In a study published in Batteries and Supercaps, researchers from Ohio State University detailed their findings centred around the construction of the battery's cathode.
"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."
With a theoretical energy density of 935Wh kg−1, potassium-oxygen batteries have been a potential alternative for energy storage since they were invented in 2013. A team of researchers from Ohio State, led by chemistry professor Yiying Wu, showed that the batteries could be more efficient than lithium-oxygen batteries while simultaneously storing about twice the energy as existing lithium-ion batteries.
Despite their potential, potassium-oxygen batteries have yet to go beyond 10 charging cycles before degrading. This is caused by molecular oxygen crossing from the cathode to anode, leading to superoxide forming on the anode.
In an effort to overcome this, Paul Gilmore, a doctoral candidate in Sundaresan's lab, incorporated polymers into the cathode to protect the anode. The functionally‐graded cathode architecture (FGC) is comprised of a carbon fibre layer, microporous carbon and polypyrrole doped with hexafluorophosphate. The third layer allows potassium ions to travel throughout the cathode but restricts molecular oxygen from getting to the anode. The design means that the battery can be charged at least 125 times.
The team's tests have not proven that the batteries can be made on the scale necessary for power-grid storage, Sundaresan said, but they do show potential.
Gilmore said potential may also exist for potassium-oxygen batteries to be useful 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.
According to Ohio State, the team proved also that the batteries can be made cheaply. Lithium-oxygen batteries, widely considered one of the most viable options for energy storage, can be expensive and many rely on scarce resources, including cobalt. The lithium-ion batteries that power many electric cars cost around $100 per kilowatt hour at the materials level. The researchers estimate 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."
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