US AFRL develops bendable Li-ion batteries

The US Air Force Research Laboratory’s (AFRL) Materials and Manufacturing Directorate is developing a new type of flexible lithium-ion battery that can be manipulated, bent and folded without voltage fluctuations, AFRL announced on 11 October.

Lithium-ion batteries last longer, charge faster and have a high energy capacity compared to traditional batteries; and with their small form factor, they can be used to power wearable battlefield devices and small sensors.

The technology uses highly conductive, flexible carbon nanotube mats to add flexibility to the Li-ion batteries, which are conventionally hard and rigid. Li-ion typically uses a thin polymer separator between their anode and cathode.

To fabricate a flexible battery, researchers replaced the common metal foil current collectors with chemical vapour deposition-grown carbon nanotube mats. Carbon nanotubes are considered highly conductive and extremely strong, both of which are necessary features for a flexible battery.

The researchers placed a separator between a carbon nanotube-based anode and cathode, which they then encapsulated in a thin, flexible plastic film. The battery was charged and mechanically tested. During the testing, the battery was bent and creased to assess if it performs consistently during extreme mechanical abuse.

According to the ARFL, the battery performed beyond expectations, maintaining a steady voltage after over 288 folds and manipulations. In contrast, a similar battery with typical metal foil current collectors showed a performance loss with each crease, and suffered a catastrophic fracture after 94 folds.

Ryan Kohlmeyer, a materials research scientist with UES, said: ‘If you’re moving around in the field, you don’t want to wear something that is bulky and rigid. Flexible batteries are conformal, meaning that they can move with the person and the device they power. The applications for this type of technology are limitless.

‘The voltage of the flexible battery remained steady even when it was flexed or creased hundreds of times. We only started to lose power when the material encapsulating our battery began to allow moisture to get inside the system and degrade the electrolyte and active materials.’