Made from inexpensive and abundant materials, an aluminum-sulfur battery could provide low-cost back-up storage for renewable energy sources.
MIT news desk
A new concept of low-cost batteries: As the world builds ever-larger wind and solar power system installations, the need for large-scale, cost-effective backup systems is rapidly increasing to provide power when the the sun has set and the air is calm. Today’s lithium-ion batteries are still too expensive for most of these applications, and other options such as pumped hydro require specific topography that is not always available.
Now, researchers at MIT and elsewhere have developed a new kind of battery, made entirely from inexpensive, abundant materials, that could help fill that gap.
The new battery architecture, which uses aluminum and sulfur as two electrode materials, with a molten salt electrolyte in between, is described today in the review Naturein a document 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 automotive. [uses]says Sadoway, who is the John F. Elliott Emeritus Professor of Materials Chemistry.
In addition to being expensive, lithium-ion batteries contain a flammable electrolyte, which makes them less than ideal for transportation. So Sadoway began to study the periodic table, looking for cheap, Earth-abundant metals that could replace lithium. The commercially dominant metal, iron, does not have the right electrochemical properties for an efficient battery, he says. But the second most abundant metal on the market – and in fact the most abundant metal on Earth – is aluminum. “So, I said, well, let’s just make this a bookend. It will be aluminum,” he says.
Then we had to decide what to combine the aluminum with for the other electrode, and what type of electrolyte to put in between to transport the ions back and forth during charging and discharging. The cheapest of all nonmetals is sulfur, which therefore became the second electrode material. As for the electrolyte, “we weren’t going to use the volatile, flammable organic liquids” that have sometimes caused dangerous fires in cars and other lithium-ion battery applications, 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 anti-corrosion measures, he says.
The three ingredients they ended up with are cheap and readily available – aluminum, no different from aluminum 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 for sure – it can’t burn,” Sadoway says.
A new concept for low-cost batteries: In their experiments, the team showed that battery cells could sustain hundreds of cycles at exceptionally high charge rates, with a projected cost per cell of around one-sixth that of comparable lithium-ion cells. They showed that the charging rate was highly dependent on working temperature, with 110 degrees Celsius (230 degrees Fahrenheit) showing rates 25 times faster than 25 C (77 F).
Surprisingly, the molten salt the team chose as the electrolyte simply because of its low melting point turned out to have a serendipitous advantage. One of the biggest battery reliability issues is the formation of dendrites, which are narrow spikes of metal that collect on one electrode and eventually grow to come into contact with the other electrode, causing a short- circuit and hindering efficiency. But this particular salt, it turns out, is very effective in preventing this dysfunction.
The chloro-aluminate salt they chose “essentially removed these runaway dendrites, while allowing for very fast charging,” says Sadoway. “We experimented at very high charging rates, charging in less than a minute, and we never lost any cells due to dendrite shorting.”
“It’s funny,” he says, because the focus was on finding a salt with the lowest melting point, but the catenated chloroaluminates they ended up with proved resistant. short circuit problem. “If we had started by trying to prevent dendritic shorting, I’m not sure I would have known how to pursue that,” Sadoway says. “I guess it was a fluke for us.”
In addition, the battery does not require any external heat source to maintain its operating temperature. Heat is naturally produced electrochemically by charging and discharging the battery. “As you load, you generate heat, which keeps the salt from freezing. And then when you unload, it also generates heat,” says Sadoway. In a typical setup used for load leveling in a solar generating facility, for example, “you store electricity when the sun is shining, then you draw electricity after dark, and you do it every day. And this charge-idle-discharge- inactivity is enough to generate enough heat to keep the thing warm.
This new battery formulation, he says, would be ideal for installations the size needed to power a single home or a small or medium-sized business, producing on the order of a few tens of kilowatt hours of storage capacity.
For larger installations, up to a utility scale of tens to hundreds of megawatt hours, other technologies might be more efficient, including the liquid metal batteries that Sadoway and his students developed several years ago and which formed the basis of a spin-off company called Ambri, which hopes to deliver its first products next year. For this invention, Sadoway was recently reward this year’s European Inventor Award.
A new concept for low-cost batteries: The smaller size of aluminium-sulfur batteries would also make them practical for uses such as electric vehicle charging stations, says Sadoway. He points out that when electric vehicles become common enough on the roads that multiple cars want to charge at the same time, as is the case today with gas pumps, “if you try to do that with batteries and you want a quick recharge, the amperages are just so high that we don’t have that amount of amperage in the line feeding the facility.” So having a battery system like this to store energy and releasing it quickly when needed could eliminate the need to install expensive new power lines to serve these chargers.
The new technology is already the basis of a new spin-off company called Avanti, which licensed the patents for 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 put it through a series of stress tests, including going through hundreds of charge cycles.
Would a sulfur-based battery cause the foul odors associated with some forms of sulfur? Not a chance, said Sadoway. “The rotten egg smell is in the gas, the hydrogen sulfide. It’s elemental sulfur, and it’s going to be locked inside the cells. If you were to try to crack open a lithium-ion cell in your kitchen, he says (and please don’t try this at home!), “the humidity in the air would react and you’d start to generate all sorts of gas too These are legitimate questions but the battery is sealed, it’s not an open container so I wouldn’t worry about that.
The research team included members from Peking University, Yunnan University, and Wuhan University of Technology, China; the University of Louisville, Kentucky; the University of Waterloo, Canada; Oak Ridge National Laboratory, Tennessee; and MIT. The work was supported by the MIT Energy Initiative, the MIT Deshpande Center for Technological Innovation and the ENN Group.
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