Researchers at Rice University have created an ultra-thin, high-performance flexible battery that is lithium-free, only a hundredth of an inch thick, and also doubles as a supercapacitor. The technology could find use in mobile and wearable electronics such as smartphones and fitness bands.

Smartwatches haven't been around for very long, and yet it is already clear that one of the most limiting factors to their performance (and, ultimately, their usefulness) is battery life. Batteries are generally rigid, bulky and heavy, which is exactly what you don't want to feel on your wrist all day long.

Flexible batteries are an attractive prospect that would lead to better wearable gadgets for consumers. We've seen examples before, but their performance hasn't been up to par when compared to their Li-ion counterparts. Now, researchers at Rice University have come up with a technology that looks very promising.

Rice chemist James Tour and colleagues created high-performance electrodes by etching a 900 nanometer-thick layer of nickel fluoride with regularly spaced holes that were only five nanometers in diameter, increasing surface area for added energy storage. They then enclosed the electrodes in an electrolyte made of potassium hydroxide in polyvinyl alcohol.

According to the scientists, the device behaves like a battery, even though it is structured as a supercapacitor. Indeed, it can be used as both; it can be charged and discharged quickly, like a supercapacitor, or it can be charged with a lower current rate, in which case it will also discharge slowly, behaving like a battery.

The device is only a hundredth of an inch thick and it was found to hold 76 percent of its original capacity after 10,000 charge-discharge cycles and 1,000 bending cycles. Energy density was measured at 384 Wh/kg, and power density at 112 kW/kg.

This technology could be used in wearable, flexible electronics such as the next generation of smartwatches and fitness bands. The scientists say they are already in talks with companies interested in large-scale production, which they say could make the battery even thinner, and also scale it up by either increasing its size or stacking layers on top of one another.

The advance is described in the latest issue of the Journal of the American Chemical Society.

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