Friday, August 24, 2012
Twist it: A cable-shaped lithium-ion battery powers an LED display even when twisted and strained.
Korean battery maker LG Chem is developing "cable batteries" that could be integrated into headphones to help power your cell phone, or woven into jewelry or textiles to recharge wearable electronics (see "Stretchable Silicon Could Make Sports Apparel Smarter").
The lithium-ion batteries keep working even when tied into knots and otherwise abused. The novel design doesn't put out very much power, but researchers at the company are developing more-efficient formulations, and they say the lithium-ion cable batteries could be ready for mass production in about five years.
Flexible batteries have been made before, but normally in the form of sheets, which limits what designers can make from them, and they usually can't store very much energy, says Je Young Kim, general manager of battery R&D at LG Chem.
Kim's team found a way around these limitations with a spring-like, spiral electrode design that incorporates the inorganic materials used in conventional lithium-ion batteries. "This structure allows the device to compensate for any external mechanical distortion while maintaining its structural integrity, and it enables the battery to be more flexible in three dimensions," he says. LG Chem's cable battery prototype is described online this month in the journal Advanced Materials.
To make the battery, the researchers started with thin strands of copper wire coated with nickel and tin—two active electrode materials. The LG group wove together strands of wire into a metal yarn and then wrapped the yarn around a circular rod. They then removed the rod and stretched the coil to create a strong spring, about a millimeter in diameter, to serve as both the battery's structural backbone and its anode.
Kim's group then wound the other parts of the battery around the anode, including an aluminum wire to serve as the basis of the cathode. The battery was next drawn through a slurry containing lithium cobalt oxide cathode material, and then dried and wrapped in protective layers. To complete the battery, the LG group poured in a liquid electrolyte to carry ions and electrons when the battery is charged and discharged.
A cable battery about 25 centimeters long can power a tiny fourth-generation iPod shuffle for 10 hours, while twisted and bent. Compared to other flexible batteries, says Kim, the discharge of the cable devices is relatively steady under stress. The battery's good performance even when knotted is "surprising," says Ray Baughman, director of the Nanotech Institute at the University of Texas in Dallas. "This is wonderfully creative."
"Cable batteries could be woven as textiles, shaped into wristbands, necklaces, belts, and so on," says Kim.
"Creating new form factors for batteries helps designers extend the power equation" for mobile devices, says Max Burton, head of industrial design at San Francisco firm Frog Design. Batteries have not been keeping pace with innovations in mobile devices, which burdens designers with finding ways to save power. Highly flexible cable batteries could be reconfigured into a variety of shapes. Designers might put these batteries into headphones or a phone case, says Burton.
Kim's group hopes to boost the performance of the cable batteries. LG Chem is working on several aspects of the cable battery design, including testing new anode materials to help these batteries put out more power faster. Kim says the company is also working to make sure the batteries will be safe if, for example, a user accidentally cuts them open.