Scientists at Rice University have managed to design a lithium-ion battery that can be painted on virtually any surface.
Created in the lab of Rice materials scientist Pulickel Ajayan, the rechargeable battery consists of spray-painted layers, each representing components of a traditional battery.
"This means traditional packaging for batteries has given way to a much more flexible approach that allows all kinds of new design and integration possibilities for storage devices," explained Ajayan.
"There has been a lot of interest in recent times in creating power sources with an improved form factor, and this is a big step forward in that direction."
Together with Ajayan, Rice graduate student Neelam Singh and her team spent numerous hours formulating, mixing and testing paints for each of the five layered components - two current collectors, a cathode, an anode and a polymer separator in the middle.
The materials were subsequently airbrushed onto ceramic bathroom tiles, flexible polymers, glass, stainless steel and even a beer stein to see how well they would bond with each substrate.
In the first experiment, nine bathroom tile-based batteries were connected in parallel. One was topped with a solar cell that converted power from a white laboratory light. When fully charged by both the solar panel and house current, the batteries alone powered a set of light-emitting diodes that spelled out "RICE" for six hours, while the batteries provided a steady 2.4 volts.
The researchers confirmed the hand-painted batteries were quite consistent in their capacities, within plus or minus 10 percent of the target. They were also put through 60 charge-discharge cycles with only a very small drop in capacity.
According to Singh, each layer of the battery can basically be described as an optimized stew. The first, the positive current collector, is a mixture of purified single-wall carbon nanotubes with carbon black particles dispersed in N-methylpyrrolidone. The second is the cathode, which contains lithium cobalt oxide, carbon and ultrafine graphite (UFG) powder in a binder solution.
The third is the polymer separator paint of Kynar Flex resin, PMMA and silicon dioxide dispersed in a solvent mixture. The fourth, the anode, is a mixture of lithium titanium oxide and UFG in a binder, and the final layer is the negative current collector, a commercially available conductive copper paint, diluted with ethanol.
"The hardest part was achieving mechanical stability, and the separator played a critical role," said Singh. "We found that the nanotube and the cathode layers were sticking very well, but if the separator was not mechanically stable, they would peel off the substrate. Adding PMMA gave the right adhesion to the separator."
Once painted, the tiles and other items were infused with the electrolyte and then heat-sealed and charged with a small solar cell. Singh says she foresees the possibility of integrating paintable batteries with recently reported paintable solar cells to create an energy-harvesting combination that would be hard to beat.
As good as the hand-painted batteries are, scaling up with modern methods will significantly improve them.
"Spray painting is already an industrial process, so it would be very easy to incorporate this into industry... We really do consider this a paradigm changer," she added.