While electronics embedded in clothing are increasingly common, the development of safe, durable energy sources to power these devices remains a challenge.

Existing lithium-ion batteries for wearable electronics, though they can be bent and rolled up, may break when twisted too far or accidentally stepped on—which happens often when being worn. This damage not only causes the battery to fail, it can also cause flammable, toxic or corrosive gases or liquids to leak out.

Chinese and South Korean scientists led by professors Yonggang Wang and Huisheng Peng, of Fudan University, Shanghai, have now developed a family of thin, flexible lithium-ion batteries with self-healing properties that can be safely worn on the body. Even after completely breaking apart, they can grow back together without significant impact on their electrochemical properties.

The capacity and charging/discharging properties of a battery “armband” placed around a doll’s elbow were maintained even after repeated break/self-healing cycles. Image credit: ©Wiley-VCH 2016. The electrodes in these batteries consist of layers of parallel carbon nanotubes. Between the layers, the scientists embedded the necessary lithium compounds in nanoparticle form (LiMn₂O₄ for one electrode, LiTi₂(PO₄)₃ for the other). According to the researchers, in contrast to conventional lithium-ion batteries, the lithium compounds cannot leak out of the electrodes, either while in use or after a break.

The thin-layer electrodes are each fixed on a substrate of self-healing polymer. Between the electrodes is a solvent-free electrolyte made from a cellulose-based gel with an aqueous lithium sulfate solution embedded in it. This gel electrolyte also serves as a separation layer between the electrodes.

After a break, the broken ends can be pressed together for several seconds and will then grow back together. The self-healing polymer and the carbon nanotubes adhere easily, and the nanotubes' parallel arrangement allows them to join together significantly better than would layers of disordered nanotubes, according to the researchers.

The electrolyte also poses no problems, the scientists report. Whereas conventional electrolytes decompose immediately upon exposure to air, the new gel is stable. Free of organic solvents, it is neither flammable nor toxic.

According to the researchers, the capacity and charging/discharging properties of a battery “armband” placed around a doll’s elbow were maintained even after repeated break/self-healing cycles.