As electric vehicles become more popular, scientists see great potential for lithium-sulfur batteries, which are a more environmentally friendly way to drive. This is because they do not rely on expensive and hard-to-access raw materials, such as cobalt, but problems such as stability have so far hindered the development of the technology.
Recently, engineers at Drexel University in the United States have made a breakthrough, they say, by using a rare chemical phase of sulfur to prevent destructive chemical reactions, which makes lithium-sulfur batteries closer to commercial use. The results of this study were recently published in the journal Communication Chemistry.
Lithium-sulfur batteries have a bright future in terms of energy storage, not only because of the abundance of sulfur reserves, but also because the source of sulfur is not a problem compared to the cobalt, manganese and nickel used in today's batteries. At the same time, lithium-sulfur batteries may also bring some significant performance gains, and their potential to store energy is several times that of current lithium-ion batteries. But one problem that has been bothering scientists is the formation of polysulfides.
When the battery is working, these substances enter the electrolyte and trigger a chemical reaction that damages the capacity and life of the battery. Scientists have successfully replaced the carbonate electrolyte with an ether electrolyte that does not react with polysulfides. But this also poses other problems, as the ether electrolyte itself is extremely volatile and contains a low boiling point component, which means that if heated above room temperature, the battery may quickly fail or melt.
As a result, chemical engineers at Drexel University have been working on another solution, starting with designing a new cathode that can work with carbonate electrolytes already in commercial applications. This cathode is made of carbon nanofibers and has been shown to slow the movement of polysulfides in the ether electrolyte. But getting it to work with carbonate electrolytes takes some experimentation.
Lead researcher Vibha Kalra said, "For commercial manufacturers, the carbonate electrolyte currently in use can act as a cathode, which is the path with the least resistance. Therefore, our goal is not to push the industry to adopt a new electrolyte, but to make a cathode that can work in existing lithium-ion electrolyte systems. ”
Scientists have tried to use a technique called steam treatment to confine sulfur to a web of carbon nanofibers to prevent dangerous chemical reactions. Although this did not achieve the desired effect, the sulfur crystallized in an unexpected way and turned it into something called monoclinic gamma phase sulfur, which is a slightly altered form of the element.
It is reported that the chemical phase of this sulfur can only be produced at high temperatures in the laboratory or observed in oil wells in nature. The researchers unexpectedly found that it did not react with the carbonate electrolyte, eliminating the risk of forming polysulfides.
"At first, it was hard to believe that this was what we detected, because in all previous studies, monoclinic crystal sulfur has been unstable at 95 °C (203 °F)," said Rahul Pai, co-author of the study. But we created it in the cathode, which went through thousands of charge-discharge cycles without degrading performance. A year later, our examination of it showed that the chemical phase remained unchanged. ”
After a year of testing and 4,000 charge-discharge cycles, the cathode remains stable, which scientists say is equivalent to 10 years of routine use. The team's prototype of the battery made with this negative electrode can provide three times the capacity of a standard lithium-ion battery, paving the way for more environmentally friendly batteries and allowing electric vehicles to travel farther after each charge.
Kalra said, "While we are still struggling to understand the exact mechanism behind this monoclinic crystalline sulfur production that remains stable at room temperature, it is still an exciting discovery that could open many doors to the development of more sustainable and economical battery technologies." ”