Supercapacitors can charge and release energy much faster than batteries, but they can't store as much. Now a team at Drexel University has used the two-dimensional material MXene to develop a new type of electrode, combining the capacitance of a regular battery with the speed of a supercapacitor, which could lead to devices that recharge in a matter of seconds.

The anxiety of a slowly-charging battery is something we're all familiar with, so it's not surprising that engineers are working on the issue. We've already seen teams tackle the problem using nanodots to improve electrode capacitance and electrolyte performance, and flexible supercapacitors with better charge times and lifespan. The Drexel team's first step towards striking a better balance between speed and storage, was to make the new electrodes out of MXene, a 2D material that's highly conductive.

"This paper 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," says Yury Gogotsi, lead researcher on the team. "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."

The structure of the electrodes is just as important as the material. To store a charge, ions are held in ports called redox active sites, so the more of these ports there are, the more energy the battery can store. Not only does the new electrode pack in more redox active sites, but it's "macroporous", meaning it has plenty of small openings to allow more ions to reach the ports at the same time.

"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," says Maria Lukatskaya, first author of 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 research was published in the journal Nature Energy.