Yarn

Researchers have shaped a rechargeable zinc-ion battery into an elastic yarn that churns out power when bent, stretched, washed with water, and even cut.

The zinc yarn battery could be woven into washable sensor-laden smart clothes and integrated into commercial textiles to power wearable displays, electronics, and medical implants.

The yarn joins a line-up of innovative flexible energy-generating and -storing devices that can be integrated into power fabrics. The list includes solar cell ribbons that can be woven into fabrics, knittable supercapacitors, and power-generating yarns that harvest mechanical energy or the triboelectric effect to generate power.

Some researchers have tried to make flexible versions of the workhorse zinc-manganese alkaline battery because of its proven high capacity, low cost, and safety. But these flexible versions have had low capacities. Plus these primary batteries can't be recharged. But researchers have recently come up with high-performance rechargeable zinc-ion batteries.

Schematic diagram of fabrication and encapsulation of the yarn ZIBIllustration: American Chemical Society

Chunyi Zhi of the City University of Hong Kong and his colleagues made their thread-like rechargeable zinc battery by twisting carbon nanotube fibers into yarn. They coat one piece of yarn with zinc to make an anode and another with manganese dioxide to serve as a cathode. Then they wind the two yarn pieces on an elastic fiber, soak it with a commonly used water-absorbing gel, and encase the device in elastic silicone and a water repellant.

The yarn battery, detailed in ACS Nano, has a energy density of 53.8 milliwatt-hours per cubic centimeter, which is around three times as much as commercial thin-film lithium-ion batteries. It retains over 98 percent of its capacity after 500 recharging cycles.

"Compared with traditional lithium-ion batteries, which suffer from intrinsic safety and cost issues, this yarn battery can work well under various severe conditions," Zhi says. It retains 95 percent of its original capacity when bent, knotted, twisted, and stretched up to three times its length. And it retained over 96 percent of its original capacity after being soaked in water for 12 hours.

As further proof of the yarn's forte and coolness, the team made a 1-meter-long yarn, cut it into eight pieces, and showed that each piece could power a watch. Then they wove the pieces into a battery textile, which could power pulse monitors, a strip of 100 LEDs, and a 10 cm x 10 cm flexible electroluminescent panel.

The researchers are now trying to integrate the yarn batteries with commercial fabrics and developing a large-scale manufacturing method for the batteries, Zhi says. "We also have a plan to develop other types of yarn batteries with more functions such as self-healing ability, or self-charge capability when combined with a solar cell component."