Li-ion battery stretches over skin to power wearables

Northwestern University’s Yonggang Huang and Illinois’ John A Rogers have demonstrated the battery that continues to work - powering a commercial light-emitting diode - even when stretched, folded, twisted and mounted on a human elbow. The stretchable battery can deliver power and voltage similar to a conventional Li-ion battery of the same size, the researchers said, but the flexible battery can stretch up to three times its original size and still function.

As reported in the journal Nature Communications, the flexible Li-ion battery completes the flexible electronics package with a cordless power source.

"Batteries are particularly challenging because, unlike electronics, it's difficult to scale down their dimensions without significantly reducing performance," Rodgers told BBC News.

"We start with a lot of battery components side by side in a very small space, and we connect them with tightly packed, long wavy lines," added Huang. "These wires provide the flexibility. When we stretch the battery, the wavy interconnecting lines unfurl, much like yarn unspooling. And we can stretch the device a great deal and still have a working battery."

The battery can work for eight to nine hours and can then be recharged wirelessly, they said.

In the paper, researchers introduce a set of materials and design concepts for a rechargeable Li-ion battery technology that exploits thin, low modulus silicone elastomers as substrates, with a segmented design in the active materials and unusual ‘self-similar’ interconnect structures between them. The result enables reversible levels of stretchability up to 300%, while maintaining capacity densities of ~1.1mAhcm⁻². Stretchable wireless power transmission systems provide the means to charge these types of batteries, without direct physical contact.

Renowned for their pioneering work with semiconductor materials and flexible, stretchable electronics, Rogers and his colleagues have applied their expertise to devise technology solutions across such broad fields as solar power, biointegrated electronics, sensing, thin film metrology and fibre optics. Incorporating battery power into these flexible systems closes the loop that will enable the technology to be used broadly. Initial targets for the technology include medical applications, but there are other applications for bendy batteries such as wearable solar cells and electric-eye cameras that can produce studio-quality photographs.