Cheap lithium sulfur batteries soon on the horizon?
NIST materials scientist Christopher Soles, said: “Five hundred cycles with the capacity we’ve shown is definitely better than what’s in your laptop today.”
Lithium-ion batteries require bulky cathodes, typically made from ceramic oxides like cobalt oxide, to house the ions, which limits the battery’s energy density. For more power-intensive applications like long-range electric vehicles lithium-ion technology has its limitations.
With lithium-sulfur batteries the cathodes are made mainly of sulfur, a waste product of petroleum processing. Sulfur weighs barely half as much as cobalt, atom for atom, and can pack more than twice as many lithium ions into a given volume as can cobalt oxide; thus, lithium-sulfur batteries have several times the energy density of lithium-ion batteries. Sulfur cathodes have two major weaknesses. Sulfur easily combines with lithium to form compounds that crystallize and gum up the battery’s insides, and sulfur tends to crack under the stress of repeated cycling. As a result, a typical lithium-sulfur battery becomes useless within a few dozen cycles – far too few for a laptop or car battery that may get cycled once a day for years.
To create a more stable cathode, the research team heated sulfur to 185 degrees Celsius, melting the element’s eight-atom rings into long chains. The researchers mixed the sulfur chains with diisopropenylbenzene, DIB, a carbon-based plastic precursor that links the sulfur chains together, creating what is known as a co-polymer.
The team described their manufacturing process as ‘inverse vulcanization’ because it resembles the process used to make rubber tires, with one crucial difference: In tires, carbon-containing material makes up the bulk, and sulfur is just sprinkled in.
Adding DIB to the cathodes prevents them from cracking as easily and keeps lithium-sulfur compounds from crystallizing. The scientists tested different mixtures of sulfur and DIB and found that the optimum mix contained between 10 and 20 percent DIB by mass: Less DIB did not provide the cathode-protecting properties while more of the electrochemically inactive DIB began to drag down the battery’s energy density.
The researchers ran their optimized battery through 500 cycles and found that it retained more than half its initial capacity.
Other experimental lithium-sulfur batteries have performed similarly, but their cathodes require more complex manufacturing processes that would be expensive to scale up, suggested Jeffrey Pyun, a chemist at the University of Arizona and Seoul National University. By contrast, the research team’s polymer cathode requires only easily available materials and moderate heat.
“We take it, we melt it in one step and pow, we get this plastic,” explained Pyun. “If you were to come to our lab, we could do this in five minutes.”
Related articles:
www.nist.gov
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