Old chemistry gives jolt to modern batteries

Manganese dioxide stops lithium-sulfur strategy from dissolving

SAN JOSE, Calif. — Some very old chemistry may provide a new trick for making better batteries.

Scientists have developed a strategy that makes sulfur-based batteries much more efficient by exploiting chemical reactions discovered in the 1800s. The research, described February 14 at the annual meeting of the American Association for the Advancement of Science, brings scientists closer to developing cheap, long-lasting batteries to power cars and computers or store energy for the electricity grid.

Lithium-sulfur batteries are a promising alternative to lithium-ion batteries, the rechargeable batteries used in cell phones, laptops and other consumer electronic devices. Sulfur is lightweight, plentiful and cheap, and lithium-sulfur batteries should be able to store much more energy than the lithium-ion variety. But sulfur-based batteries die quickly because sulfur used as a cathode tends to dissolve into the electrolyte solution that the ions move through as the battery charges and discharges. Now scientists led by Linda Nazar of the University of Waterloo in Canada have figured out a way to stabilize the sulfur cathode.

The trick was to find a chemical middleman that binds to the sulfur and keeps it from dissolving, Nazar said. Without a middleman, the sulfur exists in rings of eight atoms that, as they receive electrons, break into chains of sulfurs of various lengths. These sulfur tidbits — technically polysulfides — dissolve and tend to drift to the negative electrode of the battery, glomming up the surface and making the battery short-circuit.

“Sulfur is really loosey-goosey with electrons,” said Nazar. “All these polysulfides are doing their molecular dance and going all over the place.”

So Nazar and her team started looking for something that would trap the sulfur in place. After testing several materials, she and her colleagues found that superthin sheets of manganese dioxide do the trick, catching the intermediate sulfur compounds so they don’t dissolve away. The scientists think that as the manganese oxide grabs the sulfur chains, it spits off other sulfur chains that aren’t soluble, so they don’t wander off into the electrolyte solution.

“It binds them and drops them on the surface of the cathode,” Nazar said. The precise chemistry of the sulfur molecules isn’t completely understood, but the fundamentals of the reaction turn out to have been described by German chemist Heinrich Wilhelm Ferdinand Wackenroder in the 1840s.

The research is an example of scientists devising new strategies to overcome the hurdles of better energy storage. “New materials are very important,” said chemist Yi Cui of Stanford University.

The sulfur-manganese oxide cathodes make batteries that would allow a car to go three times as far as current car batteries would before needing a recharge, Nazar said. And batteries made with sulfur cathodes might also work well to power cell phones and to store energy in an electric power grid.

More Stories from Science News on Materials Science