Two researchers, from Northwestern University and the University of Illinois have demonstrated a stretchable lithium-ion battery that can power stretchable electronics. What makes this new creation interesting is the fact that one does not have to connect it by a cord to a physical charging outlet. Moreover, these can be used anywhere – even inside the human body.

Northwestern University’s Yonggang Huang and the University of Illinois’ John A. Rogers in their demonstration attempted to power a commercial light-emitting diode (LED), and the battery continued to work even when the researchers stretched, folded, twisted it and then mounted it on a human elbow. Researchers claim that the stretchable battery can work for eight to nine hours, and once out of power can be recharged wirelessly. 

Huang explains, “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. 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.”

What is even more interesting is that the power and voltage of the stretchable lithium-ion battery is similar to that of a regular lithium-ion battery of the same size. What sets them apart, however is that the flexible battery can stretch up to 300 percent of its original size and continue to function.

Both, Huang and Rogers developed a certain “pop-up” technology, owing to which circuits can bend, stretch and twist. They placed the circuits systematically and then connected them using a metal wire. Once the array of circuit elements is stretched, the wires pop up. However, that method cannot be applied in case of a stretchable battery. For the pop up interconnect to work, there needs to be a lot of space. Now while circuits can be placed with space between them in a systematic arrangement, battery components have to be placed such that they produce a small, but powerful battery. “There is not enough space between battery components for the “pop-up” technology to work,” the post explains. 

Huang reveals that they used metal wire interconnects – long, wavy lines and filled up the small space between battery components. The way this works is that the power travels 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 “S’s.” 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,” the post adds further. 

Huang says, “We call this ordered unraveling. And this is how we can produce a battery that stretches up to 300 percent of its original size.”

Some characteristics of the stretchable battery include the fact the stretching process is reversible. The battery’s design is such that stretchable, inductive coils can be included to allow charging through an external source, removing the need to have a physical connecting port.

During the course of their research, Huang, Rogers and their teams found that the stretchable battery is capable of 20 cycles of recharging with little loss in capacity. 

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