Donald Sadoway has been a faculty member at MIT since 1978, where he has taught a course in solid state chemistry for the past 16 years. In addition to his teaching duties, he has also been directly involved in research into how to make batteries that cost less than lithium-ion batteries. Recently, MIT announced that Sadoway and his research partners have created an aluminum sulfur battery that can do just that. The research was conducted by researchers from Peking University, Yunnan University and Wuhan University of Technology in China; University of Louisville in Kentucky; University of Waterloo in Canada; Oak Ridge National Laboratory in Tennessee; and MIT.
The new battery architecture uses aluminum and sulfur as its two electrode materials, with a molten salt electrolyte in between. With the price of lithium skyrocketing due to increasing demand, the world needs cheap alternatives. Aluminum and sulfur are plentiful and cheap. Sadoway says aluminum-sulfur battery cells will cost about $9 per kWh, which is much less than currently available lithium-ion battery cells. The new cells are not suitable for use in electric vehicles, but could reduce the cost of small batteries for households and small business customers, which could make more lithium-based cells available for the transportation sector. The research was recently published in the journal Nature.
Sadoway began by studying the periodic table, looking for cheap, abundant metals that might be able to replace lithium. Iron, which is becoming an increasingly popular choice, doesn’t have the right electrochemical properties for an efficient battery, he says. But aluminum—the most common metal on Earth—does. “So I said, well, let’s make it a bookend. It will be aluminum,” he says.
Then came the decision of what to pair the aluminum with for the second electrode and what kind of electrolyte to put in between so that the ions are transferred back and forth during charging and discharging. Sulfur is the cheapest of all non-metals, so it has become the second electrode material. As for the electrolyte, “we weren’t going to use volatile, flammable organic liquids” that have sometimes led to dangerous fires in cars and other lithium-ion battery applications, Sadoway says.
The researchers tried some polymers, but ended up looking at various molten salts, which have relatively low melting points—close to the boiling point of water instead of the nearly 1,000°F for many salts. “Once you get 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, indistinguishable from supermarket foil; sulfur, which is often a waste product from processes such as oil refining; and widely available salts. “The ingredients are cheap and the thing is safe—it can’t burn,” says Sadoway MIT News.
In their experiments, the team showed that the battery cells can last for hundreds of cycles at extremely high charging rates. The charging rate was closely related to the temperature of the electrolyte. At 110 °C (230 °F), the experimental batteries charged 25 times faster than at 25 °C (77 °F).
The researchers chose the electrolyte simply because of its low melting point, but it turned out to have a significant advantage. One of the biggest battery reliability problems is the formation of dendrites – narrow metal spikes that accumulate on one electrode and eventually grow to contact the other electrode. When this happens, it creates a short circuit that not only limits efficiency, but can lead to “thermal runaway” – which is a polite way of saying a battery fire. But the electrolyte they started with turned out to be very good at preventing dendrites from forming.
The chloroaluminate salt they chose “basically eliminated these leaky dendrites while allowing very fast charging,” says Sadoway. “We’ve done experiments at very high charging rates, charging in less than a minute, and we’ve never lost cells due to dendrite shorting.”
“It’s funny,” he says, because the whole focus was on finding the salt with the lowest melting point, but the catenated chloroaluminates they ended up with proved resistant to the short circuit problem. “If we started by trying to prevent dendritic shunting, I’m not sure I would know how to do that,” says Sadoway. “I think it was a fluke for us.
An aluminum-sulfur battery does not need an external heat source
The researchers found that the aluminum-sulfur battery they were working on required no external heat source to maintain its operating temperature. Heat is naturally produced electrochemically as the battery charges and discharges. “When you charge, you create heat, and that prevents the salt from freezing. And then when you discharge, it also generates heat,” says Sadoway.
In a typical installation used for load balancing in solar, for example, “you store electricity when the sun is shining, and then you take electricity after dark, and you would do that every day. And that charge-idle-discharge-idle is enough to generate enough heat to keep the thing up to temperature.”
The smaller range of aluminum-sulfur batteries would also make them practical for uses such as charging stations for electric cars. Sadoway says that when multiple EVs are to be charged at once, “if you want fast charging, the currents are so high that we don’t have the amount of current in the line to power the devices.” Having an aluminum sulfur battery to store energy and release it quickly when needed could eliminate the need to install expensive new electrical wiring to serve these chargers. Many places are already starting to add batteries to charging stations. The new batteries would significantly reduce the cost of adding battery storage to EV charging stations.
The new technology is already the basis of a spinoff company called Avanti, co-founded by Sadoway and Luis Ortiz, which has licensed the system’s patents. “The company’s first task is to prove that it works at scale,” says Sadoway, before putting it through a series of stress tests, including hundreds of charge cycles.
Would a sulfur-based battery run the risk of unpleasant odors associated with some forms of sulfur? Not a chance, says Sadoway. “The smell of rotten eggs is in the gas, hydrogen sulfide. This is elemental sulfur and it will be locked inside the cells.” If you tried to open up a lithium-ion cell in your kitchen, he says, “the moisture in the air would react and you’d start generating all kinds of harmful gases as well. These are legitimate questions, but the battery is sealed, not an open container. So I wouldn’t worry about that.”
New battery technologies are popping up all over the world, but anything with Donald Sadoway’s name on it is worth considering. There is a lot of hand-wringing about how invasive mining is and the environmental consequences of the search for battery materials. (Oddly enough, ripping off mountaintops and dumping them into the valleys below to get to coal never raises the same concerns.) Creating new battery technologies that use some of the most abundant minerals on Earth seems to be a no-brainer. , if the resulting performance is almost acceptable for commercial use.
This research also shows how demonizing people who happen to be Chinese can delay the critical research needed to effectively address global warming. We are all in this together, and it will require all of us working together to keep the Earth livable for future generations of people. The need is great and the time is now. Let’s stop bickering with each other and solve the problems that affect us all. That means electing representatives who will support clean energy for all of humanity.
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