With the launch of Google Glass and the Samsung Galaxy Gear wristwatch this year, wearable electronics have moved from abstract concepts to tangible products. To integrate these electronic devices seamlessly into clothing, watchbands, and backpacks, some engineers are developing flexible, powerful textile-based batteries. Now researchers in South Korea have built one of the most durable wearable batteries to date on polyester fabric (Nano Lett. 2013, DOI: 10.1021/nl403860k). The battery, which the researchers sewed into a shirt, can be folded 10,000 times without losing function.
Most attempts to make textile batteries have had limited success, says materials scientist Jang Wook Choi of the Korea Advanced Institute of Science and Technology (KAIST). The problem has been finding battery materials that can retain high function while being bent repeatedly. For example, batteries with metal foils as electrodes can bend only a few times before breaking. Electrodes made by dipping cloth in nanoparticle inks, such as solutions of carbon nanotubes, are more durable than the foils, but the electrical resistance of these cloth electrodes is relatively high, which limits the size of the batteries and the total amount of energy they can store.
To solve these challenges, Choi rethought the entire design of textile batteries, starting with the electrode. He turned to nickel, because it is a fantastic conductor. To make a flexible, but still highly conductive metal electrode, Choi came up with the idea of electroplating nickel onto polyester fabric. The process is simple, and the nickel-coated textile retains the mechanical properties of the fabric. The electrodes had a very low electrical resistance, about 0.35 ohms per square, comparable to that of a pure nickel metal foil.
The other critical component is the polymer used to bind the anode and cathode materials onto the electrodes in the battery. If this binder material fails, the battery will peel apart and stop functioning. Choi found that polyurethane had the right mechanical properties. To complete the battery, Choi’s group used conventional lithium-ion battery materials for the anodes and cathodes.
Choi’s group put the polyester-based batteries through their paces. Other groups have demonstrated bending and flexing of batteries, but the KAIST team thought the real test of mechanical durability would be to fold the device with firm creases. They powered an array of light-emitting diodes with the battery and folded it repeatedly. After 10,000 folding and unfolding cycles, the textile battery still worked. Batteries built with aluminum foil electrodes broke after three cycles and stopped working altogether after 100 cycles.
The KAIST group showed that their textile batteries can be sewn into a sweatshirt and a watchband. They also integrated the batteries with flexible solar cells so the batteries could recharge without needing to be removed from the clothing. “It’s quite comfortable to wear,” Choi says, adding that the battery is sealed so people could wash the fabric with the battery still attached.
“I’m really impressed,” says Yi Cui, a battery researcher at Stanford University. The KAIST group has successfully put their batteries through much harsher mechanical tests than others have been able to, he says.
The next step, Cui says, is to use battery materials that can store more energy to further improve the performance. So far, the KAIST team has used lithium iron phosphate for the cathode and lithium titanium oxide for the anode. Cui says that using a carbon anode material in the textile battery would increase the battery’s voltage, which determines how much power the device can deliver and how fast it can recharge. The voltage of the textile battery is about 2.5 V, and Choi says it should be about 3.8 V for practical applications.
Indeed, Choi’s group is experimenting with other materials, in collaboration with an unnamed South Korean battery maker that is interested in scaling up production of the wearable batteries.