Stretchable battery for wearable electronics

RESEARCHERS at Stanford University, US have developed a soft and stretchable battery that could be used to power wearable electronics. They claim it is one of the first demonstrations of an intrinsically stretchable lithium-ion battery.

Wearable electronics combine sensors and wireless communications to enable remote collection of information and more seamless interface between humans and technology. They can be used for purposes such as data collection, live monitoring and feedback, and communication. The emergence of wearable electronics which bring batteries in close contact with human skin has exacerbated the need for battery materials that are robust, highly ionically conductive, and stretchable.

The researchers developed a solid and stretchy polymer that can act as an electrolyte. Previously, polymer electrolytes have been in the form of flowable gels that are flammable and could leak. In their work the researchers also used the polymer as a stretchable electrode binder, which resulted in stretchable lithium-ion battery electrodes with a strain capability of more than 900%.

In lab tests, the stretchable battery maintained constant power even when it was squeezed, folded, or stretched. The batteries – typically with an active material area of 1 cm² – had a capacity of 1.1 mAh/cm² and could function even when stretched up to 70% strain. The capacity of the battery is high compared to most stretchable batteries and is similar to commercially-available flexible batteries. Additionally, it is not reduced significantly as compared to conventional electrode materials, which have capacity ranging from 2–4 mAh/cm².

The battery is based on a supramolecular lithium-ion conductor (SLIC). The macromolecule contains a soft segment based on the ion-conducting polymer poly(propylene glycol)-poly(ethylene glycol)-poly(propylene glycol) (PPG-PEG-PPG). Hydrogen-bonding motifs in the backbone of the macromolecule interact with each other to impart high mechanical strength. When the material is stretched, the polymer can mechanically dissipate stress by breaking the reversible hydrogen bonds, while maintaining ion-transport pathways.

The polymer electrolyte has “unprecedented” toughness of 29.3 MJ/m³, which according to the study is at least three-fold higher than most robust electrolytes reported to date. Additionally, the researchers achieved high ionic conductivity (1.2×10^–4 S/cm at 25°C). The ionic conductivity competes with those of the highest reported ionic conductivities and is “acceptable” for use in lithium ion battery applications.

Zhenan Bao, Professor of Chemical Engineering at Stanford, said: “Until now we haven’t had a power source that could stretch and bend the way our bodies do, so that we can design electronics that people can comfortably wear.”

According to David Mackanic, Graduate Research Fellow and lead author of the study, the group is now working to increase the battery’s energy density; build larger versions; and, in future, run experiments to demonstrate the battery’s performance outside of the lab.

Nature Communications: http://doi.org/ggd6dj