Rare Form of Sulfur Offers a Key To Triple-Capacity EV Batteries

Engineers at Drexel University have made a breakthrough they say takes [lithium-sulfur batteries] closer to commercial use, by leveraging a rare chemical phase of sulfur to prevent damaging chemical reactions. New Atlas reports: [T]here 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. 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.

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. The research was published in the journal Communications Chemistry.

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