2 July 2012
Imagine spray painting the side of your house and it not only produces power from the sun, but can store the energy for later as well. A novel approach to battery design from Rice University researchers could enable that and other types of spray-on batteries.
The research, published last week in Nature, seeks a new approach to battery fabrication by using materials that can be spray-painted onto various surfaces. Combined with flexible printed circuits and research in spray-on solar cells, the technique offers the prospect of turning common objects into smart devices with computing power and storage. Another possibility is consumer electronics, such as cell phones or cameras, with a battery coating.
Conventional lithium-ion batteries are made with multiple layers of different components, including the anode, separator, cathode, and metal foils to collect charge. This “jelly roll” structure means that lithium ion batteries are limited to cylindrical and rectangular, or prismatic, shaped cells.
The Rice group focused on using liquid components paintable with an air brush. In one experiment, the five traditional battery components were applied as layers onto nine ceramic bathroom tiles and other household objects, including a beer mug, for testing. They found the technique could be used on ceramics, stainless steel, and flexible polymers.
The bathroom tile batteries were wired in parallel and connected to solar photovoltaic panel to light an LED light for six hours. The batteries supplied a steady 2.4 volts.
The core science behind the effort was finding a suitable liquid composition for the different battery components. Lithium ion batteries often use an aluminum foil to collect positive charge but conductive aluminum micro powders pose health concerns if sprayed, so the group used single-walled nanotubes to collect current.
The group encountered fabrication issues as well, where the different layers peeled apart after spraying. For a separator, the researchers settled on a polymer paint made up of a mix of resin, Plexiglas, and silicon dioxide in a solvent. “The hardest part was achieving mechanical stability, and the separator played a critical role,” said Neelam Singh who was lead author of the paper, in a statement. The final thickness was about 200 microns, or the thickness of a few strands of hair.
Singh describes the work as a new concept in batteries that frees designers from the constraints of today’s shapes. “Now we have the flexibility in the choice of substrate and shapes of batteries. We can make printable batteries on practically any material,” she said in a video released by professor Pulickel Ajayan’s lab at Rice.
The performance of the batteries is comparable with today’s lithium-ion batteries and the capacity of experimental batteries was close to predictions, which suggests that the process could be adapted for hand-held spray cans, according to Rice.
But the paper points out one significant limitation to the idea of buying spray cans of lithium cobalt oxide at the hardware store for a do-it-yourself battery. Lithium ion battery components are sensitive to air and moisture which means producing spray-on batteries directly on outdoor without costly equipment would be dangerous.
Still, because the spray-on technique could be used with industrial spray guns and doesn’t require exotic materials, the research could conceivably be commercialized if a useful application can be found. In addition to improving on basic battery performance—charge cycles, capacity, safety—the costs need to be estimated as well.