Stretchability may not be what you think of when you think of batteries. But this shape-changing property is necessary for batteries to be integrated into flexible electronics, which are gaining attention for wearable health monitors. ACS Energy Letter A lithium-ion battery with fully stretchable components, including an electrolyte layer that can expand by up to 5000%, has been reported, which maintains its charge storage capacity even after nearly 70 charge-discharge cycles.
Electronics that bend and stretch need batteries with similar properties. Most researchers who have tried to make such batteries so far have done so using conductive textiles or rigid components folded into stretchy shapes like origami. But a truly malleable battery would require all of its components to be stretchable, including the electrodes that collect the charge and the intermediate electrolyte layer that balances the charge. So far, prototypes of truly stretchable batteries have suffered from moderate stretchability, complicated assembly processes, and limited energy storage capacity over time, especially when repeatedly charged and discharged. The latter could be due to weak connections between the electrolyte layer and the electrodes, or the instability of a fluid electrolyte that moves around when the battery changes shape. So instead of using a liquid, Wen-Yong Lai and colleagues wanted to create a fully solid-state stretchable battery by incorporating the electrolyte in a polymer layer fused between two flexible electrode films.
To create a fully stretchable battery electrode, the team spread a thin film of a conductive paste containing silver nanowires, carbon black, and lithium-based cathode or anode material onto a plate. They then applied a layer of polydimethylsiloxane, a flexible material commonly used in contact lenses, on top of the paste. The researchers added lithium salts, highly conductive liquids, and materials that make the stretchable polymer directly on top of this film. When activated with light, these components combine to form a solid, rubbery layer that can stretch up to 5,000 percent of its original length and transport lithium ions. Finally, they placed another electrode film on top of the stack and sealed the whole device with a protective coating.
When the stretchable solid-state battery design was compared to a similar device using a conventional liquid electrolyte, the new version had an average charge capacity about six times higher during fast charging. Similarly, the solid-state battery maintained a more stable capacity during operation over 67 charge-discharge cycles. In other prototypes made with solid electrodes, the polymer electrolyte maintained stable operation for over 1000 cycles, with a 1% drop in capacity over the first 30 cycles compared to a 16% drop with the liquid electrolyte. While there is still room for improvement, this new method of creating fully stretchable solid-state batteries could be a promising step forward for wearable and implantable devices that bend and move with the body.
The authors gratefully acknowledge funding from the National Key Research and Development Program of China, the National Natural Science Foundation of China, the Natural Science Foundation of Jiangsu Province, the Key Laboratory Fund for Flexible Electronics of Zhejiang Province, the Special Professor Program of Jiangsu Province, the NUPT “1311 Project” and Science Foundation, the China Postdoctoral Science Foundation, the NJUPT State Key Laboratory Project of Organic Electronics and Information Display, and the NJUPT Natural Science Foundation.
The paper abstract will be available on July 17 at 8 a.m. ET here: http://pubs.acs.org/doi/abs/10.1021/acsenergylett.4c01254
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