Recently, researchers from China have demonstrated practical lithium-ion battery fibres that can be woven into textiles. What challenges do typical power sources face in wearables, what have the researchers achieved, and how could woven electronics change the wearable industry?

What challenges do power sources face in wearables?

Of all Industries on this planet, the one industry that would benefit most from electronics is wearables. If wearables could be made mainstream, most modern portable devices would be instantly replaced as wearable electronics offer greater convenience and usability.

However, for wearable electronics to become mainstream, multiple challenges need to be faced. The first challenge is that electronics are inherently ridged, while applications involving wearables are generally required to be flexible and move organically with the user.

The second challenge is that electronics constructed from flexible materials need to have the ability to create complex circuits as the resulting device needs to be practical (i.e. be able to perform modern functions). Flexible circuits have already been demonstrated, but almost all of these are inappropriate for commercial electronics use. They are either too power-hungry or use logic technologies inappropriate for modern applications (such as NMOS).

The third, and most important challenge, is that wearable electronics need to be powered by an energy source that is also wearable. Some researchers are looking into the possibility of obtaining energy from the surrounding environment using energy harvesters. However, such sources universally produce minimal amounts of power and, therefore, cannot power even the most essential circuits.

Researchers develop fibre lithium-ion batteries

Recently, researchers from China have developed high-performing woven lithium-ion fibre batteries that can be woven into textiles and other materials.

While such fibre batteries have been produced previously, it was thought that this type of battery internal resistance is too great to be used as a practical power source. Furthermore, previous research has shown that producing such fibres presents significant challenges beyond a few centimetres in length (high internal resistance, for example).

However, research from the Chinese team shows that the resistance of such fibres is not linear and instead follow a hyperbolic cotangent function relationship meaning that the internal resistance of such batteries levels off as the battery length increases. The researchers also noted that fibre batteries using different technologies have the same resistance characteristic where the resistance levels off as the battery length increases.

The battery fibres produced by the Chinese team have an energy density of 85.69 watt-hours per kilogram, which is significantly higher than standard fibre batteries that typically have an energy density of just one watt-hour per kilogram. The fibres are constructed by depositing lithium manganate particles onto carbon nanotubes, separated from a silicone coated carbon nanotube sheet via a gel electrolyte.

What is of particular interest in the research is that the fibre was demonstrated to retain 90.5% of its capacity after 500 charging cycles and over 80% of its capacity after 100,000 mechanical bending Cycles. As such, the researchers were able to demonstrate that the new fibre could be safely woven into textiles, washed, and worn while retaining its ability to hold a charge.

Future of wearable power

Of all power sources being developed for wearable electronics, the fibre batteries developed by the Chinese research team is by far the most practical for wearable electronics. The ability to create a power source that is not only flexible but able to retain is charged even under mechanical stress is a significant step forward in producing wearable electronics.

While this research is still in the early stages, it demonstrates that future wearable electronics could be powered by textiles that integrate batteries in fibres. Such technology could be taken further with conductive fibres woven into textiles that would allow wearable devices to either connect to the battery directly or via inductive coupling to take advantage of the close distance to increase the overall efficiency.