A Battery You Can See Through

ENERGY

Transparent batteries could lead to designs for cell phones and other gadgets.

• BY KATHERINE BOURZAC

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Researchers at Stanford University have made fully transparent batteries, the last missing component needed to make transparent displays and other electronic devices.

Stanford materials science professor Yi Cui, who led the work, says a tremendous amount of research goes into making batteries store more energy for longer, but little attention has been paid to making them "more beautiful, and fancier."

Researchers have previously made transparent variations on other major classes of electronics, including transistors and the components used to control displays, but not yet batteries. "And if you can't make the battery transparent, you can't make the gadget transparent," says Cui.

Some battery components are easier to make using transparent materials than others. The electrodes are the tricky part, says Cui. One way to make a transparent electrode is to make it very thin, on the order of about 100 nanometers thick. But a thin electrode typically can't store enough energy to be useful.

Another approach is to make the electrode in the form of a pattern that has features smaller than the naked eye can see. As long as there is enough total electrode material in the battery, this type of electrode can still store a significant amount of energy. Cui designed a mesh electrode where all the lines of the mesh are on the order of 50 micrometers, effectively invisible, and the squares inside the mesh contain no battery materials.

Fabrication is also tricky, since the usual methods for making components at this resolution require harsh chemical processes that damage battery materials. The Stanford group instead used a relatively simple method to make the transparent mesh electrodes, which are held together inside a clear, squishy polymer called PDMS.

They start by using lithography to make a mold on a silicon wafer. Then they pour liquid PDMS over the mold, cure the polymer to solidify it, and peel it off the mold. The PDMS sheet is then engraved with a grid of narrow channels. Next they drip a solution of electrode materials onto the surface of the PDMS. Capillary action pulls the materials in until they have filled all the channels to create the mesh. The researchers used standard lithium-ion battery materials to make their electrodes.

To make the complete battery, they sandwich a clear gel electrolyte between the two electrodes, and put it all inside a protective plastic wrapping. The Stanford researchers created prototypes, and used them to power an LED whose light can be seen through the battery itself.

Cui says these batteries should, in theory, be able to store about half as much energy as an equivalent-sized opaque battery, because there is a trade-off between energy density and transparency. They can lay down a thicker mesh of electrode materials to store more energy, but that means less light will get through.

So far, his lab's prototypes can store 20 watt-hours per liter, about as much energy as a nickel-cadmium battery, but Cui expects to improve this by an order of magnitude, in part by reducing the thickness of the polymer substrate, and by making the trenches that hold the electrode materials deeper.

Another way to store more energy without sacrificing transparency would be to stack multiple cells on top of one another in such a way that the grid of the electrodes lined up, allowing light to pass through. So far, the group has made electrodes that are about an inch across, but Cui says they could be made much larger, and the material could simply be cut to the desired size.