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Monday, 23 September 2019
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Raw materials & technologies, Science today - coatings tomorrow

Painting turns any surface into a lithium-ion battery

Thursday, 23 August 2012

Sounds easy to do: Spray the layers of the paint and create a battery or a solar cell. Not depending on the surface. Thats what researchers developed now.

Rice University graduate student Charudatta Galande, Professor Pulickel Ajayan and graduate student Neelam Singh show off the first test device for their paintable batteries, an array of standard ceramic tiles combined with a solar cell and an array of LEDs, which the batteries powered for six hours.

Source: Jeff Fitlow/Rice University
Rice University graduate student Charudatta Galande, Professor Pulickel Ajayan and graduate student Neelam Singh show off the first test device for...

Li-ion batteries power most of our portable electronics by virtue of their high energy and power density. Commercial Li-ion batteries are multilayer devices, fabricated by tightly rolling up sandwiched battery components and packaging them into metal canisters.

Lithium Cobalt and Titanium Oxide used as electrodes

Fabrication of batteries by spray painting requires formulation of component materials into liquid dispersions, which can be sequentially coated on substrates to achieve the multilayer battery configuration. Commercial Li-ion batteries have positive and negative electrode materials coated on appropriate current collectors, sandwiching an ion conducting separator.

Aluminum and copper foils are commonly used current collectors (CC) (positive and negative CC respectively), while electrode materials and separators are chosen based on desired voltage, current capacity, operating temperature and safety considerations. Neelam Singh and her team from Rice University chose Lithium Cobalt Oxide [LiCoO2] (LCO, positive electrode) and Lithium Titanium Oxide [Li4Ti5O12] (LTO, negative electrode), for which the effective cell voltage is ∼2.5 V.

Nanotubes dispersable without surfactants

Single-walled nanotube (SWNT) current collectors have been used in batteries due to their high electrical conductivity and electrochemical stability at potentials above 1 V vs. Li/Li+. The researchers found that high concentrations (∼0.5–1% w/v) of SWNTs can be readily dispersed without the use of surfactantsor polymeric bindersn by bath ultrasonication in 1-methyl-2-pyrrolidone (NMP) to form viscous, highly consistent inks suitable for spray painting.

The experts chose NMP due to its ability to solvate pristine SWNTs without altering its electronic properties or requiring any post-treatments such as surfactant removal. A 20 % w/w of Super PTM (SP) conductive carbon additive lowers the sheet resistance of the spray-painted SWNT films (∼2 mg/cm2), sufficient for use as current collectors.

Ultrafine graphite into PVDF also included in the paint

LCO paint was made by adding a mixture of LCO, SP carbon and ultrafine graphite (UFG) into Polyvinylidine fluoride (PVDF) binder solution in NMP. Spray painted electrodes with only SP carbon as conductive additive had poor capacity retention, possibly due to inhomogeneous distribution of the small SP carbon particles (∼50 nm) in films composed of far larger LCO particles (7–10 µm). Addition of UFG (particle size ≤5 µm, comparable to LCO) gave more homogeneous distribution of conducting pathways, improving capacity retention.

Bathroom tiles used in the first experiments

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.

A video gives shows the development about Rice’s paintable batteries.

Layer by layer to become a battery

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 whole development was published at nature.com

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