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Researchers are looking towards the sea to find a way to increase the capacity of lithium-sulfur batteries.

Scientists from the U.S. Department of Energy’s Lawrence Berkeley National Laboratory have discovered that carrageenan—a substance extracted from red seaweeds—acts as a stabilizer in lithium-sulfur batteries.

“There's a lot of demand for energy storage but there's very little chemistry that can meet the cost target,” Gao Liu, the corresponding author of the study, said in a statement. “Sulfur is a very low-cost material—it's practically free.

“And the energy capacity is much higher than that of lithium-ion,” he added. “So lithium-sulfur is one chemistry that can potentially meet the target.”

Rechargeable lithium-sulfur batteries are limited in commercial applications because the sulfur starts to dissolve, creating a polysulfide shuttling effect.

The researchers had the sulfur chemically react with the binder—the substance that holds all the active materials in a battery cell together—to overcome the issue with the sulfur dissolving.

“A binder is like glue and normally battery designers want a glue that is inert,” Liu said. “This binder we tried worked really well.

“We asked why and we discovered it's reacting—it reacted immediately with the polysulfide,” he added. “It formed a covalent bonding structure.”

After seeing this reaction, the researchers turned to carrageenan—which is in the same functional group as the synthetic polymer they used in the initial experiments.

“We looked for something that was economical and readily available,” Liu said. “It turns out carrageenan is used as a food thickener.

“And it actually worked just as well as the synthetic polymer--it worked as a glue and it immobilized the polysulfide, making a really stable electrode."

According to Liu, the discovery may open up new avenues for battery chemistry, specifically that a binder can be reactive rather than inert.

The next step for Liu and his team is to improve the lifetime of lithium-sulfur batteries to get to the thousands of charge/re-charge cycles benchmark. They also plan on continuing to work on understanding the chemical reactions in the cell.

“After this polymer binds with sulfur, what happens next?” Liu said. “How does it react with sulfur and is it reversible?

“Understanding that will allow us to be able to develop better ways to further improve the life of lithium-sulfur batteries.”

The study was published in Nano Energy.

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