Researchers at Rice University have developed a lithium-ion battery that can be painted on virtually any surface. The spray-on power source consists of five spray-painted layers, each representing components in a traditional battery — two current collectors, a cathode, an anode, and a polymer separator in the middle.
Construction of a cell starts by spraying purified single-wall carbon nanotubes with carbon black particles onto a substrate to form a positive current collector. The cathode of the battery is sprayed as the second layer in a binder containing lithium-cobalt oxide, carbon, and ultrafine-graphite (UFG) powder. A polymer separator makes up the third layer using Kynar flex resin, poly(methyl methacrylate) or PMMA, and silicon dioxide. The fourth layer, the anode, is a mix of lithium-titanium oxide and UFG in a binder. The final layer is the negative current collector, a commercially available conductive copper paint that’s diluted with ethanol. Once painted, the cell is infused with electrolyte, heat-sealed, and then charged.
The polymer separator solves one of the major design problems: maintaining the mechanical stability of the cell. Unless the separator is stable, the nanotube and cathode layers would peel off the substrate when the cell was flexed.
In one experiment, nine of the lithium-ion cells sprayed onto bathroom tiles were connected in parallel. The cells were topped with a solar cell that converted power from a white light to charge the cells. When fully charged, the battery provided a steady 2.4-V output for 6 hr that powered a set of LEDs arranged to spell out “RICE.”
Combining paintable batteries with the recently reported paintable solar cells might create an energy-harvesting combination that would be hard to beat. Mass production should scale easily as spray painting is already an industrial process.
Researchers reported that the hand-painted batteries were remarkably consistent in their capacities, within ±10% of the target. The cells were subjected to 60 charge-discharge cycles where they displayed just a small drop in capacity. And “batteries” were airbrushed onto ceramic bathroom tiles, flexible polymers, glass, stainless steel, and even a beer stein to see how well they would bond with different substrate.