Fully elastic lithium-ion battery

Traditionally, batteries have been rigid. However, batteries need elasticity to be incorporated into flexible electronic devices, which are gaining ground in the field of wearable health monitors.

A team including Wen-Yong Lai and Shijun Xiao of the Nanjing University of Posts and Telecommunications in China has created a lithium-ion battery with fully stretchable components, including an electrolyte layer that can expand by 5,000 percent, and that retains its charge storage capacity after nearly 70 charge-discharge cycles.

Electronic devices that bend and stretch require batteries with similar properties. Most researchers who have tried to design such batteries have created them with conductive fabrics or rigid components folded into stretchable shapes. But for a battery to be truly malleable, all of its parts — the electrodes that pick up the charge and the middle layer of electrolyte that balances the charge — must be elastic.

So far, prototypes of truly elastic batteries have suffered from either moderate elasticity, complex assembly processes, or limited energy storage capacity, particularly over time and with repeated charging and discharging. The latter can be due to a weak connection between the electrolyte layer and the electrodes or to the instability of the fluid electrolyte, which can shift when the battery changes shape.

So instead of using a liquid, Wen-Yong Lai and his collaborators set out to embed the electrolyte in a molten polymer layer between two flexible electrode films to create a completely solid yet elastic battery.

To fabricate the electrodes for the fully elastic battery, the team spread a thin film of conductive paste containing, among other components, silver nanowires as well as lithium-based cathode or anode materials onto a plate.

A layer of polydimethylsiloxane, a flexible material commonly used in contact lenses, was then applied to the paste.

Directly on top of this film, the researchers added a lithium salt, a highly conductive liquid and the ingredients to make an elastic polymer.

When these components were activated by light, they combined to form a solid, rubbery layer capable of stretching up to 5,000% of its original length and transporting lithium ions. Finally, the stack was covered with another film of electrodes and the entire device was sealed with a protective layer.

Demonstration of the elasticity of the new lithium-ion battery. (Photos: adapted from ACS Energy Letters 2024, DOI: 10.1021/acsenergylett.4c01254)

When comparing the solid stretch battery design to a similar device using a traditional liquid electrolyte, the new version had an average charge capacity about six times higher at a fast charging rate. Similarly, the solid battery maintained a more stable capacity over 67 charge-discharge cycles. In other prototypes made with solid electrodes, the polymer electrolyte maintained stable operation for more than 1,000 cycles with a 1% capacity drop in the first 30 cycles, compared to a 16% drop for the liquid electrolyte. There are still improvements to be made, but this new way of making fully stretchable solid batteries could be a promising advance for wearable or implantable devices that flex and move with the body.

Wen-Yong Lai and colleagues present the technical details of their stretchable lithium-ion battery in the journal ACS Energy Letters, under the title “Elastic Polymer Electrolytes Integrated with In Situ Polymerization-Transferred Electrodes toward Stretchable Batteries.” (Source: American Chemical Society)