Researchers have mass-produced meters of fiber-shaped lithium batteries using standard industrial equipment (Nature 2021, DOI: 10.1038/s41586-021-03772-0). The high-performance fiber batteries pack more than 80 times as much energy by weight than previous ones.
Textiles woven from the batteries safely charged devices even after washing, being punctured, bent and twisted, and over a temperature range of 20–60 °C.
Flexible and fiber-shaped batteries that can be integrated into textiles offer a convenient way to charge gadgets like fitness bands, smart watches, and phones. Researchers have made fiber-shaped batteries by twisting or winding together different battery materials, or coating them in layers onto polymer fibers or metal wires.
Previous fiber batteries have been just centimeters in length, impossible to weave into large fabrics, and had low energy densities. “For practical applications, we need reels of high-performing fiber batteries that can be woven into large-area textiles,” says Peining Chen, a chemist and chemical engineer at Fudan University.
Making fiber batteries that perform well boils down to the quality of the battery materials’ coatings. It is challenging to maintain the quality of these coatings when depositing them over hundreds of meters of curved wires, Chen says. Researchers have assumed that as a battery fiber gets longer, its resistance would keep increasing, bringing down performance.
Chen, chemical engineer Huisheng Peng and their colleagues at Fudan found a way to make robust, uniform coatings, and saw that as the fiber length increases, resistance increases at first but then levels off.
The team coated metal wires with lithium cobalt oxide (LCO) to make the positive electrode, and other metal wires with graphite to make the negative electrodes. The wires are pulled through mixtures of the materials at a constant speed. The key to robust coatings, Peng says, are special polymer binders added to the LCO and graphite mixtures, which improve adhesion between the material and the wire surface. They used polyvinylidene fluoride binders for the positive LCO material, and a sodium carboxymethyl cellulose and butadiene styrene rubber emulsion for the graphite.
After wrapping the negative electrode with separator film to prevent short circuit, they twisted the two wires together, and encapsulated them in a polypropylene tube. Finally, they injected a gel electrolyte into the tube.
They made batteries up to 100 m in length, and the devices had an energy density of 85.69 W h/kg. These fiber batteries could be woven to make large energy-storing fabrics.
In one demonstration, they integrated a battery textile patch into a cotton shirt along with a wireless power-transmitting coil, and showed it could recharge a cell phone in 40 min. They also showed that the battery textile could power sweat sensors and a read-out display integrated into a jacket. The sensor detected the concentrations of sodium and calcium ions in the wearer’s sweat and sent the data to the textile display.
Wen Lu of Yunnan University, says that the remarkably high energy density and large-scale production of these fiber batteries make them very competitive for real-world use. A major limitation though, could be the use of metal wires, which are heavy and inherently rigid and fragile, Lu says. But the fiber-battery production method reported here could be extended to other materials and battery chemistries.