….And S T R E T C H
Imagine a world where your mobile phone could be wrapped around your wrist, your GPS could be bent around your bike handles, or your tablet rolled up and put in your pocket... this is a vision of a future powered by flexible electronics.
And it's not all that futuristic. Across the world, companies are developing flexible displays capable of being rolled and bent without damage. But until now, there has been a major bottleneck - the lack of an equally-flexible battery to reliably power them.
A team, led by Yonggang Huang at Northwestern University and John Rogers from the University of Illinois have finally filled that gap - they have developed a battery that can not only twist and bend, but can stretch and return to its normal shape, all without damaging its performance. Their rechargeable lithium ion battery is based on a silicone sheet containing one hundred rigid disks of storage material, all electrically connected by long, tightly-packed, S-shaped metal wires which carry the electrical charge between the disks.
The team chose to use lithium, a well-known battery storage material, and focus instead on the design and development of truly deformable interconnects. The stretchiness of this battery stems from these flexible wires; when the device is stretched, the wires undergo 'ordered unravelling'- i.e. they do the 'stretching' - while the battery's precious storage disks remain rigid and undamaged by the process.
The serpentine interconnects have 'self-similar' geometry - each S-shaped wire is made up of many smaller 'S's'. When you begin to stretch the device, you start to unravel the large 'S'. As you continue to stretch it, you begin to unravel the smaller 'S's', until eventually the wires become taut, and the battery cannot stretch further. Even under this extreme stretching, the interconnects are are under such a low mechanical strain that the device can easily return to its original shape.
To demonstrate the performance of their battery, Huang and his team connected it to an LED, stretched by 300%, folded it, twisted it and mounted it onto a human elbow - and the battery continued to operate. The power and voltage outputs of the stretchable battery are comparable to a conventional lithium-ion battery of the same size, with capacity density of ~1.1 mAhcm-2. It can also work for eight to nine hours before being recharged, and this can be done wirelessly in just 6 minutes, thanks to a flexible charging circuit developed by the same team.
Huang's stretchy battery still has to overcome some challenges - its performance has been measured for just 20 charge/discharge cycles, or seven days of operation, so further development will be needed to improve this lifetime, and the materials have yet to be optimised. But with this work, this team have reopened the door to real-world flexible electronic systems that you can bend, twist and stretch as much as you want - with this technology, you can power through.