MIT makes longest lithium battery fiber

Massachusetts Institute of Technology engineers have developed a rechargeable lithium-ion battery in the form of an ultra-long fiber that can be woven into fabrics and even washed. This latest innovation could enable a wide variety of wearable electronic devices and might even be used to make 3D-printed batteries in virtually any shape.

The team envisions 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 components.

In a proof of concept, the researchers behind this fibrous electrical storage technology have produced the world's longest flexible battery, 140 meters (459 feet) long, to demonstrate that the material can be manufactured to arbitrarily long lengths.

The work is described in the journal "Materials Today," with MIT postdoc Tural Khudiyev, former MIT postdoc Jung Tae Lee, and Benjamin Grena, who were lead authors on the paper, along with 10 others.

At present, researchers, including members of this discovery team, have demonstrated fibers that contain a wide variety of electronic components, including light-emitting diodes (LEDs), photosensors, communications, and digital systems.

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

The new fiber battery is manufactured using novel battery gels and a standard fiber-drawing system that starts with a larger cylinder containing all the factors necessary 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 also maintaining their original arrangement.

While many others have attempted to make batteries in fiber form, Khudiyev says, those were structured with key materials on the outside of the fiber. In contrast, this system embeds the lithium and other battery materials inside the fiber with a protective outer coating, thus directly making this version stable and waterproof.

This is the first demonstration of a sub-kilometer-long fiber battery which is both sufficiently long and highly durable to have practical applications, he said.

Furthermore, the fact that the research team was able to make a 140-meter fiber battery shows that "there's no obvious upper limit to the length. We could definitely do a kilometer-scale length," added Khudiyev.

A demonstration device using the new fiber battery incorporated a "Li-Fi" (light fidelity) communications system – one which pulses of light are used to transmit data and include 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 fiber, that means sensitive battery components already have a good sealing, and all the active materials are very well-integrated, so they don't change their position," Khudiyev said referring to the drawing process.

In addition, the resulting fiber battery is much thinner and more flexible, yielding an aspect ratio – the length-to-width fraction – up to a million, which is beyond other designs. This makes it advantageous and capable of using standard weaving equipment to create fabrics that incorporate the batteries, as well as electronic systems.

The 140-meter fiber battery has an energy capacity of 123 milliamp-hours, which is capable of charging smartwatches or phones. Additionally, the fiber device is only a few hundred microns in thickness, thinner than any previous attempts to produce batteries in fiber form.

"The beauty of our approach is that we can embed multiple devices in an individual fiber," said Lee. "Unlike other approaches which need integration of multiple fiber devices."

The researchers demonstrated the integration of LED and a Li-ion battery in a single fiber, and it is believed more than three or four devices can be combined in such a small space in the future.

"When we integrate these fibers containing multi-devices, the aggregate will advance the realization of a compact fabric computer," finished Lee.

In addition to individual one-dimensional fibers, 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 casing that could provide both the structure of a device and its power source.

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

"This is the first 3D printing of a fiber battery device," said Khudiyev. "If you want to make complex objects" through 3D printing that incorporates a battery device, he added, 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 fiber, all the metals, all the active materials. It's just a one-step printing. That's a first."

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

"The shape flexibility of the new battery designs and applications that have not been possible before," said Martin Winter, a professor of physical chemistry at the University of Muenster, Germany.

Although not involved in the development, he stated the work is "very creative," adding: "As most academic works on batteries look not at grid storage and electric vehicles, this is a wonderful deviation from mainstream."