New Cathode for Potassium-Oxygen Battery Enables >125 Recharge Cycles

May 25, 2019 by Scott McMahan

Researchers have devised a more efficient, more reliable potassium-oxygen battery, a step toward a potential solution for energy storage for the grid.

In a study published in the April issue of the journal Batteries and Supercaps, researchers from Ohio State University detailed their findings related to the construction of a potassium-oxygen battery's cathode.  (See image above of Paul Gilmore and Vishnu-Baba Sundaresan).

The findings, according to the researchers, the findings could lead to cheaper and more efficient energy storage, thereby making intermittent renewable energy sources such as wind and solar more viable for the power grid.

"If you want to go to an all-renewable option for the power grid, you need economical energy storage devices that can store excess power and give that power back out when you don't have the source ready or working," said Vishnu-Baba Sundaresan, co-author of the study and professor of mechanical and aerospace engineering at Ohio State. "Technology like this is key, because it is cheap, it doesn't use any exotic materials, and it can be made anywhere and promote the local economy."

Potassium-oxygen batteries have been a possible alternative for energy storage since they were first developed in 2013. A researcher team from Ohio State University, led by chemistry professor Yiying Wu, demonstrated that the batteries could achieve higher efficiency than lithium-oxygen batteries while at the same time storing about twice the energy as existing lithium-ion batteries.

However, potassium-oxygen batteries have not been widely adopted for energy storage because, so far, they haven't been able to support enough recharge cycles.

They found that the battery degraded with each charge, never lasting longer than five or 10 charging cycles.

The degradation occurred when oxygen got into the battery's anode. The oxygen caused the anode to break down, making it ineffective in supplying a charge.

Polymers Incorporated into Cathode

Paul Gilmore, a doctoral candidate in Sundaresan's lab, tried incorporating polymers into the cathode to see if he could protect the anode from oxygen. He thought, if he could find a way to do that, it might give potassium-oxygen batteries a shot at longer lives.

He was proven right.

The team realized that swelling in the polymer played a critical role in its performance. The solution, Gilmore said, was finding a way to bring oxygen into the battery that was necessary for it to work without allowing it to seep into the anode.

This design is somewhat analogous to human lungs. Air comes into the battery through a fibrous carbon layer. Then, it meets a second, slightly less porous layer. Finally, the oxygen ends up at a third layer, which is barely porous at all.

The third layer made of the conducting polymer, allows potassium ions to travel throughout the cathode, but it also helps prevent molecular oxygen from getting to the anode.

The design means allows the battery to be charged at least 125 times, more than 12x the previous longevity of potassium-oxygen batteries they had with low-cost electrolytes.

So far, the team's tests haven't proven that the batteries can be made on the scale needed for power-grid storage, Sundaresan indicated.

However, it does show potential.

Gilmore said potassium-oxygen batteries also have the potential to be used in other applications.

"Oxygen batteries have higher energy density, which means they can improve the range of electric vehicles and battery life of portable electronics, for example, though other challenges must be overcome before potassium-oxygen batteries are viable for these applications," he said.

And the finding provides a lower cost alternative to lithium-ion batteries and others that use cobalt, a material that has been referred to as "the blood diamond of batteries." The mining of Cobalt is so troublesome that major companies, such as TESLA, have announced their intentions to eliminate it from batteries completely.

"It is very important that batteries intended for large-scale applications do not use cobalt," Sundaresan said.

Also, the battery must be produced inexpensively. The researchers estimated that their potassium-oxygen battery will cost about $44 per kilowatt hour.

"When it comes to batteries, one size does not fit all," Sundaresan said. "For potassium-oxygen and lithium-oxygen batteries, the cost has been prohibitive to use them as grid power backup. But now that we've shown that we can make a battery this cheap and this stable, then it makes it compete with other technologies for grid power backup.

"If you have a smallish battery that is cheap, then you can talk about scaling it up. If you have a smallish battery that is $1,000 a pop, then scaling it up is just not possible. This opens the door for scaling it up."

This work was funded via a grant from the National Science Foundation.