Paintable batteries offer potential form-factor solution

Rice University researchers have developed a lithium-ion battery that can be painted on any surface. The rechargeable battery was created in the lab of Rice materials scientist Pulickel Ajayan. It consists of spray-painted layers, each representing a particular component in of a regular battery.

Ajayan states that traditional packaging for batteries have given way to a much more flexible approach that allows all kinds of new design and integration potential in storage devices. There has been a growing demand for the creation of power sources that possessed an improved form factor and the findings of this study should allow future development of this goal.

A team led by Rice graduate student Neelam Singh spent 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. These materials were then 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; the batteries provided a steady 2.4V.

The researchers reported that the hand-painted batteries possessed remarkably consistent capacities. They were also put through 60 charge-discharge cycles with only a very small drop in capacity.

Layer breakdown of the painted battery

Each layer is an optimized mix. The first layer is the positive current collector. It is a mixture of purified single-wall carbon nanotubes with carbon black particles dispersed in N-methylpyrrolidone. The second layer is the cathode. It contained lithium cobalt oxide, carbon and ultrafine graphite (UFG) powder in a binder solution. The third layer is the polymer separator paint of Kynar Flex resin, Poly-methyl methacrylate (PMMA) and silicon dioxide dispersed in a solvent mixture. The fourth layer is the anode. It is a mixture of lithium titanium oxide and UFG in a binder. The final layer is the negative current collector, which was made with a commercially available conductive copper paint that was diluted with ethanol.

Mechanical stability issue solved
The study did face some challenges, particularly achieving mechanical stability with the separator. Initially, the team found that the nanotube and the cathode layers were sticking very well but without mechanical stability in the separator, the two components would peel off the substrate. Adding PMMA provided the right adhesion to the separator. Once painted, the tiles and other items were infused with the electrolyte and then heat-sealed and charged.

Singh added that the batteries were easily charged with a small solar cell. She foresaw the possibility of integrating paintable batteries with recently reported paintable solar cells to create an energy-harvesting combination that would be hard to beat.

The researchers have already filed a patent for the newly developed technique, which they plan on further refining. Singh said they are actively looking for electrolytes that would make it easier to create painted batteries in the open air. They also believe that in the future, their batteries can be made into snap-together tiles which can be configured in a number of ways. Findings of the research are currently available in Nature's online.