When you think of a battery, stretchability is probably not the first thing that comes to mind. However, this ability will be crucial for integration into flexible electronics, which are becoming increasingly popular in wearable health devices. Researchers from ACS Energy Letters have introduced a lithium-ion battery with fully stretchable components, including an electrolyte layer that can expand by an incredible 5000%, while retaining energy storage capacity even after nearly 70 charge and discharge cycles.

Electronics that can bend and stretch require batteries with similar characteristics. Most scientists who have attempted to create such batteries have used fabrics with conductive properties or rigid components bent into stretchable shapes, similar to origami. However, for a truly stretchable battery, every part - from the electrodes that collect charge to the middle electrolyte layer that balances the charge - must be elastic. Previous prototypes of batteries with moderate elasticity had complex assembly processes or limited energy storage capacity, especially during long-term use with repetitive charge and discharge cycles. This can be due to poor bonding between the electrolyte layer and the electrodes or the instability of a liquid electrolyte that can move when the battery changes shape. Instead of using a liquid electrolyte, Wen-Yong Lai and his colleagues decided to embed the electrolyte in a polymer layer placed between two flexible electrode film layers, creating a fully solid, stretchable battery.

To make electrodes for the fully elastic battery, the team spread a thin film of conductive paste containing silver nanowires, carbon black, and lithium-based cathode or anode materials onto a plate. They then applied a layer of polydimethylsiloxane, a flexible material often used in contact lenses, on top of the paste. Directly on top of this film, researchers added lithium salt, a highly conductive liquid, and ingredients to create a stretchable polymer. Activated by light, these ingredients formed a solid, rubbery layer capable of stretching up to 5000% of its original length and transferring lithium ions. Finally, another electrode film was added on top of the layer, and the entire device was sealed with a protective layer.

Comparing the design of the solid stretchable battery with a similar device using a traditional liquid electrolyte, the new version showed approximately six times the average charging capacity during rapid charging. Likewise, the solid battery maintained a more stable capacity over 67 charge and discharge cycles. In other prototypes made with solid electrodes, the polymer electrolyte maintained stable operation over 1000 cycles, with a 1% capacity drop in the first 30 cycles, compared to a 16% drop for the liquid electrolyte. Although improvements are still needed, this new method of creating fully stretchable, solid batteries could be a promising step forward for wearable or implantable devices that bend and move with the body.

The authors acknowledged financial support from the National Key Research and Development Program of China; the National Natural Science Foundation of China; the Jiangsu Provincial Natural Science Foundation; the Zhejiang Provincial Key Laboratory of Flexible Electronics Foundation; the Jiangsu Specially-Appointed Professor Program; the NUPT "1311 Project" and Science Foundation; the China Postdoctoral Science Foundation; the State Key Laboratory of Organic Electronics and Information Displays, NJUPT Project; and the NJUPT Natural Science Foundation.

Source: American Chemical Society