Multilayered electrodes help batteries flex and conduct

Although researchers have been making progress on building electronics that bend and flex, they often face a roadblock: electronic components can be stretchy, or they can be conductive, but they are rarely both.

A new study, however, reports the use of multi-scale engineering to fabricate a durable, stretchy battery that maintains 72% of its charge capacity while enduring 30% strain (Sci. Adv. 2019, DOI: 10.1126/sciadv.aaw1879). To make the battery, an international research team initially tried flexible electronic fabrication methods published in the scientific literature. But they couldn’t reproduce the previous results, says Minsu Gu, a postdoctoral scholar at Yonsei University and the lead author of the study. So the team switched their focus to developing their own layer-by-layer assembly method.

They made composite sheets containing negatively charged gold nanoparticles suspended in positively charged polyurethane. By changing the percentage of nanoparticles in these composites, the researchers could tune both the conductivity and the stretchiness of the sheets. Still, they faced the familiar conductivity-stretchiness trade-off. A sheet containing 90% nanoparticles by mass had comparable electrical resistance to a metal conductor but could stretch only 2%; sheets with lower-mass fractions of the nanoparticles could stretch up to 380% but were unable to conduct.

The breakthrough came when the team tried stacking sheets with varying nanoparticle concentrations. The stacks could then stretch and flex while still conducting electricity.

“These properties are not easy to combine. They are contrarian to each other,” says Nick Kotov, one of the team leaders and a chemical engineer at the University of Michigan. “By multi-scale engineering of these component materials, we are able to resolve these conundrums.”

When the scientists imaged the stacked conductor under different strain rates with small-angle X-ray scattering, they found something surprising. As the material stretched, the gold nanoparticles in the various layers self-assembled in the direction of the strain. According to Kotov, this self-assembly is what allows the material to continue conducting, even under 100% strain rates.

To test the usability of their conductor, Kotov and the rest of the team incorporated it into the electrodes of a water-based lithium-ion battery. Under strain, microcracks appeared in the battery materials, but the electrodes recovered to their original state after the strain was released.

According to Kotov, there is a “very clear case” that batteries like these could power implantable devices in the next few years. He points to urinary-tract implants as a case where a stretchable battery could significantly improve life for a patient with incontinence or other bladder-control issues.

Jodie Lutkenhaus, a professor of chemical engineering at Texas A&M University, has some reservations about batteries like these being used in implants in their current form. Because the battery uses a water-based electrolyte, she says, it poses a leakage risk. Still, Lutkenhaus praises the new study for demonstrating a fabrication method for electronic components that could extend to a variety of applications, such as wearable sensors, optical coatings, and—one day—implantable medical devices engineered for safety.