Step towards transparent gadgets - now, where did I put my phone?

Posted by Emma Woollacott

Stanford University scientist Yi Cui says he wants a see-through iPhone - and he's done his bit to help create one by developing a transparent battery.

There are currently a few partically transparent gadgets on the market - digital photo frames and cellphone keyboards - but they've all had to be powered by boring visible batteries.

Cui's idea was that because various important components can't actually be made transparent, his team needed a way to build a battery that allowed its nontransparent components to be too small to see.

"If something is smaller than 50 microns, your eyes will feel like it is transparent," says graduate student Yuan Yang.

Yang and Cui devised a mesh-like framework for the battery electrodes, with each 'line' in the grid just 35 microns wide. Because the individual lines are so thin, the entire meshwork area appears transparent.

The two settled upon a transparent, slightly rubbery compound known as polydimethylsiloxane (PDMS) for the grid.

"PDMS is pretty cheap, and already being used in plastic surgery and contact lenses," said Yang. "But it is not conductive, so we had to deposit metals onto it to make it conductive."

Next, Yang developed a special transparent substance to be sandwiched between electrodes. He modified an existing gel electrolyte to make it serve as both an electrolyte and a separator. Stacking up layers created the battery.

As long as the gridlines are matched accurately, the battery's pretty transparent - 62 percent in visible light, and about 60 percent  transparent even with three full cells stacked on top of each other.

It is, unfortunately, only half as powerful as comparably sized lithium-ion counterparts.

"The energy density is currently lower than lithium batteries," said Yang. "It is comparable to nickel-cadmium batteries right now."
But Yang and Cui believe that advancements in materials science will soon improve this, and that there's the potential for commercial application. Cui's filed a patent.

"Its cost could be similar to those of regular batteries," he says. "Especially if we use low cost metals as current collectors, there is no reason this cannot be cheap."