Bendy batteries

Scientists have produced a bendy battery that can stretch elastically to three times its normal length without affecting its performance.

US researchers Yonggang Huang from Northwestern University and John Rogers, from the University of Illinois, have made the new battery by using long S-shaped wires, dubbed serpentine interconnects, to link together in series many tiny 2mm-diameter individual circular lithium cells.

These are sandwiched inside a stretchy silicon polymer. In tests, the batteries could be extended to over 300% of their starting dimensions.

The key to this impressive performance is that the S-shaped wires are themselves made of many repeating smaller s-shapes, meaning that each interconnect contains a significant reserve of metal.

Stretching the battery temporarily unravels some of these connections, meaning that they lengthen but without perturbing the power-producing lithium cells themselves.

The energy output of the batteries, at 1.1 mAh per square centimetre, is on par with standard lithium cells, and doesn't deteriorate when the cells are deformed.

The cells still have some challenges to overcome; for a start, long term stability still needs to be proven, because the team have only discharged and re-charged them - using the built-in wireless recharger they have also implemented - about 20 times over a 7 day period.

Published in Nature Communications, the team suggest that their devices will pave the way for flexible power supply applications in areas ranging from "wearable photovoltaics to epidermal health / wellness monitors", to "sensitive robotic skins, soft surgical tools and electronic eyeball imaging devices."

In all cases, they say, "stretchability represents a key enabling characteristic..."