Scientists have created the first wirelessly rechargeable stretchable lithium-ion battery to work with flexible electronics

Stretchable battery paves way for flexible electronics

Scientists have created a stretchable lithium-ion battery capable of powering their innovative flexible electronics.

Northwestern University's Professor Yonggang Huang and the University of Illinois' Professor John Rogers have been working together for the last six years on stretchable electronics and designing a cordless power supply has been a major challenge.

Now the pair are the first to demonstrate a flexible battery removing the requirement for their stretchable electronic devices to be connected by a cord to an electrical outlet meaning they could now be used anywhere, including inside the human body

The implantable electronics could monitor anything from brain waves to heart activity – succeeding where flat, rigid batteries would fail.

"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," said Prof 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."

For their stretchable electronic circuits, the two developed "pop-up" technology, an array of tiny circuit elements connected by metal wire "pop-up bridges", so that when the array is stretched, the wires not the rigid circuits pop up.

This approach works for circuits but not for a battery as a lot of space is needed in between components for the "pop-up" interconnect to work.

Circuits can be spaced out enough in an array, but battery components must be packed tightly to produce a powerful but small battery so there is not enough space between battery components for the "pop-up" technology to work.

They solved the problem with a space filling technique using metal wire interconnects that are long, wavy lines, filling the small space between battery components with the power travelling through the interconnects.

The unique mechanism is a "spring within a spring" – the line connecting the components is a large S shape and within that S are many smaller Ss, so when the battery is stretched the large S first stretches out and disappears, leaving a line of small squiggles.

The stretching continues, with the small squiggles disappearing as the interconnect between electrodes becomes taut.

"We call this ordered unravelling," Prof Huang said. "And this is how we can produce a battery that stretches up to 300 per cent of its original size."

The battery continues to work – powering a commercial LED – even when stretched, folded, twisted and mounted on a human elbow and can work for eight to nine hours before it needs recharging.

The power and voltage of the stretchable battery, a square array of 100 electrode disks connected in parallel, are similar to a conventional lithium-ion battery of the same size, but the flexible battery can stretch up to 300 per cent of its original size and still function.

The stretching process is reversible and the battery can be recharged wirelessly as the battery's design allows for the integration of stretchable, inductive coils to enable charging through an external source but without the need for a physical connection.

Prof Huang, Prof Rogers and their teams found the battery was capable of 20 cycles of recharging with little loss in capacity.

Prof Huang led the portion of the research focused on theory, design and modelling, while Prof Rogers led the group that worked on the experimental and fabrication work of the stretchable battery.

Details of the device were published yesterday by the online journal Nature Communications.

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