Bright Sparks

Paintable Battery Technology

Some years ago I travelled to Johns Hopkins University, not far from Boston in the US, to interview an English professor at the Department of Materials Science and Engineering who was leading research into polymer batteries. The concept was to try to find an energy source that would store and distribute electrical charge without the need for the conventional materials that add weight and pollution. As the subsequent report showed, elimination of any metallic components or liquids and the lightweight and flexible construction provided a unique alternative for secondary battery technology.

I had in mind applications such as motor vehicles. A normal car’s battery is a big, heavy and (by today’s standards) extremely dirty piece of very old technology. One of the reasons for the eventual failure of the Wankel rotary car engine to gain a foothold in the 1970s was its poor fuel consumption, but another reason was that while it was wonderfully space-efficient – only a quarter the dimensions of a normal internal combustion engine – all its ancillaries were of normal size. The battery weighed almost as much as the engine and designers of an innovative bent just gave up.

I envisaged things like a sedan’s interior headlining being made of polymer batteries instead of plastic (today, many headliners are made from processed hemp, believe it or not, and I did wonder about the legal niceties – fancy being stuck in a traffic jam and getting arrested for smoking car components…), the power storage being scattered about the vehicle in all sorts of wasteful areas (door pockets) and poorly shaped corners. It was technically possible and that’s one direction in which the team was heading, my professor told me.

Unfortunately the Johns Hopkins research was sponsored by the US Air Force and some large defence contractors such as Raytheon. It’s highly probable that the latest Stealth Fighter has a smokeable headliner, but they consistently refuse to answer my questions.

Which makes the latest battery power development all the more interesting because it is (as far as we can tell) neither defence-related nor top secret. At the Department of Mechanical Engineering and Materials Science of another US university, Rice, in Houston, a team has just published its findings on the subject of paintable batteries – literally spraying batteries onto the substrate of your choice. Now, from a very different direction, my 15-year-old dream has been revived. I can have my car painted in power.

Well, not quite. At least, not quite yet. But to be serious, paintable batteries have a very feasible future. The research appears in Nature’s online, open-access journal Scientific Reports and its summary reads: “If the components of a battery, including electrodes, separator, electrolyte and the current collectors, can be designed as paints and applied sequentially to build a complete battery, on any arbitrary surface, it would have significant impact on the design, implementation and integration of energy storage devices.”

The project looked at the “possibility of interconnected modular spray painted battery units to be coupled to energy conversion devices such as solar cells, with possibilities of building stand-alone energy capture-storage hybrid devices in different configurations.” The team established what they claim is a “paradigm change in battery assembly” by fabricating rechargeable Li-ion batteries solely by multi-step spray painting of its components on a variety of materials such as metals, glass, glazed ceramics and flexible polymer substrates.

The five basic components of a battery are reproduced as individual coats of paint and it was found that this needs to be applied in a set order; determining the order and the optimum composition of the layers was what took most of the painstaking research. 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, fourth is the anode, a mixture of lithium titanium oxide and UFG in a binder, and the ‘topcoat’ is the negative current collector, a commercially available conductive copper paint, diluted with ethanol.

The team hand-painted a variety of objects including bathroom tiles, flexible polymers, glass, stainless steel and even a ceramic beer glass. 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 delivered 2.4 volts and powered a set of LEDs that spelt out ‘RICE’ for six hours.

The hand-painted batteries were remarkably consistent in their capacities, within plus or minus 10 per cent of the target, according to the report’s lead author Neelam Singh, a postgraduate student at Rice. They were also put through 60 charge-discharge cycles with only a very small drop in capacity, she added. Don’t forget: these were experimental hand-painted ‘batteries’ with limited function and performance; as good as the hand-painted batteries were, points out Singh, they would be made much more efficient once a methodology has been agreed upon to produce them bigger and better. 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,” she said.

With a little experimentation it should be perfectly feasible to make and bring to mass production the combination of paintable batteries with paintable solar cells. If you don’t know about the latter, take heart, for the Australian National University was in the forefront of research into paintable solar cells back in 2006, while a variety of techniques have since been developed including colloidal quantum dot solar cells: tiny nanoscale semiconductors that can be placed in a liquid carrier that acts as a paint and can be sprayed on – say – the side of a building to give it most of the properties of a ‘conventional’ photovoltaic solar array. It may sound far-fetched but, within a few years, research such as this could well develop a coating for exterior building walls that combines weatherproofing with power generation and storage.

That kind of energy harvesting combination would be hard to beat. With or without the power generation aspect, the paintable battery gives designers ideas about form factors they have hitherto been unable to contemplate – one, very possibly, being a mobile phone that actually fits one’s face without its dimensions being dominated by the brick of a conventional Li-ion battery pack (admittedly a fraction of the size of a decade ago, but still a brake on creativity).

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, according to Pulickel Ajayan, professor in Mechanical Engineering and Materials Science, and Chemistry, at Rice. “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.” The ‘green’ qualities of substrates, liquids and other materials involved in production of paintable batteries have yet to be established, but given the proven principle, the rest should flow fairly fast.

I showed the paintable-battery idea to my teenage son, for whom design technology is a favourite school subject. As he took in the possibilities afforded by the technology, his eyes – LIT UP.

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September 25, 2018, 8:18 AM AEST