New designs for electronics are including more micro-components that are attached to flexible substrates such as polymer films, metal foils, paper sheets, and rubber surfaces. Elastic substrates like rubber provide the potential to create products that bend and also stretch—opening up a much wider range of concepts, designs, and applications for energy storage devices and electronics.
Stretchable batteries are also a big challenge for mechanical engineers. These systems must accommodate large strain deformations, including bending, twisting, stretching, and compressing without breaking the delicate components and connections. Another high-value feature for these batteries is rechargeability with high-storage capacity and wireless connection to external power supplies.
Electronics with these features may become a reality, thanks to a research breakthrough at Northwestern University in Evanston, IL. As reported in his article in the February 26, 2013 issue of Nature Communications entitled “Stretchable Batteries with Self-Similar Serpentine Interconnects and Integrated Wireless Recharging Systems,” Yonggang Huang, professor of civil and mechanical engineering at Northwestern University (and also editor of ASME’s Journal of Applied Mechanics), and his research team have created a highly flexible battery that embeds tiny lithium-based batteries and flexible connections in a silicone sheet that can also be recharged wirelessly.
“This is a brand-new, stretchable and rechargeable lithium-ion battery that has similar performance compared to conventional rigid lithium-ion batteries, but can be twisted, bent, compressed, and even stretched up to four times its original size,” says Huang.
The ultra-stretchy battery could be used in bioengineering and medicine. Image: Engineering.illinois.edu
Key Design Concepts
Huang’s new rechargeable lithium-ion battery technology uses thin, low-modulus silicone elastomers as a substrate with a segmented design in the active materials, as well as innovative connections between them. The individual batteries are connected in a framework of conducting wires that are arranged in a repeating pattern and manufactured on a silicone substrate. The wires are coiled, allowing them to stretch without breaking when the battery is flexed.
“We can achieve these characteristics by exploiting segmented layouts and deformable electrical interconnects in specialized, ‘self-similar’ geometries,” says Huang. By using self-similar structures of wires in serpentine configurations, these layouts provide both high system-level stretchability and low interconnect resistance. The self-similar geometry leads to hierarchical buckling physics that ensures ultra-low strains in the materials, even under extreme stretching.
The resulting prototype provides biaxial stretchability up to strains of 300%, with capacity densities of B1.1mAhcm2, and little loss in capacity for up to 20 cycles of recharging. It also enables integration of stretchable, inductive coils that provide wireless charging through external supplies without the need for physical connections, notes Huang.
“Stretchability greater than 300% and reversibility greater than 200% significantly exceed those of previous reports in stretchable batteries and/or battery electrodes,” Huang says. “They are also greater than those of any other reports of stretchable interconnects that use lithographically defined patterns of conventional metals.”
Huang’s stretchable batteries enable true integration with stretchable electronics in small packages that are wirelessly rechargeable. Examples of stretchable electronics include epidermal health/wellness monitoring electronics, wearable photovoltaics, electronic “eyeball” imaging devices, and implantable light-emitting systems.
“This may open the door for flexible/stretchable electronic devices that can be implanted not only on the human body, but also inside it, to monitor anything from brain waves to heart activity,” says Huang. “The information we present on stretchable interconnects has broad value beyond batteries, creating possible applications for many kinds of electronic and optoelectronic systems.”
Mark Crawford is an independent writer.
This is a brand-new, stretchable and rechargeable lithium-ion battery that has similar performance...but can be twisted, bent, compressed, and even stretched up to four times its original size.
Prof. Yonggang Huang, Northwestern University