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

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.

“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.”

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