Sulfur, the key to tripling the capacity of electric car batteries

Lithium-sulfur batteries don’t rely on the same expensive and hard-to-source raw materials, such as cobalt, as today’s batteries. But other stability-related issues have slowed the technology down until now.

Drexel University engineers have made a breakthrough that they say brings these batteries closer to commercial use, by harnessing a rare chemical phase of sulfur that prevents harmful chemical reactions.

Lithium-sulfur batteries hold great promise when it comes to energy storage, and not just because sulfur is abundant and less of a problem to obtain than the cobalt, manganese, and nickel used in today’s batteries ( although they said the same about sodium ones months ago).

In addition, they can offer higher performance than current ones, with the potential to store several times the energy of traditional lithium-ion batteries. But there is a problem that scientists continue to run into. : the formation of chemical compounds called polysulfides.

As the battery works, they find their way into the electrolyte (the solution that carries charge between the anode and cathode) where they trigger chemical reactions that reduce the capacity and life of the battery.

Scientists have succeeded in changing the carbonate electrolyte to an ether electrolyte, which does not react with polysulfides. But this poses other problems, as ether electrolyte is highly volatile and contains components with low boiling points, which means the battery could fail or melt.

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Chemical engineers at Drexel University have been working on another solution, and this one starts with the design of a new cathode, which can work with carbonate electrolytes already in commercial use.

This cathode is made of carbon nanofibers and had already been shown to slow down the movement of polysulfides in an ether electrolyte. But making it work with a carbonate electrolyte required some experimentation.

Scientists attempted to confine the sulfur in the mesh of carbon nanofibers to prevent dangerous chemical reactionsusing a technique called steam arrangement. This didn’t have the desired effect, but instead crystallized the sulfur in an unexpected way, turning it into something called monoclinic sulfur.

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This chemical phase of sulfur has only been produced at high temperatures in the laboratory or observed in oil wells in nature. Fortunately for scientists, it is non-reactive with the carbonate electrolyte, thus eliminating the risk of polysulfide formation.

The cathode was stable over a year of testing and 4,000 charge/discharge cycles, which scientists say is equivalent to 10 years of regular use.

The prototype battery the team made with this cathode offered three times 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 load.