Potassium-ion batteries offer promise for renewable energy storage

Technology News | June 23, 2017

By Rich Pell

“With the growth of rechargeable batteries for electronic devices, electric vehicles and power grid applications, there has been growing concern about the sustainability and cost of lithium,” says Vilas G. Pol, an associate professor in the Davidson School of Chemical Engineering at Purdue.

“The intermittent energy generated from solar and wind requires new energy storage systems for the grid,” he says. “However, the limited global availability of lithium resources and high cost of extraction hinder the application of lithium-ion batteries for such large-scale energy storage. This demands alternative energy storage devices that are based on earth-abundant elements.”

He has been working with US Department of Energy’s Oak Ridge National Laboratory and National Cheng Kung University in Taiwan.

“We initiated this program almost a year ago, and there are not many groups in the world working on potassium-ion batteries,” he says.

The researchers have developed a new anode design using carbon nanofibers rather than graphite. According to Pol, “The carbon nanofibers demonstrate great potential as an anode material for potassium-ion batteries. We studied batteries for up to 1,900 charge-discharge cycles, which is pretty remarkable.”

The carbon nanofibers were created with a process called electrospinning, where high voltage is used to pull a charged polymeric solution between two electrodes, and the polymer fibres are then turned into carbon fibres.

The batteries showed “reasonable capacity” after being charged for only six minutes, having a capacity of 110 milliamp hours per gram, one-third of the capacity achieved after 10 hours of conventional lithium ion battery charging.

“Usually, you would get very limited capacity after only six minutes because that’s not enough time to charge a conventional battery,” says Pol. “However, carbon nanofibers allow the battery to be charged much faster because the ions only have to travel a very short distance. The typical cellphone battery contains particles that are 15-20 micrometers in diameter, which is a much greater distance then this fiber-like architecture, and that’s why it takes two to three hours to charge your phone.”

The design eliminates the need for a binder or a polymer gel to hold graphite powder in place. A nanofiber network instead holds the potassium particles, an approach that could decrease the battery’s weight.

Plate-like structures of ceramic, electrically conductive materials called MXenes were also used to create a new type of potassium-ion battery. During the charging process, ions are forced by electric current to move between layers of the carbon anode. The larger atomic diameter of potassium makes it is more difficult, and MXenes based on titanium carbonitride may allow researchers to sidestep this obstacle.

“Usually ceramics are not electrochemically active, but MXenes are because you have a brick-like structure, and the ions can be inserted between these bricks during charging,” says Pol. “We have ashown that potassium does charge and discharge effectively if you tune the material’s morphology and structure.”