click to enlarge An electron microscope image of a spray-painted lithium-ion battery developed at Rice University shows its five-layer structure. Image: Ajayan Lab/Rice University

Researchers at Rice
University have developed
a lithium-ion battery that can be painted on virtually any surface.

The rechargeable battery created in the laboratory of Rice
materials scientist Pulickel Ajayan consists of spray-painted layers, each
representing the components in a traditional battery. The research appears
today in Scientific Reports.

“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,” said Ajayan, Rice’s Benjamin M.
and Mary Greenwood Anderson Professor in Mechanical Engineering and Materials
Science and of chemistry. “There has been 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.”

Lead author Neelam Singh, a Rice graduate student, and her
team spent painstaking 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 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.

click to enlarge Conventional lithium-ion batteries wrap active layers into a canister or other portable container. But Rice University researchers have found a way to paint those layers onto any surface, which opens up the possibility of turning those surfaces into storage devices. Image: Neelam Singh/Rice University

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 2.4 V.

The researchers reported that the hand-painted batteries
were remarkably consistent in their capacities, within plus or minus 10% of the
target. They were also put through 60 charge-discharge cycles with only a very
small drop in capacity, Singh said.

Each layer is 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.

click to enlarge Rice University graduate student Charudatta Galande wires an array of LEDs to a set of batteries painted on the surface of ceramic tiles. A team of Rice researchers has invented a form of paintable lithium-ion battery that can be applied to virtually any surface. Image: Jeff Fitlow/Rice University

“The hardest part was achieving mechanical stability, and
the separator played a critical role,” Singh said. “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.

Singh said the batteries were easily charged with a small
solar cell. 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, she said, scaling up with modern methods will improve them by leaps and
bounds. “Spray painting is already an industrial process, so it would be very
easy to incorporate this into industry,” Singh said.

The Rice researchers have filed for a patent on the
technique, which they will continue to refine. Singh said they are actively
looking for electrolytes that would make it easier to create painted batteries
in the open air, and they also envision their batteries as snap-together tiles
that can be configured in any number of ways.

“We really do consider this a paradigm changer,” she said.

Source: Rice University