One-atom-thick filter helps lithium–sulfur batteries keep their charge

Longer-lasting phones, lighter drones, electric cars that drive farther. These are just some of the possibilities thanks to a new battery separator design from University of Florida researchers and their partners.

Think of a tiny coffee filter, but this one works inside a battery. The team recently showed that a one-atom-thick filter can block sulfur chains from shuttling within the battery, potentially unlocking the long-awaited promise of lithium–sulfur batteries.

While lithium–sulfur batteries are lighter and pack more power in a lighter package compared to the more conventional lithium-ion batteries, their fatal flaw is the sulfur doesn’t cooperate well inside the system. It clumps into long chains that clog up the works, draining the battery’s power and cutting its lifespan.

Engineers from UF, Purdue University and Vanderbilt University developed a high-performance filter that works at a molecular level to fix the problem.

“It’s like a microscopic coffee filter or a bouncer at a club,” said Piran Kidambi, an associate professor of mechanical and aerospace engineering at UF and author of the study published this month in ACS Applied Materials & Interfaces. “Tiny lithium ions slip through easily, but bulky sulfur chains get blocked.”

Today’s lithium-ion batteries power nearly everything we carry and drive, from smartphones to electric vehicles. They work well, but they’ve hit limits on how much energy they can store for their weight.

“Lithium-ion batteries have been perfected over a long time, and they work very well,” Kidambi said. “But lithium–sulfur can be much lighter and can store more electrical energy.”

An average electrical vehicle battery weighs about 1,000 pounds and gets around 200 to 250 miles of range. With lithium–sulfur at the same weight, a driver can go farther on one charge, Kidambi said. Cellphones would last longer between charges, and lighter batteries would allow drones to stay in the air longer.

A filter at the atomic scale

To make the filter, the researchers used a method called chemical vapor deposition. They start with a copper foil, heat it and flow a vapor over it. The chemical reaction leaves behind a film of graphene with perfectly defined openings to separate the lithium from the sulfur chains.

When they tested the design, the difference was striking. Unfiltered batteries began losing performance quickly. With the one-atom-thick filter, batteries held nearly all their capacity over more than 150 charge-discharge cycles.

“They performed quite well,” Kidambi said. “The others dropped off with each charge and discharge, but the ones with our filter held steady.”

Beyond cars and phones

The potential applications extend well beyond consumer devices. Kidambi pointed to freight trucks, trains and even ships, where battery weight becomes a major obstacle.

“As you move from cars to trucks, trains or ships, the battery weight rises exponentially because you need more energy to move them,” he said. “That’s called weight compounding. The battery starts to weigh almost as much as the load it’s supposed to move.”

Much work remains before lithium–sulfur batteries with atom-thin filters could be manufactured at scale and placed into everyday devices. But Kidambi is encouraged by their progress.

“There’s real scientific success in being able to show that we can solve a problem, and we can solve it by engineering at the atomic level,” he said. “That is exciting for me.”