The rechargeable battery can be woven and washed, and could provide power for fibre-based electronic devices and sensors.

Researchers have developed a rechargeable lithium-ion battery in the form of an ultra-long fibre that could be woven into fabrics. The battery could enable a wide variety of wearable electronic devices, and might even be used to make 3D-printed batteries in virtually any shape. 

This submarine drone is powered by a 20-metre-long fibre battery that is wrapped on its surface. Image: Courtesy of the researchers

The researchers envision new possibilities for self-powered communications, sensing, and computational devices that could be worn like ordinary clothing, as well as devices whose batteries could also double as structural parts.

Can be manufactured to arbitrarily long lengths

In a proof of concept, the team behind the new battery technology has produced the world’s longest flexible fibre battery, 140 metres long, to demonstrate that the material can be manufactured to arbitrarily long lengths.

The work is described in the journal Materials Today. MIT postdoc Tural Khudiyev (now an assistant professor at National University of Singapore), former MIT postdoc Jung Tae Lee (now a professor at Kyung Hee University), and Benjamin Grena SM ’13, PhD ’17 (currently at Apple) are the lead authors on the paper. Other co-authors are MIT professors Yoel Fink, Ju Li, and John Joannopoulos, and seven others at MIT and elsewhere.

Researchers, including members of this team, have previously demonstrated fibres that contain a wide variety of electronic components, including light emitting diodes (LEDs), photosensors, communications, and digital systems.

Many of these are weavable and washable, making them practical for use in wearable products, but all have so far relied on an external power source. Now, this fibre battery, which is also weavable and washable, could enable such devices to be completely self-contained.

The new fibre battery is manufactured using novel battery gels and a standard fibre-drawing system that starts with a larger cylinder containing all the components and then heats it to just below its melting point. The material is drawn through a narrow opening to compress all the parts to a fraction of their original diameter, while maintaining all the original arrangement of parts. 

The fibre battery continues to power an LED even after partial cutting indicating that the fibre battery system is free from electrolyte loss and from short-circuiting

While others have attempted to make batteries in fibre form, says Khudiyev, those were structured with key materials on the outside of the fibre, whereas this system embeds the lithium and other materials inside the fibre, with a protective outside coating, thus directly making this version stable and waterproof. This is the first demonstration of a sub-kilometre long fibre battery which is both sufficiently long and highly durable to have practical applications, he says.

'Li-Fi' communications system

The fact that they were able to make a 140-metre fibre battery shows that “there’s no obvious upper limit to the length. We could definitely do a kilometre-scale length”, he says. A demonstration device using the new fibre battery incorporated a 'Li-Fi' communications system – one in which pulses of light are used to transmit data, and included a microphone, pre-amp, transistor, and diodes to establish an optical data link between two woven fabric devices.

“When we embed the active materials inside the fibre, that means sensitive battery components already have a good sealing,” says Khudiyev, “and all the active materials are very well integrated, so they don’t change their position” during the drawing process.

In addition, the resulting fibre battery is much thinner and more flexible yielding an aspect ratio, that is the length-to-width fraction, up to a million, which is way beyond other designs, which makes it practical to use standard weaving equipment to create fabrics that incorporate the batteries as well as electronic systems.

The thermally drawn fibre battery (right) is fire-resistant due to the gel electrodes and gel electrolyte, whereas the control fibre battery with liquid electrolyte (left) instantly catches fire and expands

The 140-metre fibre produced so far has an energy storage capacity of 123 milliamp-hours, which can charge smartwatches or phones, he says. The fibre device is only a few hundred microns in thickness, thinner than any previous attempts to produce batteries in fibre form.

“The beauty of our approach is that we can embed multiple devices in an individual fibre," says Lee, “unlike other approaches which need integration of multiple fibre devices.”

They demonstrated integration of LED and Li-ion battery in a single fibre and he believes more than three or four devices can be combined in such a small space in the future. “When we integrate these fibres containing multi-devices, the aggregate will advance the realisation of a compact fabric computer.”

In addition to individual one-dimensional fibres, which can be woven to produce two-dimensional fabrics, the material can also be used in 3D printing or custom-shape systems to create solid objects, such as casings that could provide both the structure of a device and its power source.

Improve efficiency

To demonstrate this capability, a toy submarine was wrapped with the battery fibre to provide it with power. Incorporating the power source into the structure of such devices could lower the overall weight and so improve the efficiency and range they can achieve.

“This is the first 3D printing of a fibre battery device,” says Khudiyev. “If you want to make complex objects” through 3D printing that incorporate a battery device, he says, this is the first system that can achieve that. “After printing, you do not need to add anything else, because everything is already inside the fibre, all the metals, all the active materials. It’s just a one-step printing. That’s a first.”

That means that now, he says, “computational units can be put inside everyday objects, including Li-Fi”.

The team has already applied for a patent on the process and continues to develop further improvements in power capacity and variations on the materials used to improve efficiency. Khudiyev says such fibre batteries could be ready for use in commercial products within a few years.

“The shape flexibility of the new battery cell allows designs and applications that have not been possible before,” says Martin Winter, a professor of physical chemistry at the University of Munster in Germany, who was not involved in this work. Calling this work “very creative”, he adds, “as most academic works on batteries look now at grid storage and electric vehicles, this is a wonderful deviation from mainstream”.