur? Not a chance, Sadoway says. “The
rotten-egg smell is in the gas, hydrogen sulfide. This is elemental sulfur, and
it’s going to be enclosed inside the cells.” If you were to try to open up a
lithium-ion cell in your kitchen, he says (and please don’t try this at home!),
“the moisture in the air would react and you’d start generating all sorts of
foul gases as well. These are legitimate questions, but the battery is sealed,
it’s not an open vessel. So I wouldn’t be concerned about that.”
The research team included members from Peking University, Yunnan
University and the Wuhan University of Technology, in China; the University of
Louisville, in Kentucky; the University of Waterloo, in Canada; Oak Ridge
National Laboratory, in Tennessee; and MIT. The work was supported by the MIT
Energy Initiative, the MIT Deshpande Center for Technological Innovation, and
ENN Group.
https://news.mit.edu/2022/aluminum-sulfur-battery-0824
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