Just recently an array of light-emitting diodes in a laboratory at Rice University in Houston, Texas was illuminated for six hours to spell out the name of the institution in capital letters – RICE. That might seem unexceptional were it not for the fact that the 40mA of current at 2.4V that powered the display was provided by a set of bathroom tiles.
To be precise, it was provided by a lithium-ion battery charged up by a mix of mains current and a small solar panel that – and this is where the situation really was innovative and unusual – had been painted on to the surface of the tiles and which was just 0.5mm in depth.
The exercise formed part of an 18-month project at the university that aimed to investigate a potential method of satisfying the increasingly urgent demand for appropriately slight batteries to power the increasingly lighter and thinner mobile devices, most obviously phones and computers, that consumer electronics companies are bringing to market. It is a demand confirmed by Neelam Singh: “There is now a lot of interest in unconventional battery designs,” she says. Singh is a graduate student at the university, who led the project as part of her work towards gaining a PhD. She says that, although the technique has only been demonstrated at laboratory scale, the project has proven that the concept is quite practicable.
Singh explains that the project, which got under way early last year, focused from the start on creating a paintable lithium-ion battery because that is the formulation “with the highest energy density of any battery technology,” an obvious priority if it was to be exceptionally thin. The research team – Singh mentions her tutor Professor Pulickel Ajayan and Charudatta Galande, co-author of the resultant paper, although others were also involved – came up with a composition involving five layers. These were in sequence:
- the positive current collector – carbon nanotubes and carbon black particles in N-methylpyrrolidone
- the cathode – lithium cobalt oxide, carbon and ultrafine graphite powder (UFG) in a binder solution
- a separator layer – Kynar Flex resin, PMMA polymer and silicon dioxide dispersed in a solvent
- the anode – lithium titanium oxide and UFG in a binder
- the negative current collector – conductive copper paint diluted with ethanol.
In each instance, says Singh, the team faced the challenge of achieving the required electrical and mechanical performance, which in the latter case meant ensuring that each layer of material would adhere to those next to it. But she says that the task was particularly acute for the separator layer that forms the core of the battery, not least because it had to be infused with electrolyte. Ensuring the adhesion of the separator was a “critical” factor, she says, because of the absolute requirement to avoid short-circuiting between the anode and cathode.
The team managed to create working batteries in which the materials were applied to a diverse range of surfaces, including a beer stein as well as the bathroom tiles. The tiles were the most complex of the power sources they achieved. Nine tiles were wired together in parallel, with one of them also carrying the solar cell. Not only did the battery work as a power source, it also proved capable of going through as many as 60 charging and discharging cycles with only a small drop in capacity.