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 in the journal Nature.
In addition to being expensive, lithium-ion batteries contain a flammable electrolyte,
making them less than ideal for transportation.
So, the researcher 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.
But the second-most-abundant metal in the marketplace — and actually the most abundant
metal on Earth — is aluminum.
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, the researchers were not willing to use the volatile, flammable
organic liquids that have sometimes led to dangerous fires in cars and other applications
of lithium-ion batteries.
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.
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.
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 showing 25 times faster rates than 25 C.
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.
The researchers did experiments at very high charging rates, charging in less than a minute,
and they never lost cells due to dendrite shorting.
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.
This new battery formulation, 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 the researchers
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.
The smaller scale of the aluminum-sulfur batteries would also make them practical for uses such
as electric vehicle charging stations.
The researcher 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.
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.