Yuan Yang, a graduate student in materials science, holds one of the transparent batteries he developed with Professor Yi Cui. Photo: L.A. Cicero |
It
sounds like something out of a cheesy science fiction movie, but thanks to new
research by several Stanford scientists, transparent cell phones are one step
closer to becoming a reality.
Several
companies have successfully created partially transparent gadgets such as
digital photo frames and cell phones with see-through keyboards. However, fully
transparent e-book readers or cell phones have remained largely in the realm of
conceptual art due to one last missing puzzle piece.
“If you want to make everything transparent, what about
the battery?” says Yi Cui, an associate professor of materials science and
engineering and of photon science at SLAC National Accelerator Laboratory,
renowned for his work with batteries.
With graduate student Yuan Yang, who is the first author of
the paper “Transparent lithium-ion batteries” in the Proceedings of
the National Academy of Sciences, Cui set out to create
a clear battery suitable for use in consumer electronics.
“I can make the battery more powerful, but I also want
to make the battery look fancier,” says Cui, who praised Yang for coming
up with this unusual research idea.
Since key active materials in batteries cannot yet be made
transparent or replaced with transparent alternatives, Yang and Cui realized
that they had to find a way to construct a battery such that its nontransparent
components were too small to be seen by the naked eye.
“If something is smaller than 50 microns, your eyes
will feel like it is transparent,” says Yang, because the maximum
resolving power of the human eye is somewhere between 50 to 100 microns.
Yang and Cui devised a mesh-like framework for the battery
electrodes, with each “line” in the grid being approximately 35
microns wide. Light passes through the transparent gaps between the gridlines;
because the individual lines are so thin, the entire meshwork area appears
transparent.
This was easier said than done. The pair finally came up
with an ingenious three-step process that utilized low-cost, commonly available
substances.
First, since regular materials such as copper or aluminum
were out of the question, a transparent alternative had to be found. Yang and
Cui settled upon a transparent, slightly rubbery compound known as
polydimethylsiloxane (PDMS).
“PDMS is pretty cheap, and already being used in
plastic surgery and contact lenses,” says Yang. “But it is not
conductive, so we had to deposit metals onto it to make it conductive.”
To do so, PDMS was poured into silicon molds to create
grid-patterned trenches. A metal film was evaporated over the trenches,
creating a conductive layer.
The researchers then dropped a liquid slurry solution
containing minuscule, nano-sized active electrode materials into the trenches.
Next, Yang developed a special transparent substance to be
sandwiched between electrodes. He modified an existing gel electrolyte to make
it serve double-duty as both an electrolyte and a separator. Since all of the
materials used to make separators in regular batteries are nontransparent, this
was a vital step.
By precisely placing an electrolyte layer between two
electrodes, one functional battery is created. Multiple layers can be added in
order to create a larger and more powerful battery.
As long as the gridlines are matched accurately,
transparency is maintained. Yang and Cui’s light transmittance tests showed a
62% transparency in visible light, and approximately 60% transparency even with
three full cells stacked on top of each other. The entire battery is also
highly flexible.
Perhaps best of all, the transparent battery is less
expensive than one might expect.
“Its cost could be similar to those of regular
batteries,” says Cui. “Especially if we use low cost metals as
current collectors, there is no reason this cannot be cheap.”
The only current limitation is that the transparent battery
is only about half as powerful as comparably sized lithium-ion counterparts.
“The energy density is currently lower than lithium
batteries,” says Yang. “It is comparable to nickel-cadmium batteries
right now.”
Most laptops and cell phones are powered by lithium-ion
batteries, while nickel-cadmium batteries are often found in cameras and other
less energy-intensive devices.
Yang and Cui are optimistic that advancements in materials
science will enable the improvement of the energy density of the transparent
battery. The manufacturing process is definitely scalable, and there is
potential for commercial application, says Cui, who has filed a patent for the
battery.
But wait, one might ask—what is the reason to have a
transparent device, or even a transparent battery?
“It’s very exciting for doing fundamental scientific research,”
says Cui. “You can study what is happening inside batteries since they are
transparent now.”
Grand contributions to science aside, though, there was
definitely a bigger motivating force behind Yang and Cui’s research.
“It just looks cool,” says Cui. “I want to
talk to Steve Jobs about this. I want a transparent iPhone.”
