Accordion-inspired supercapacitor blends flexibility and high capacity
Flexible electronics could open up some interesting possibilities in bendy displays, wearable devices or solar energy, but developing flexible energy storage devices to power them is another challenge entirely. Scientists in China have demonstrated a creative solution to this problem, designing a novel supercapacitor that maintains high capacity when stretched and twisted thanks to accordion-style wrinkles.
While today's lithium batteries have a high energy density and can therefore store energy for long periods of time, they have a low power density, meaning they can only deliver small trickles of power and take a long time to charge up. Supercapacitors are the yin to their yang, in the sense that they can be charged up very quickly and offer tremendous power density on discharge, but can't hold nearly as much energy.
This makes supercapacitors an attractive proposition for some energy storage applications, and one where scientists are making inroads is in powering stretchable electronics. Some of the progress made so far centers on the use of a family of two-dimensional, highly conductive materials known as MXenes. These are transitional metal carbides, carbonitrides or nitrides, and researchers have had some success in using sheets of them to form multi-layered supercapcitor electrodes with large surface areas and therefore high energy storage potential.
But MXene-based electrodes are prone to breaking when bent, as they would be in flexible or stretchable electronic devices, so scientists have had to integrate polymers or other materials that make the more pliable. One downside of these additions, however, is that it winds up lowering the storage capacity of the material.
The authors of the new study may have come up with a solution to this problem, by taking inspiration from the accordion. Led by Desheng Kong from Nanjing University, the team started by fabricating a textured film made of pure titanium carbide nanosheets, which was than layered onto a piece of acrylic elastomer that had been pre-stretched to 800 percent of its relaxed size.
Releasing the elastomer caused it to shrink back to its original size, with the nanosheets taken along for the ride and crumpling up into accordion-style wrinkles in the process. This stretchy MXene served as the electrode for the team's supercapacitor, with a pair of three-micrometer-thick layers of the material sandwiching an electrolyte made of polyvinyl(alcohol)-sulfuric acid gel in between.
The team's subsequent experiments showed that the accordion-inspired supercapacitor could be stretched and relaxed repeatedly without incurring damage, and without compromising its ability to store a charge. The capacity was comparable to other supercapcitors constructed with MXenes, but with the key difference being it could be stretched up to 800 percent without cracking. Stretching the material over 1,000 times saw its energy storage capacity only drop to 90 percent.
This kind of high capacity and extreme stretchability could one day see the team's supercapacitor used in wearable electronics or other applications where energy storage devices need to undergo deformation.
The video below provides an overview of the research, while the study was published in the journal Nano Letters.