The rechargeable battery created in the lab of Rice materials scientist Pulickel Ajayan consists of spray-painted layers, each representing the components in a traditional battery. The research appears today in Nature’s online, open-access journal Scientific Reports. Technique could turn any surface into a lithium-ion battery; may be combined with solar cells.
Paintable Battery concept – (a) Simplified view of a conventional Li-ion battery, a multilayer device assembled by tightly wound ‘jellyroll’ sandwich of anode-separator-cathode layers. (b) Direct fabrication of Li-ion battery on the surface of interest by sequentiall
“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.”
They formulated, mixed and tested 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.
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 volts.
The researchers reported that the hand-painted batteries were remarkably consistent in their capacities, within plus or minus 10 percent 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.
“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.
The 9 fabricated cells were connected in parallel to store a total energy of about 0.65 Wh, equivalent to 6 Wh/m2 (about 80 lego units). This ‘lego unit’ system can also be subsequently integrated with other devices. For example, an array of cheap polycrystalline silicon solar cells was glued on top of one of the finished cells and connected to it through a current limiter circuit. This cell was simply charged by illuminating with white light, while the rest eight cells were charged using a galvanostat. The fully charged battery pack (9 parallel cells) delivered enough energy to power 40 red LEDs for more than 6 h (at about 40 mA) and could be easily reconfigured to supply different voltages and current capacities. Such ‘lego’ cells, when combined with solar cells, could be used to convert any outdoor surface to an energy conversion-storage device.
In summary, battery materials can be engineered into paint formulations and simple spray painting techniques can be used to fabricate batteries directly on surfaces of various materials and of different shapes. We also integrated a photovoltaic panel with this technology to demonstrate energy capture-storage hybrid devices, which could be integrated into large outdoor surfaces and objects of daily use without constraints of space or form factor. The technique could be applied to virtually any multilayer energy storage or conversion devices such as supercapacitors or paintable solar cells. In the present work, we targeted Li-ion batteries due to their high energy and power density. However, fabrication and conditioning of Li-ion batteries requires use of toxic, flammable and potentially corrosive liquid electrolytes and an oxygen and moisture-free environment. Applying our technique, in its present form, to build Li-ion batteries directly on outdoor objects could be costly due these stringent requirements. Battery components (electrolyte and electrode) that are less sensitive to moisture and oxygen along with development of moisture and oxygen barrier paints would enable assembly of batteries without use of dry rooms, possibly even by non-specialists, enabling widespread renewable energy capture, storage and utilization.
SolarWindow™ technology utilizes an organic solar array composed of a series of ultra-small solar cells measuring less than ¼ the size of a grain of rice each. They are fabricated using environmentally-friendly hydrogen-carbon based materials, and successfully produce electricity, as demonstrated in a published peer-reviewed study in the Journal of Renewable and Sustainable Energy of the American Institute of Physics.
New Energy Technologies, a solar energy startup here in the US, has developed a technique to manufacture “spray-on” photovoltaic windows. The technique should ramp up production speed and bring down costs.
First of all, what’s meant by a spray-on window? New Energy Technologies gives a good run-down of the product, which they call SolarWindow, on their site. The tech uses an organic solar array made up of extremely small solar cells–they measure about a quarter of the size of a grain of rice.
The Christian Science Monitor, in a story back on Earth Day, further explains that NET developed plastic polymers that, when sprayed on a window, would produce electricity. The stuff is so effective as to harvest light even from northern exposure, and indeed even from indoor fluorescent lighting. “It will generate electricity even in low light conditions,” John Conklin, NET’s CEO, told the Monitor. NET teamed up with the National Renewable Energy Laboratory and the University of Florida to develop the tech.
The Engineer, which reported on the manufacturing breakthrough, says that the film can be sprayed on in an ultra thin, sub-micrometer layer. The breakthrough is important for American industry, because as the Monitor pointed out in April, while American firms experimenting in spray-on solar had the technical edge, Chinese companies were so far able to produce the stuff more cheaply.
And spray-on solar is more than just an eye-catching innovation. It’s potentially a revolution in solar power, a move away from the traditional rooftop solar array. “It puts energy harvesting everywhere,” said Ken McCauley of Konarka, an NET competitor, to the Monitor.
Everywhere, that is, assuming the cost of production could come down. The traditional method to make spray-on solar panels was something called vacuum deposition, which was time-consuming and expensive. But NET found a way to do what the Engineer calls “high-speed roll-to-roll and sheet-to-sheet manufacturing,” and it made the process possible at low temperatures and at ambient pressure.