Energy

Rare form of sulfur offers a key to triple-capacity EV batteries

Rare form of sulfur offers a k...
Lithium-sulfur batteries hold great potential when it comes to powering electric vehicles of the future
Lithium-sulfur batteries hold great potential when it comes to powering electric vehicles of the future
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An up-close look at the carbon nanofiber cathode developed at Drexel University
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An up-close look at the carbon nanofiber cathode developed at Drexel University
Scientists have stabilized a rare form of sulfur within a novel cathode, paving the way for a new breed of high-capacity batteries
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Scientists have stabilized a rare form of sulfur within a novel cathode, paving the way for a new breed of high-capacity batteries
Lithium-sulfur batteries hold great potential when it comes to powering electric vehicles of the future
3/3
Lithium-sulfur batteries hold great potential when it comes to powering electric vehicles of the future
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As electric vehicles continue to grow in popularity, scientists see great potential in lithium-sulfur batteries as a more environmentally friendly way to power them. This is because they don't rely on the same expensive and difficult-to-source raw materials, such as cobalt, but other problems relating to their stability has held the technology back so far. Engineers at Drexel University have made a breakthrough they say takes these batteries closer to commercial use, by leveraging a rare chemical phase of sulfur to prevent damaging chemical reactions.

Lithium-sulfur batteries hold a lot of promise when it comes to energy storage, and not just because sulfur is abundant and less problematic to source than the cobalt, manganese and nickel used in today's batteries. They may offer some significant performance gains, too, with the potential to store several times the energy of today's lithium-ion batteries. But there is one problem that scientists keep running into, which is the formation of chemical compounds called polysulfides.

As the battery operates, these make their way into the electrolyte – the solution that carries the charge back and forth between the anode and cathode – where they trigger chemical reactions that compromise the battery's capacity and lifespan. Scientists have had some success swapping out the carbonate electrolyte for an ether electrolyte, which doesn't react with the polysulfides. But this poses other problems, as the ether electrolyte itself is highly volatile and contains components with low boiling points, meaning the battery could quickly fail or meltdown if warmed above room temperature.

The chemical engineers at Drexel University have been working on another solution and it starts with the design of a new cathode, which can work with the carbonate electrolytes already in commercial use. This cathode is made from carbon nanofibers and had already been shown to slow the movement of polysulfides in an ether electrolyte. But making it work with a carbonate electrolyte involved some experimentation.

An up-close look at the carbon nanofiber cathode developed at Drexel University
An up-close look at the carbon nanofiber cathode developed at Drexel University

“Having a cathode that works with the carbonate electrolyte that they’re already using is the path of least resistance for commercial manufacturers,” said lead researcher Vibha Kalra. “So rather than pushing for the industry adoption of a new electrolyte, our goal was to make a cathode that could work in the pre-existing Li-ion electrolyte system.”

The scientists attempted to confine the sulfur in the carbon nanofiber mesh to prevent the dangerous chemical reactions using a technique called vapor disposition. This didn't quite have the desired effect, but as it turned out, actually crystallized the sulfur in an unexpected way and turned it into something called monoclinic gamma-phase sulfur, a slightly altered form of the element. This chemical phase of sulfur had only been produced at high temperatures in the lab or observed in oil wells in nature. Conveniently for the scientists, it is not reactive with the carbonate electrolyte, thereby removing the risk of polysulfide formation.

“At first, it was hard to believe that this is what we were detecting, because in all previous research monoclinic sulfur has been unstable under 95 °C (203 °F),” said Rahul Pai, co-author of the research. “In the last century there have only been a handful of studies that produced monoclinic gamma sulfur and it has only been stable for 20-30 minutes at most. But we had created it in a cathode that was undergoing thousands of charge-discharge cycles without diminished performance – and a year later, our examination of it shows that the chemical phase has remained the same.”

Scientists have stabilized a rare form of sulfur within a novel cathode, paving the way for a new breed of high-capacity batteries
Scientists have stabilized a rare form of sulfur within a novel cathode, paving the way for a new breed of high-capacity batteries

The cathode remained stable across a year of testing and 4,000 charge-discharge cycles, which the scientists say is equivalent to 10 years of regular use. The prototype battery the team made featuring this cathode offered triple the capacity of a standard lithium-ion battery, paving the way for more environmentally friendly batteries that allow electric vehicles to travel much farther on each charge.

“While we are still working to understand the exact mechanism behind the creation of this stable monoclinic sulfur at room temperature, this remains an exciting discovery and one that could open a number of doors for developing more sustainable and affordable battery technology,” Kalra said.

The research was published in the journal Communications Chemistry.

Source: Drexel University

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10 comments
10 comments
Karmudjun
Thanks Nick - it appears these Drexel University Engineers have stumbled onto the Holy Grail of Lithium Ion development into the next phase. One year of testing! All they have to do is figure out how they stumbled into the stable monoclinic sulfur, how the monofiliments of carbon nanofibers filled with the monoclinic sulfer degrade or transform over a very long time, and how to recycle these batteries without environmental damage - or any other damage! Good write up!
dan
this technology offers up to 350 Wh/kg in the laboratory today. So, exciting what scientist may discover, but from theory to lab to real life it is a very long journey...
Chase
It may have 3 times the capacity, but how do the weights compare? I'm all for a battery that can fit under the back seats like fuel tanks of old, but if they have a similar power to weight ratio that will dampen my enthusiasm.
EH
If it requires lots of carbon nanofibers, then it's likely not going to be economical soon. If they can get the same sulfur behavior out of easily producible forms of carbon such as graphene foam (e.g. from sugars) then it could be a big deal.
guzmanchinky
I eagerly await batteries with 3x the capacity. Electric motorcycles with useful range!
vince
INstead of reporting on a small upgrade of a factor of 3 why not report on a STUNNING development at University of Technology at Sydney with Lithium Air that was able to store 46 times the energy of a LiON battery with a 1400 cycling reliability with only a few percent degradation. That's stunning news.
christopher
@vince - Lithium Air is 46x more *current*, not 46x more *energy storage*.
Ornery Johnson
Outstanding eureka moment! Several times the charge capacity, long battery life, and renewable, cheap materials.
vince
chistophere: According to Wang at UTS

"The battery exhibits a 46-fold increase in discharge capacity, a low charge overpotential of 0.7 V, and an ultralong cycle life greater than 1400 cycles.
vince
Christopher: So then a Lithium Air can discharge at a 46 times higher rate than LiON means that it almost becomes a lot like a supercapacitor in that it can discharge a lot faster to dump the energy to a motor then? That certainly would be beneficial but your right then that means nothing about it's storage capacity. I get it.