When you think of a battery, you probably don’t think of its elasticity. But batteries will need this shape-shifting ability to be integrated into flexible electronics, which are gaining traction for wearable health monitors. Now, researchers in ACS Energy Letters report a lithium-ion battery with fully stretchable components, including an electrolyte layer that can stretch 5,000 percent, and that retains its charge storage capacity after nearly 70 charge/discharge cycles.
Electronic devices that bend and stretch need batteries with similar properties. Most researchers who have tried to build such batteries have created them from woven conductive fabric or rigid components folded into stretchable shapes, like origami. But for a truly malleable battery, every part—the electrodes that collect the charge and the middle electrolyte layer that balances the charge—must be elastic. So far, prototypes of truly stretchable batteries have had moderate elasticity, complex assembly processes, or limited energy storage capacity, especially over time with repeated charging and discharging. The latter may be due to a weak connection between the electrolyte layer and the electrodes or the instability of the fluid electrolyte, which can shift as the battery changes shape. So, rather than using a liquid, Wen-Yong Lai and his colleagues wanted to embed the electrolyte in a polymer layer fused between two flexible electrode films, to create a completely solid-state, stretchable battery.
To make the electrodes for the fully elastic battery, the team spread a thin layer of conductive paste containing silver nanowires, carbon black, and lithium-based cathode or anodic materials on a plate. A layer of polydimethylsiloxane, a flexible material commonly used in contact lenses, was then applied on top of the paste. Directly on top of this film, the researchers added a lithium salt, a highly conductive liquid, and the ingredients to make a stretchable polymer. When activated by light, these components combined to form a tough, rubbery layer that could stretch up to 5,000 percent of its original length and carry lithium ions. Finally, the stack was covered with another electrode film, and the entire device was sealed in a protective coating.
Comparing the expandable solid-state battery design to a similar device with a traditional liquid electrolyte, the new version had an average charge capacity about six times higher at a fast charge rate. Similarly, the solid-state 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 1,000 cycles, with capacity decreasing by 1% over the first 30 cycles, compared to a 16% decrease for the liquid electrolyte. There is still room for improvement, but this new way of creating fully expandable solid-state batteries could be a promising advancement for wearable or implantable devices that flex and move with the body.
The authors gratefully acknowledge the National Key Research and Development Program of China, the National Natural Science Foundation of China, the Natural Science Foundation of Jiangsu Province, the Foundation of Key Laboratory of Flexible Electronics of Zhejiang Province, the Program for Specially Appointed Professors of Jiangsu, the “1311 Project” and the Science Foundation of NUPT, the China Postdoctoral Science Foundation, the Project of State Key Laboratory of Organic Electronics and Information Display, NJUPT, and the Natural Science Foundation of NJUPT.
The article abstract will be available on July 17 at 8 a.m. EST here: http://pubs.acs.org/doi/abs/10.1021/acsenergylett.4c01254
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