- The components used to build an aluminum-sulfur battery are all readily available and reasonably priced.
- Aluminum and sulfur serve as the battery’s two electrode components, with a layer of molten salt serving as the electrolyte.
- Some other inventions, like the liquid metal batteries Sadoway and his students created a number of years ago, served as the basis for a spinoff business named Ambri, which aims to release its first products in the next year.
Aluminum-sulfur battery could operate at extraordinarily high charging rates
The need for affordable, large-scale backup systems to provide electricity when the sun goes down and the weather is calm is increasing quickly as the world develops ever larger installations of wind and solar power systems. The majority of these applications still cannot afford today’s lithium-ion batteries, and alternative solutions like pumped hydro require a particular topography that is not always available.
Researchers from MIT and other institutions have created a brand-new type of battery that could help close that gap. It is constructed entirely from readily available and affordable components.
Donald Sadoway, an MIT professor, and 15 other researchers from MIT, as well as those in China, Canada, Kentucky, and Tennessee, describe the new battery architecture in a paper that was published today in the journal Nature. The battery uses aluminum and sulfur as its two electrode materials, with a molten salt electrolyte in between.
“I wanted to invent something that was better, much better, than lithium-ion batteries for small-scale stationary storage, and ultimately for automotive [uses],” stated Sadoway, who is the John F. Elliott Professor Emeritus of Materials Chemistry.
Lithium-ion batteries are not the best for transportation because they are pricey and their electrolyte is combustible. Sadoway then started going through the periodic table for low-cost, readily available metals that could perhaps take the place of lithium. According to him, iron, which now holds the majority of commercial use, lacks the proper electrochemical characteristics for an effective battery. Aluminum, on the other hand, is the second-most plentiful metal on the market and the most abundant metal overall.
“So, I said, well, let’s just make that a bookend. It’s gonna be aluminum,” he added.
The next step was choosing the other electrode to go with the aluminum and the electrolyte that would go between them to transport ions back and forth during charging and discharging. Sulfur was chosen as the second electrode component since it is the least expensive non-metal. The volatile, flammable organic chemicals that have occasionally resulted in dangerous fires in cars and other applications of lithium-ion batteries were not going to be used as the electrolyte, according to Sadoway.
In the end, they looked at a variety of molten salts with relatively low melting points — close to the boiling point of water as opposed to roughly 1,000 degrees Fahrenheit for many salts. They had initially examined various polymers before creating the aluminum-sulfur battery. Making batteries that don’t need additional insulation and anticorrosion methods becomes feasible once temperatures are close to body temperature, he states.
Aluminum, which is identical to the foil from the grocery store, sulfur, which is frequently a waste product from activities like petroleum refining, and generally accessible salts are the three materials they ultimately settled on. “The ingredients are cheap, and the thing is safe — it cannot burn,” Sadoway explains.
In their tests, the scientists demonstrated that the aluminum-sulfur battery could operate at extraordinarily high charging rates for hundreds of cycles with a predicted cost per cell that was roughly one-sixth that of comparable lithium-ion cells. The operating temperature has a significant impact on the charging rate, with 110 degrees Celsius (230 degrees Fahrenheit) demonstrating 25 times faster rates than 25 C. (77 F).
Unexpectedly, the team’s decision to employ molten salt as an electrolyte simply because of its low melting point ended up having an unexpected benefit. The growth of dendrites—narrow metal spikes that develop on one electrode and eventually spread across to make contact with the other electrode, short-circuiting the battery and impairing efficiency—is one of the main issues with battery reliability. However, it so happens that this specific salt is excellent at avoiding the such issue.
The salt they used, chloro-aluminate, “essentially retired these runaway dendrites, while also allowing for very rapid charging,” according to Sadoway. “We did experiments at very high charging rates, charging in less than a minute, and we never lost cells due to dendrite shorting.”
The catenated chloro-aluminates they eventually came up with turned out to be immune to the shorting problem, even though the entire focus was on finding a salt with the lowest melting point.
“If we had started off with trying to prevent dendritic shorting, I’m not sure I would’ve known how to pursue that. I guess it was serendipity for us,” says Sadoway.
Additionally, the aluminum-sulfur battery can maintain its operating temperature without using an external heat source. The charging and discharging of the battery naturally generate heat through electrochemistry.
“As you charge, you generate heat, and that keeps the salt from freezing. And then, when you discharge, it also generates heat. 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,” explains Sadoway.
According to him, this new aluminum-sulfur battery composition would be excellent for installations that produce storage capacity on the order of a few tens of kilowatt-hours and are roughly the size needed to power a single home or small to medium business.
Other technologies, such as the liquid metal batteries Sadoway and his students developed several years ago and which served as the foundation for a spinoff company called Ambri, which hopes to deliver its first products within the next year, may be more effective for larger installations, up to utility-scale of tens to hundreds of megawatt hours. Sadoway recently received the 2018 European Inventor Award for that creation.
According to Sadoway, the aluminum-sulfur batteries’ lower size would also make them useful for applications like electric vehicle charging stations. He makes the observation that when electric vehicles are widely used on the roadways, many automobiles will want to charge at once, just like with gasoline fuel pumps now.
“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 it may not be necessary to create expensive new power lines to supply these chargers if a battery system like this can store energy and swiftly release it when needed.
The new technology is already the foundation for a new spinoff business named Avanti, which was created by Sadoway and Ambri co-founder Luis Ortiz (’96 ScD ’00), and which has leased the rights to the system. “The first order of business for the company is to demonstrate that it works at scale,” says Sadoway, adding that after that, it will go through a number of stress tests, such as undergoing hundreds of charging cycles.
Would a sulfur-based battery be susceptible to emitting unpleasant smells connected to various kinds of sulfur? According to Sadoway, the answer is absolutely no.
“The rotten-egg smell is in the gas, hydrogen sulfide. This is elemental sulfur, and it’s going to be enclosed inside the cells. 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,” he explains.