Flexible electronics of the future, such as wearable computers or displays that can roll into a tube, will need flexible power sources. Unfortunately, researchers developing such batteries have had a limited toolkit of electrode materials. This limitation has meant that today’s bendy batteries have low energy storage capacities. Now researchers have devised a way to use a high-quality battery material to make a flexible power source that can run a display (Nano Lett., DOI: 10.1021/nl302254v).
When researchers build a flexible battery, they assemble the electrode materials on a bendable substrate such as plastic, paper, or textile. But these substrates are temperature sensitive. As a result, they don’t work with the high-temperature processes required to build electrodes using the high-quality inorganic materials found in conventional, rigid batteries. For example, manufacturers anneal electrodes made of lithium cobalt oxide, a material commonly found in the lithium-ion batteries for portable electronics, at 700 ºC, a temperature that would melt a flexible substrate.
Keon Jae Lee, professor of materials science at Korea Advanced Institute of Science and Technology and his colleagues wanted to build a battery from high performance materials found in conventional rigid batteries, and then apply it to a flexible substrate.
Their solution was to build the battery on mica, which can tolerate high temperatures. The annealing process that they used to produce a high-quality lithium cobalt oxide cathode also helped separate it from the mica. After annealing the cathode, they completed the battery with a solid electrolyte and a lithium-metal anode. The researchers could remove the battery from the mica using tape.
To create a flexible substrate for the battery, Lee and his colleagues then encapsulated the peeled-off battery in flexible sheets of polydimethylsiloxane (PDMS). Their batteries were about 10 µm thick.
When the researchers bent the device, the resulting strain built up in the squishy PDMS, not in the battery itself. The batteries stored the most energy ever reported for a flexible battery, with an energy density of 2,200 µWh/cm3. The researchers also compared the performance of the thin-film battery to that of a similar battery on an inflexible substrate. The bendable battery discharged power at 4.2 V, a value comparable to the power produced by the rigid one. Finally, Lee’s group used the batteries to power a flexible organic light-emitting diode display. Lee says his work is the first to demonstrate a flexible display with a flexible power source.
Yi Cui, a materials scientist at Stanford University, says these flexible batteries show potential. However, he says, batteries for portable electronics such as cell phones store more energy than Lee’s flexible batteries can so far. One way to increase energy storage, Cui says, would be to make the batteries thicker. But, he notes, “If it’s too thick, it won’t be flexible anymore.”
Lee’s group is working on ways to make their batteries more energy dense, including stacking thin-film cells in a way that will maintain their flexibility.
They’re also developing a one-step process using laser pulses to lift the batteries from the mica substrate. Lee says that compared to the tape method, the laser technique will be more practical for large-scale fabrication.