Scientists at the Fudan University in Shanghai, China, have developed a high-performance Li-ion battery made of carbon nanotube fiber yarns. Roughly one 1 mm in diameter, the fiber shaped lithium-ion batteries are reported lightweight enough to create weavable and wearable textile batteries that could power various devices. The researchers say that the yarn is capable of delivering nearly 71 mAh/g of power, and can also be woven into existing textiles to create novel electronic fabrics.

To make the fiber batteries, the team had to develop functional cathode and anode composite yarns. Lithium manganate (LMO) particles were deposited on a carbon nanotube (CNT) sheet and scrolled up to create a CNT-LMO composite yarn which functions as the cathode. The anode composite yarns were made by sandwiching a CNT sheet between two silicon-coated CNT sheets and scrolling them up. When the two yarns, which are separated by a gel electrolyte for safety, are wound, it results in a CNT-based fiber shaped Li-ion battery (LIB).

"The two yarns can be twisted directly to form a battery, or can also be wound onto any commercial polymer fiber as they both have small diameters and good flexibility," Dr Wei Wang, the lead researcher, tells Gizmag. "The fiber battery can then be added into an existing textile or just woven into a textile directly."

Though researchers have successfully created wearable supercapacitors, creating Li-ion battery fibers has been a challenge, the Fudan team reports, even though they're more favorable for mobile electronics than supercapacitors, since they possess lower self-discharge losses and higher energy densities. The difficulty until now has been a matter of finding appropriate workable materials and designing the necessary composite structures.

For instance, one of the biggest issues in designing LIB battery fibers is said to be dealing with the silicon expansion that happens in the fibers, when they charge and discharge power. The silicon expands in volume up to 300 percent, causing the silicon layer to peel away when used, damaging the battery fiber.

To overcome this issue, the team created a hybrid yarn structure incorporating CNTs between silicon-coated CNT sheets, which buffered the silicon's volume change and clamped it in place. The resulting LIB fiber, was found to be capable of adjusting to the silicon volume changes, thus preserving the integrity of its silicon layers. According to the researchers, the fiber LIBs have a linear energy density of 0.75 mWh cm-1, and can be woven into flexible textile batteries with an area energy density of 4.5 mWh cm-2, capable of delivering adequate levels of power.

"Based on the two yarns alone, we can achieve nearly 71 mAh/g of power," Weng tells us. "For a commercial Li-ion battery, you get about 50 mAh/g with the packaging. At this time, we can get about 6.8 Wh power in 1 ft by 1 ft (0.3 x 0.3 m), if the fiber batteries are woven into a textile. This is enough power to run a mobile phone for 24 hours."

With further development, the researchers say that they expect to get a 1 ft x 1 ft (0.3 x 0.3 m) section of the textile battery to store and deliver around 10-20 Wh. While it's possible to currently charge the LIB fiber with two connecting lines, the possibility of charging them wirelessly is also being investigated.

Other than improving the battery yarn's performance, the team is also looking into giving it more properties, such as making it stretchable, before going into production on a larger-scale. The current focus is making it safe enough to wear.

"Safety is the first important thing for wearable electronics," Weng explains. "Here, carbon nanotubes with good mechanical and electrical properties were used as the skeleton material. We are investigating the battery structure, the electrolyte and the packaging."

Ultimately, the team hopes to create flexible fiber-based power sources that can be easily woven into textiles, and to smoothly integrate with electronic devices that users carry.

"We want to create clothes and blankets to charge our daily electronics, e.g., mobile phones," Weng tells us. "We also see them as an emergency energy source when you hike or camp, for instance, because they are convenient to take by rolling-up. Or the textile batteries could directly be woven into your sleeping bag or tent."

A paper describing the research was recently published at the Journal Nanoletters.

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