Researchers at the Korea Advanced Institute of Science and Technology (KAIST) have developed a promising solid state, thin-film lithium-ion battery that claims the highest energy density ever achieved for a flexible battery. The new design, which showed for the first time that high-performance thin films can be used for flexible batteries, may be commercialized as early as next year.
Lithium-ion batteries are a strong candidate for powering the flexible electronics of the future. A high-performance lithium-ion flexible battery would be a giant step toward fully-fledged flexible electronics systems and would open the door to flexible e-paper, wearable devices, and better piezoelectric systems that harvest energy from mechanical forces.
Research is progressing, but seems to have hit an invisible – though very real – performance wall. This is because most designs employ either low-performance flexible organic materials, or polymer binders that take up too much space and decrease the battery's power density. In addition, the cathodes have to be treated at high temperatures to improve performance, but this can't be done effectively on substrates made of flexible polymers.
The new approach developed at KAIST uses high energy density inorganic thin films that can be treated at high temperatures, resulting in the highest-performance flexible lithium-ion batteries yet. "There is no performance difference in energy density, capacity, and cycle life between our flexible battery and bulk batteries," Prof. Keon Jae Lee, who led the research efforts, told Gizmag. "On the contrary, performance is improved by about 10 percent because of the stress release effect."
The batteries are built by sequentially depositing several layers – a current collector, a cathode, an electrolyte, an anode, and a protective layer – on a brittle substrate made of mica. Then, the mica is manually delaminated using adhesive tape, and the battery is enclosed between two polymer sheets to improve mechanical resistance.
Bending the battery affects performance, but not to disastrous levels. With the battery constantly bent at a radius of sixteen millimeters (about the same curvature of a fifty-cent coin) the discharge capacity drops by about seven percent after 100 charge-discharge cycles, compared to a three percent drop when the battery is not bent. Voltage was shown to remain almost constant, dropping by a very modest 0.02 V after the battery was bent and released 20,000 times.
"The technology for commercializing this battery could come in a relatively short time, about a year," says Prof. Lee. But first, the researchers need to find a better, automated way to delaminate the mica substrate – the manual method, involving adhesive tape, is very unpractical and can take up to ten minutes per battery.
"We are investigating a laser lift-off [delamination] process to facilitate mass production of large area flexible lithium-ion batteriesr" says Lee. "Its feasibility is already proven and will be reported in a later paper."
The team is also interested in stacking the structures on top of each other to improve charge density.
A paper describing the battery was recently published on the journal Nano Letters. The video below illustrates the voltage performance of the batteries under mechanical stress.
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