MXenes are what are dubbed “two-dimensional” inorganic compounds. They consist of few atoms-thick layers of transition metal carbides, nitrides, or carbonitrides. MXenes have only been around since 2011.

The interesting thing about MXenes is that they have demonstrated a high capacity for reversible charge storage. So some MXenes have been investigated experimentally for use in lithium-ion batteries.

The latest work in this area took place at Drexel University. There, researchers led by Yury Gogotsi created an electrode out of MXene.

Gogotsi says that the team’s work “refutes the widely accepted dogma that chemical charge storage, used in batteries and pseudocapacitors, is always much slower than physical storage used in electrical double-layer capacitors, also known as supercapacitors. We demonstrate charging of thin MXene electrodes in tens of milliseconds. This is enabled by very high electronic conductivity of MXene. This paves the way to development of ultrafast energy storage devices than can be charged and discharged within seconds, but store much more energy than conventional supercapacitors.”

One key to fast battery charging is to use electrode materials that have a lot of sites for charge storage. The formal name for these charge storage sites are “redox active sites.” Researchers produced a hydrogel electrode design with a lot of redox active sites, which lets the hydrogel store as much charge for its volume as a battery. This measure of capacity, termed “volumetric performance,” is an important metric for judging the utility of any energy storage device.

To make those plentiful hydrogel electrode ports even more attractive to ion traffic, the Drexel-led team designed electrode architectures with open macroporosity — many small openings — to make each redox active sites in the MXene material readily accessible to ions.

“In traditional batteries and supercapacitors, ions have a tortuous path toward charge storage ports, which not only slows down everything, but it also creates a situation where very few ions actually reach their destination at fast charging rates,” said Drexel researcher Maria Lukatskaya, the first author on the paper. “The ideal electrode architecture would be something like ions moving to the ports via multi-lane, high-speed ‘highways,’ instead of taking single-lane roads. Our macroporous electrode design achieves this goal, which allows for rapid charging — on the order of a few seconds or less.”

The overarching benefit of using MXene as the material for the electrode design is its conductivity. MXenes are conductive, just like metals, so not only do ions have a wide-open path to a number of storage ports, but they can also move quickly to meet electrons there.

The full paper is available at: https://www.nature.com/articles/nenergy2017105.