Lithium ion batteries that can be painted onto any surface

A lithium-ion battery that can be painted onto virtually any surface has been developed at Rice University. The rechargeable battery consists of spray-painted layers, each representing the components in a traditional battery. Lead author Neelam Singh, a graduate student, and her team experimented with the formulation, mixing and testing of paints for each of the five layered components- namely two current collectors, a cathode, an anode and a polymer separator in the middle. The first layer, 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,’ said Singh. ‘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. Now researchers have succeeded in painting these batteries onto a diverse range of surfaces, converting almost any object to a battery. 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 the house current, the batteries alone powered a set of light-emitting diodes that spelled out RICE for six hours. One of these battery tiles was topped with a solar power cell that helped charge the batteries, suggesting the researchers could give any surface the capability to both harvest and store energy. The researchers reported that the hand-painted batteries were consistent in their capacities, within ±10% of the target. They were also put through 60 charge-discharge cycles with only a very small drop in capacity. The batteries were easily charged with a small solar cell. The team foresees the possibility of integrating paintable batteries with recently reported paintable solar cells to create an energy-harvesting combination. Until now, Lithium-ion batteries power most portable electronics nowadays, but their spiral, jelly-roll-like design generally limits them to rectangular or cylindrical shapes. With this development, the rechargeable lithium-ion batteries now found in many mobile phones and laptops may one day be sprayed like paint onto virtually any surface, potentially ushering in a new generation of thin, flexible devices. Unlike existing batteries, the paintable version does not require an extra compartment for storage, and could thus be more easily integrated into existing designs for battery-powered devices. It also opens up exciting possibilities for solar-power generation and storage. The researchers have filed for a patent on the technique, which they continue to refine.

Research scientist Andreas Mershin and his team at MIT's Center for Bits and Atoms, along with University of Tennessee biochemist Barry Bruce, have worked out a process that extracts functional photosynthetic molecules from common yard and agricultural waste. If all goes well, in a few years it should be possible to gather up a pile of grass clippings, mix it with a blend of cheap chemicals, paint it on your roof and begin producing electricity. Inspired by how light is harvested by densely-packed trees, Mershin and his team have fabricated tiny forests of zinc oxide nanowires interspersed with titanium dioxide "sponges". When this mini array is then coated with the photosynthetic molecule extract, the electricity produced can actually be harvested to do work. The major drawback currently is the low 0.1% efficiency of the experimental cells, but the ongoing flurry of research into biophotovoltaics is expected to boost that to commercially viable levels (at least 1-2%) fairly soon. One of the team's biggest hurdles was figuring out how to keep the light-gathering molecules functional outside the cellular environment. Eventually, they managed to stabilize the chlorophyll-rich plant extract, also known as Photosystem I, with specially designed surfactant peptides, but then had to contend with the fact that some of its components are susceptible to damage by UV light. Luckily, both zinc oxide and titanium dioxide absorb UV and so afford protection as well as scaffolding for the delicate light-harvesting mixture.