Using liquid versions of the same components found in conventional lithium-ion batteries, the team airbrushed their invention, in several layers, on to a glass slide, a stainless steel sheet, glazed ceramic tiles, and the curved surface of a mug.
"Basically, using this approach, we can convert any object or surface to a battery," said lead author Neelam Singh, an engineering student at Rice University in Texas.
Lithium-ion (or Li-ion) batteries work by transferring a charge between a negative and positive electrode. Compared to other rechargeable batteries, they are light, have a high power output and storage capacity, and are safer.
The team developed liquid, paintable versions of the five layered components — two electrodes, a cathode, an anode and a polymer separator.
"We first converted all the components of the battery into paints. We could then use these paints to literally paint batteries on any surface and using nothing but just a spraygun," said Singh.
Singh says her team’s work is filling a need in the socially critical field of energy storage for new battery designs. "We find that the focus of research is now shifting towards integration of batteries," she says. That is, people are trying to design batteries that can be built into a variety of different objects. Several teams are working to make thin and flexible batteries as well as batteries that can be incorporated into textiles. Solar energy is one of the applications that researchers are particularly interested in. Solar panels can require large surface areas, and the Rice team’s design is an efficient way to collect and store energy in this realm.
To test their design, they applied the battery paints onto ceramic bathroom tiles, glass, a flexible transparency film, stainless steel and the side of a beer stein. In each case, the battery worked. In one experiment, they hooked a solar cell to one of the batteries and powered an LED display.
Singh said the biggest challenge was to make a battery that was both stable and powerful: "It was not very easy to get all the layers on top of each other without interfering with their capacity or compromising the performance of the battery." There were safety concerns as well. Many lithium ion batteries use aluminum as a positive current collector, but aluminum microparticles can be lung irritants, so using them in aerosol paint would be hazardous. Instead, the researchers relied on carbon nanotubes.
Lithium cobalt oxide was used as the cathode, commercially available gel electrolytes as the separator, lithium titanium oxide as the anode, and copper as the negative current collector.
Singh thinks that the Rice team’s battery is a game-changer because it is energy efficient for its volume and can be applied to objects of many different compositions and shapes.
Vilas Pol, a materials scientist at Argonne National Laboratory who was not affiliated with the study, agrees that the new design is exciting, describing it as "an exceptional and notable concept in the arena of battery design and integration."
But for now paint-on batteries are not quite ready to hit the shelves at your local hardware store. For one, the electrolyte separator layer is not yet oxygen stable. It would explode if it came into contact with air, so special conditions are necessary when creating the battery.
Singh says the team currently is working to make all the materials less reactive to air and moisture and more environmentally friendly. She adds that other groups are working on developing paint-on solar cells. Then, she envisions "paintable solar cells on top of paintable solar batteries." Houses could become solar-energy capture-and-storage devices.