New nanofiber electrode material increases energy storage.

The next generation of wearable and flexible devices will necessitate the development of more robust, lightweight energy storage systems and researchers at the Korea Institute of Science and Technology (KIST) have answered the call – developing modified carbon nanotube fibers in an electrode-like material that can offer 3.3 times the strength and 1.3 times the conductivity over regular carbon nanotubes.

Technology classified as wearables demands specific requirements: it must retain power and energy density under rigorous conditions and operate despite potential damage, such as being pierced, dropped, crushed, or experiencing mechanical strain from rapid acceleration and deceleration, depending on the application.

Fiber-shaped supercapacitors (FSSCs) lend themselves to wearable energy storage due to their similarity to regular fibers, with strong, flexible, and highly conductive carbon nanotube fibers (CNTFs) incorporating seamlessly into almost any shape.

According to the KIST team, CNTFs made using a wet-spinning process in a liquid crystal phase demonstrate these qualities due to the fiber's tightly packed and aligned structure with fewer defects.

However, the surface of regular CNTFs is too smooth for layering on useful chemical reactions, which limits their function as FSSCs. Researchers have attempted to work around this flaw by coating the carbon surfaces with materials like conductive polymers or metal oxides to increase the surface area for chemical reactions.

This method has its own disadvantages – materials that do not fully bond might come off during use, and the current process is unsuitable for making long, continuous fibers.

Future of fiber-based batteries

KIST scientists have discovered a way to improve CNTFs for energy storage without adding extra materials, making them cheaper and more practical. They first treated the surface of carbon nanotubes, turning them into a special liquid crystal form and then spinning them into fibers that can store energy well without needing extra steps or materials.

To solve the common lessening of the fibers' conductivity by various coatings, the researchers also made their liquid crystal solution extra concentrated, which improved strength and conductivity.

The modified carbon nanotubes were finally transformed into fiber-like supercapacitors and tested in applications like a digital watch. The materials showed 33 times more energy storage, 3.3 times more strength, and 1.3 times more conductivity than regular carbon nanotube fibers.

After being bent, folded, and cleaned, they also functioned well when woven into wrist straps using a blend of normal and carbon nanotube fibers. In further studies using fiber-shaped supercapacitors, the materials maintained 95% of their performance after 5,000 bending tests and nearer to 100% when knotted.

"We have confirmed that carbon nanotubes, which have recently started to attract attention again as a conductive material for secondary batteries, can be used in a much wider range of fields," said Kim Seung-min, a researcher at KIST.

The team plans to move beyond supercapacitors and extend its research into unconventional energy storage applications, exploring ways to adapt this technology for fiber-type batteries with increased energy density.

The details of the team's research were published in the journal Advanced Energy Materials.