A battery that is nontoxic and stretchable that could be useful in a variety of IoT applications, has been developed by a team of researchers at Rice University and North Carolina State University. The applications range from wearables to advanced surveillance monitors with built-in kill switches.

The battery operates by extracting moisture from the ambient environment — even in the desert and other dry climates. Emerging technologies, such as wearable monitors, miniature robotics and other IoT devices, need lightweight, flexible power sources.

Right now, the best power source options are conventional batteries, but they are often too rigid and heavy to be useful, and they contain toxic materials that can leak. So-called energy harvesters, which capture energy from the surrounding environment and convert it into electrical power, are lighter, but their performance is limited.

“The new moisture-activated battery (MAB) includes a magnesium anode and a silver/silver chloride cathode, with a cellulose membrane loaded with lithium chloride salts that serves as a separator,’’ the researchers explained. “The separator harvests moisture from ambient air which dissolves the salts and creates the electrolyte, allowing charge to flow through the battery.”

The MAB essentially runs on salt water, which eliminates toxic and flammable electrolytes, according to Amay Bandodkar, assistant professor of electrical and computer engineering at NC State and co-corresponding author of the research. “And since it only activates once it’s exposed to ambient air, it remains inactive while within sealed packaging, giving it an extended shelf life.”

The researchers also enhanced the battery’s performance while it is being stretched. Most stretchable batteries leverage a series of serpentine interconnectors that still allow the current to flow when stretched, they said. However, gaps are created by stretching the device, thus lowering energy density. The MAB’s design is able to eliminate most of those gaps by utilizing a pangolin-inspired structure of densely packed overlapping scales.

“Mechanics plays a central role in making these batteries both stretchable and practical,” said Raudel Avila, assistant professor of mechanical engineering at Rice University and co-corresponding author of the study, “Our modeling revealed how bioinspired stacking and stretchable interconnectors can redistribute deformation throughout the battery, preserving performance under bending, twisting and stretching while minimizing the empty space that typically reduces energy density.”

The researchers demonstrated how the battery’s lifespan is comparable to that of conventional batteries by using the MAB to run a wireless Bluetooth oximeter for up to 30 hours.

“This battery is far more than an academic proof of concept; it is a practical energy source capable of powering everyday IoT and medical devices,” said Abraham Vázquez-Guardado, assistant professor of electrical and computer engineering at NC State and co-corresponding author of the research. “That level of performance proves this battery technology is ready to power a whole new generation of electronic devices and applications.”

The research team also showed how a unique “kill switch” feature based on the moisture-harvesting technology, can serve as an anti-tampering safeguard and when triggered, quickly kill a device. This makes the MAB potentially useful in surveillance monitoring used in covert intelligence-gathering missions.

The research appears in Science Advances and is supported by NC State’s Advanced Self-Powered Systems of Integrated Sensors and Technologies (ASSIST) Center Industry Seed Fund and Chancellor’s Innovation Fund, as well as by Rice University’s ENRICH office.