By Duncan Graham-Rowe of Nature magazine

Flexible, transparent lithium-ion batteries have been made by a team of researchers at Stanford University in California, a technological leap that could spawn see-through electronic gadgets such as translucent iPads.

Many electronic components can be fabricated to be transparent, but so far this hasn't been possible for the power supply, says materials scientist Yi Cui, who led the work, which is published today in Proceedings of the National Academy of Sciences.

Batteries are normally made up of a pair of electrodes separated by an electrolytic solution, with something to conduct the current to an external circuit, and packaging to hold it all together. But only the electrolyte is naturally transparent, says Cui. And to make matters worse, he says, these components need to be piled on top of each other, which means all of them must be clear for the device to be transparent.

Nothing to see here

While transparent materials can easily be found for the casing and conductors, the electrodes tend to be naturally opaque. Cui's solution was to make the components so small that they are beyond the resolution of human eyes. This is done by creating each electrode out of a very fine mesh, instead of a solid sheet or rod of the material. Because no individual strand within the mesh is wider than 35 micrometers, they are too small for the eye to detect and so when stacked they appear transparent, much like looking through two very fine sieves.

Creating such tiny features out of active electrode materials is tricky, says Cui. Lithographic fabrication techniques are capable of creating structures at this scale but these processes involve using solvents that would be harmful to the electrodes.

So Cui's group used microfabrication techniques to first create a grid pattern mould out of silicon. Then a 100-micrometre-thick layer of polydimethylsiloxane (PDMS), a flexible, transparent polymer, was applied using a technique called electro-spinning. When the PDMS was removed from the silicon an aqueous slurry solution containing the active electrode material was applied to it, which filled the grid-like trenches to form a mesh, says Cui.

Cui and his colleagues show that they can create lithium-ion batteries that let 60% of the light that hit them pass right through, and an energy density of 10 watt-hours per liter. "That's very transparent," says Cui. Not as good as glass, but clear enough to read text through.

Low density

But 10 watt-hours per liter is a low energy density. The energy densities of conventional Lithium-ion batteries are typically an order of magnitude greater than this, says Hiroyuki Nishide, a chemist at Waseda University in Tokyo, whose own group is working on a type of transparent battery that uses nitroxide radicals as charge carriers.

Cui agrees, but says it should be relatively easy to push the energy density to roughly half that of conventional batteries. Making the electrode meshes deeper will increase the volume of the active material and the amount of stored energy without making the battery any more visible, he says.

Besides the aesthetic appeal of creating transparent gadgets, this sort of battery could also help reduce the size of portable devices, says Nishide. For example integrating the power supplies of smart phones within their displays should make them more compact. But the main innovation here seems to be the fabrication technique, which may not be limited to just lithium-ion batteries. This strategy seems pretty versatile and may be applicable to a range of different battery types, says Nishide.

This article is reproduced with permission from the magazine Nature. The article was first published on July 25, 2011.