Engineers at the Massachusetts Institute of Technology have developed a battery from inexpensive, abundant materials that could provide low-cost backup storage for renewable energy sources. Less expensive than lithium-ion battery technology, the new architecture uses aluminum and sulfur as the two electrode materials with a molten salt electrolyte in between.
The new battery architecture is described in a Nature journal article by MIT Professor Donald Sadoway along with 15 others at MIT and in China, Canada, Kentucky and Tennessee.
Wind and solar advocates are building larger and larger installations of wind and solar power systems, so the need for economical, large-scale backup systems to provide power when the sun is down and the air is calm is growing rapidly. Today’s lithium-ion batteries are still too expensive for most of these applications, and other options like pumped hydro require specific landscapes that are not always available.
The chemistry architecture of the MIT-led group could help close the disruption gaps.
Sadoway, John F. Elliott Professor Emeritus of Materials Chemistry, said, “I wanted to invent something that would be better, much better than lithium-ion batteries for small stationary storage and ultimately for the automotive industry [uses].”
In addition to being expensive, lithium-ion batteries contain a flammable electrolyte, making them less convenient for transportation. So Sadoway began studying the periodic table in search of cheap, earth-abundant metals that could potentially replace lithium. The commercially dominant metal, iron, doesn’t have the right electrochemical properties for an efficient battery, he said. But the second most common metal on the market – and indeed the most abundant metal on earth – is aluminum.
“Well, I said, let’s just make a bookend out of it. It will be aluminum,” he commented.
The next thing to decide was what to pair the aluminum with for the other electrode and what type of electrolyte to put in between to transport ions back and forth during charging and discharging. The cheapest of all non-metals is sulfur, so it became the second electrode material.
Regarding the electrolyte, “we didn’t want to use volatile, flammable organic liquids,” which have sometimes led to dangerous fires in cars and other lithium-ion battery applications, Sadoway said. They tried a few 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°F for many salts. “Once you get close to body temperature, it becomes practical” to make batteries that don’t require special insulation and anti-corrosion measures, he noted.
The three ingredients they ended up with are cheap and readily available – aluminum, not unlike the foil you find in the supermarket; sulfur, which is often a waste product from processes such as petroleum refining; and common salts. “The ingredients are cheap, and the thing is safe — it can’t burn,” Sadoway said.
In their experiments, the team showed that the battery cells can endure hundreds of cycles at exceptionally high charge rates, with a projected cost per cell of about one-sixth that of comparable lithium-ion cells. They showed that the charge rate is highly dependent on the working temperature, with 110 °C (230 °F) having 25 times faster charge rates than 25 °C (77 °F).
Surprisingly, the molten salt, which the team chose as the electrolyte solely because of its low melting point, turned out to have an accidental advantage. One of the biggest problems with battery reliability is the formation of dendrites, narrow spikes of metal that build up on one electrode and eventually overgrow to contact the other electrode, causing a short circuit and affecting efficiency. But this particular salt is very good at preventing this malfunction.
The chloroaluminate salt they chose “essentially pulled those runaway dendrites back while still allowing for very fast charging,” Sadoway said. “We’ve run experiments with very high charge rates, charged in less than a minute, and we’ve never lost cells due to dendrite shorting.”
“It’s weird,” he said, because the whole focus was on finding a salt with the lowest melting point, but the chained chloroaluminates they ended up with proved resistant to the short circuit problem. “If we had started trying to prevent dendritic shorting, I’m not sure I would have known how to go about it,” Sadoway said. “I think it was a lucky coincidence for us.”
In addition, the battery does not require an external heat source to maintain its operating temperature. The heat is naturally generated electrochemically by charging and discharging the battery. “When charging, heat is generated that keeps the salt from freezing. And then when you discharge, it also generates heat,” Sadoway said.
For example, in a typical load-balancing installation in a solar array, “you store electricity when the sun is out, and then you draw electricity after dark, every day. And that charge-idle-discharge-idle is enough to generate enough heat to keep the thing up to temperature.”
This new battery formulation, he says, would be ideal for installations about the size needed to power a single-family home or small-to-medium-sized business and produce on the order of tens of kilowatt-hours of storage capacity.
For larger installations, up to tens to hundreds of megawatt hours of supply scale, other technologies might be more effective, including the liquid metal batteries that Sadoway and his students developed a few years ago and that formed the basis of a spin-off company called Ambri, the first hopes deliver products within the next year. Sadoway was recently awarded this year’s European Inventor Award for this invention.
The smaller scale of aluminium-sulfur batteries would also make them practical for applications such as electric vehicle charging stations, Sadoway noted. He points out that when electric vehicles are so widespread on the roads that multiple cars want to charge at the same time, as is the case with petrol pumps today, “if you’re going to try batteries and want fast charging, the current levels are fair high that we don’t have that level of amperage on the line feeding the facility.” A battery system like this, which stores power and quickly releases it when needed, could eliminate the need to install expensive new power lines for these chargers.
The new technology is already the basis for a new spinoff company called Avanti, which has licensed the patents for the system, which was co-founded by Sadoway and Luis Ortiz ’96 ScD ’00, who also co-founded Ambri. “The first task for the company is to demonstrate that it works at scale,” Sadoway said, and then subject it to a series of stress tests, including running it through hundreds of charge cycles.
Would a sulfur based battery run the risk of producing the foul odors associated with some forms of sulfur? Not a chance, Sadoway said. “The smell of rotten eggs is in the gas, hydrogen sulfide. That’s elemental sulfur, and it gets trapped in the cells.” If you tried to open a lithium-ion cell in your kitchen, he says (and please don’t try that at home!), “the moisture would get in react with the air and you would start producing all kinds of putrefaction including gases. Those are legitimate questions, but the battery is sealed, it’s not an open vessel. I wouldn’t worry about that.”
The research team included members from Peking University, Yunnan University and 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 with. The work was supported by the MIT Energy Initiative, the MIT Deshpande Center for Technological Innovation, and the ENN Group.
That sounds pretty good! But no mention of how many watt-hours by battery size or weight. Then there are those “scale” things that show up as you exit the lab and get to a factory facility.
One hopes for further refinement. The press release makes it sound like the whole thing just fell apart because of low cost decisions. You can be sure that a lot of thought went into these decisions.
However, what we do know is that this technology needs to be used to stay warm enough to work well. There has to be an energy price and that was not revealed or discussed. One should be sure that these are really cheap as continuous operation with only cycles in the “hundreds” might not really cut the economic launch tape.
By Brian Westenhaus on New Energy and Fuel
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