“There’s a lot of demand for energy storage, but there’s very little chemistry that can meet the cost target,” said Gao Liu who led the team. “Sulfur is a very low-cost material—it’s practically free. And the energy capacity is much higher than that of lithium-ion. So lithium-sulfur is one chemistry that can potentially meet the target.”
Rechargeable lithium-sulfur batteries are used in underwater vehicles and the 14-day solar-powered flight of the unmanned Zephyr aircraft in 2010, as well as a new generation of electric scooters, but have problems when the sulfur starts to dissolve. Liu was experimenting with the binder to hold all the active materials in the cell together.
“A binder is like glue, and normally battery designers want a glue that is inert,” said Liu. “This binder we tried worked really well. We asked why, and we discovered it’s reacting—it reacted immediately with the polysulfide. It formed a covalent bonding structure.”
By chemically reacting with the sulfur, the binder was able to stop it from dissolving. The team then looked for a naturally occurring material that would do the same thing. Carrageenan is extracted from red seaweed and in the same functional group as the synthetic polymer they used in their initial experiments.
“We looked for something that was economical and readily available,” he 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.”
General Motors, an industry research partner of Berkeley Lab’s Energy Storage & Distributed Resources Division, confirmed Liu’s research findings. “They independently tested it and saw the same effect we saw—in fact the stability was even better,” Liu said.
The results open up a new way of thinking about battery chemistry says Liu. “Scientifically, it’s a totally different concept, of a binder that is reactive rather than inert,” he said. “People don’t think that way. They think a binder’s function is to physically hold things together. We found, no, we need a way to chemically bind the polysulfide.”
Liu is now seeking to improve the lifetime of lithium-sulfur batteries even further. “We want to get to thousands of cycles,” he said.
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