A new low-cost aluminum-sulfur battery
MIT-led team selects abundant materials for battery backup Metal Tech News – August 31, 2022
Last updated 4/16/2023 at 7:04am
Seeking an affordable and safer alternative to lithium-ion batteries for the storage of intermittent clean energy from wind and solar, a global team of researchers led by an award-winning chemist at the Massachusetts Institute of Technology has developed a new rechargeable battery made with affordable and readily available materials – aluminum, sulfur, and molten salts.
"I wanted to invent something that was better, much better, than lithium-ion batteries for small-scale stationary storage, and ultimately for automotive (uses)," said Donald Sadoway, the John F. Elliott Professor Emeritus of Materials Chemistry at MIT.
This is a tall order when you consider that automotive and electronic manufacturers, scientists, and governments around the world have adopted lithium-ion batteries as the crème de la crème when it comes to the storage of electricity.
With lithium batteries going into virtually every electric vehicle being manufactured today, however, automakers are having a tough time finding enough of the minerals and metals for these batteries, which is increasing the costs of these battery ingredients.
Adding to the supply chain and cost issues being driven by the EV sector, lithium batteries are currently the most common way to store renewable energy when the sun is not shining or the wind is not blowing.
In addition to being expensive, lithium-ion batteries contain a flammable electrolyte that has caused problems for both EVs and stationary storage.
The aluminum-sulfur batteries developed by Sadoway and his team offer a safer and less expensive alternative made with plentiful materials.
"The ingredients are cheap, and the thing is safe - it cannot burn," the MIT professor said.
Scouring the periodic table
Wanting to create a battery that overcomes the safety and supply chain shortfalls of lithium batteries, Sadoway began scouring the periodic table for an inexpensive and plentiful alternative to lithium. This led the MIT professor to aluminum, an element with the required electrochemical properties and happens to be the most abundant metal in Earth's crust.
"So, I said, 'well, let's just make that a bookend. It's gonna be aluminum'," he recalled.
Then came deciding what to pair the aluminum with for the other electrode bookend and what kind of electrolyte to put in between to carry ions back and forth during charging and discharging.
Sulfur, the cheapest of all the non-metals, was selected as the second electrode.
After immediately eliminating organic liquids that could catch fire and trying various polymers, the Sadoway-led team selected molten salts that melt at temperatures below the boiling point of water as the electrolyte of choice.
"Once you get down to near body temperature, it becomes practical to make batteries that don't require special insulation and anticorrosion measures," Sadoway explains.
Due to the low melting point of the selected molten salts – made up of sodium chloride, potassium chloride, and aluminum chloride – the aluminum-sulfur battery requires no external heat source to maintain its operating temperature.
"As you charge, you generate heat, and that keeps the salt from freezing. And then, when you discharge, it also generates heat," Sadoway said.
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," the MIT professor went on to explain.
A serendipitous advantage
The selected molten salts also offered up an unexpected and welcome benefit; they helped make the batteries more durable and longer lasting.
One of the biggest obstacles scientists have come up against when developing rechargeable batteries is the formation of dendrites, narrow spikes of metal that build up on one electrode and eventually grow across to contact the other electrode, which short-circuits the cell and lowers the efficiency and lifespan of the battery.
The molten salts chosen by the research team for their low melting point also happen to be very good at preventing this short-circuiting malfunction in the aluminum sulfur battery.
"We did experiments at very high charging rates, charging in less than a minute, and we never lost cells due to dendrite shorting," said Sadoway.
"If we had started off with trying to prevent dendritic shorting, I'm not sure I would've known how to pursue that," he added. "I guess it was serendipity for us."
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.
The high charging rate, however, is highly dependent on the working temperature, with the aluminum-sulfur batteries charging 25 times faster at 230 degrees Fahrenheit (110 degrees Celsius) than at 25 C (77 F). Fortunately, the heat produced from the electrochemical reaction of charging and discharging helps maintain the roughly water-boiling-point temperatures needed for these higher charge rates.
Homes and rapid chargers
Sadoway said aluminum-sulfur batteries would be ideal for applications that require a few tens of kilowatt-hours of electrical storage capacity, which would be enough to power a single home or small to medium businesses.
These safe and inexpensive batteries would also be ideal for ultra-fast charging stations for EVs.
As more EVs pull up to chargers to fill up their batteries, electrical grids will have a tough time delivering the amperages needed. Batteries at charging stations would level the load while offering the ability to deliver a quick burst of charging power. This would speed up charging with less need to install expensive new power lines.
For megawatt-hour scale storage, such as would be needed for grid- or industrial-scale storage, other battery technologies might be more effective.
It so happens that Sadoway and former students at MIT developed a liquid-metal battery that is proving to be ideal for storing wind and solar power at this scale.
MIT spun this technology out into Ambri Inc., a public company that hopes to deliver its first products within the next year.
"The first order of business for the company is to demonstrate that it works at scale," said Sadoway, a co-founder of Ambri.
Xcel Energy, a utility company that generates enough electricity to power roughly 23 million homes across eight states, recently selected Ambri's molten-salt batteries as a potential long-term clean energy storage solution.
Further details can be read at US utility testing liquid metal batteries in the current edition of Metal Tech News.
Sadoway was recently bestowed the 2022 European Inventor Award for the molten salt battery technology being commercialized by Ambri.
The aluminum-sulfur battery 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.
A paper on their findings, Fast-charging aluminium–chalcogen batteries resistant to dendritic shorting, was published in the journal Nature.