Stretchable Batteries Make Flexible Electronics More So

The stretchable battery is gaining momentum in the electronics industry, where it might one day serve as an energy storage medium in fitness trackers, wearable electronics, and even smart clothing.

Researchers believe the concept will become more valuable in the next decade, as electronic devices migrate closer and closer to human skin. For many applications, such as wearables, stretchability is necessary since our skin stretches as we move," said James Pikul, a professor of mechanical engineering at the University of Wisconsin-Madison. A battery that only flexes would feel uncomfortable to wear."

A stretchable battery behaves like a rubber band, whereas flexible batteries are more like a piece of paper, which can bend but not stretch.

Pikul and others around the world are now working on batteries that stretch. The new batteries differ from commonly known flexible batteries" in that they withstand axial tension forces-longitudinal forces on a body that include tension and compression-and will stretch elastically when such force is applied. In essence, a stretchable battery behaves like a rubber band, whereas flexible batteries are more like a piece of paper, which can bend but not stretch.

Patches Argue for Stretches

Recent interest in stretchable batteries stems from growing use of unpowered wearable patches that monitor blood and even sweat. Gatorade, for example, now markets a skin patch called the Gx Sweat Patch, which helps track personal hydration. Numerous other companies offer wearable medical patches, many of which would benefit from the integration of a power source. We're seeing microelectronics being used everywhere," said Thierry Djenizian, a professor in the flexible electronics department at the School of Mines Saint-Etienne in France. And those electronics need power. One solution is the development of microbatteries that can be completely invisible."

We stretched the battery, twisted it, hit it with a hammer, and still we showed that it could consistently power a servo motor under all of those deformations." -James Pikul, University of Wisconsin-Madison

Djenizian is part of a group that published a paper in February in the journal Advanced Materials Technologies on a stretchable lithium-ion wire battery. The wire battery, which measures 1.4 millimeters in diameter and more than 20 centimeters long, uses a twisted copper fabric as a current collector. It has been fabricated using conventional battery chemistries such as lithium cobalt oxide (LCO) and lithium nickel cobalt aluminum (NCA, popular for a time in Tesla cars). The researchers report that their battery can be stretched up to 22 percent and can be used in applications including biomedical patches, health trackers, smart textiles, and wristwatches. Djenizian said that a big part of the battery's appeal is simple comfort. If you're doing a yoga stretch, your shirt can pull back on you. And if you have batteries that are not stretchable, you'll feel it."

Similarly, Pikul is part of a group that published a paper in March in the journal Advanced Functional Materials on stretchable metal-air batteries. The new metal-air battery addresses a simple fact of battery life-that hard metals may make good anodes and cathodes, but they don't stretch. The solution to that dilemma is an architecture in which the battery's metal electrodes are allowed to slide freely between its enclosure and its electrolyte, both of which do stretch.

The result is that the active parts of the battery-the anode and cathode-don't need to stretch. In essence, they slide across the face of the electrolyte. The electrolyte is made from a hydrogel, a substance roughly the consistency of a soft contact lens. The anode and the cathode are blocks, and they just slide across the other components that are doing the stretching," Pikul explained. The wire battery uses a metal anode, typically zinc, and a carbon cloth loaded with platinum, as the air" cathode. The battery is not rechargeable, and its applications include medical patches and hearing aids.

Adding Zinc to the Mix

Researchers are also developing stretchable batteries engineered for safety, which can even be used in applications where the wire battery comes in contact with, for example, the wet skin of a perspiring user. In a paper published in February in the journal Small, authors Zhao Wang and Jian Zhu say the key to such batteries is a stretchable zinc-ion chemistry that uses an aqueous electrolyte. Such batteries are safer than lithium-ion, which uses an inherently flammable" organic electrolyte, they say. Stretchable batteries with aqueous electrolytes can give us absolute safety and reliable power during deformation," Zhu wrote in an email.

The authors describe numerous zinc-ion chemistries, mostly involving a zinc anode and a manganese oxide cathode or a silver cathode. Energy capacity of stretchable zinc-ion ranges from a few milliampere-hours per gram to as much as 300 mAh per gram. In comparison with conventional lithium-ion batteries, stretchable zinc-ion batteries have a lower energy density, but they can drive most power-consumption modules," including sensors, transistors, and displays, Zhu said. With careful engineering, he said, the batteries can be stretched more than 900 percent.

Unlike cellphone batteries, which consume much of the volume and weight of the overall product, the new breed of stretchable batteries is expected to be virtually invisible. Most are less than 2 millimeters in diameter, and weigh just a few grams.

Moreover, durability does not seem to be an issue with any of the thin, stretchable batteries. Researchers said they subjected their stretchable batteries to substantial abuse without incident. We stretched the battery, twisted it, hit it with a hammer, and still we showed that it could consistently power a servo motor under all of those deformations," Pikul said.

Battery experts believe the stretchable concept is viable, and will likely find a market. Yes, in principle a stretchable battery could be made, provided there is a suitable anode," said Donald Sadoway, a retired materials science professor from MIT and founder of Sadoway Labs Foundation, a nonprofit research institution aimed at new battery discoveries. But maybe flexible is what is needed, not necessarily stretchable." Sadoway added that he built a stretchable wristwatch battery in the 1990s, but found it was too early for the market.

None of today's researchers know when the new breed of batteries will reach the market, but they expect demand for them to grow. In the past 10 years, there's been all these advances in stretchable electronics, and now there are a lot of new applications," Pikul said. So there's a need to power these stretchable devices, and the logical solution is to have stretchable batteries."