STANFORD (US) — Researchers have developed a highly-flexible, low-cost transparent lithium-ion battery that has potential for use in a variety of consumer electronics, including cell phones.
Partially transparent gadgets including digital photo frames and cell phones with see-through keyboards are already in existence, but fully transparent electronics have been elusive.
“If you want to make everything transparent, what about the battery?” says Yi Cui, associate professor of materials science and engineering and of photon science at Stanford University. “I can make the battery more powerful, but I also want to make the battery look fancier.”
Since key active materials in batteries cannot yet be made transparent or replaced with transparent alternatives, Cui and graduate student Yuan Yang, the study’s first author, realized that they had to find a way to construct a battery so that the nontransparent components are too small to be seen by the naked eye.
The research is published in the journal Proceedings of the National Academy of Sciences.
“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.
Using a three-step process, the researchers came up with a mesh-like framework for the battery electrodes, where each line in the grid is 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.
First, a transparent, slightly rubbery compound known as polydimethylsiloxane (PDMS) was found to replace regular materials like copper or aluminum.
“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. A liquid slurry solution containing minuscule, nano-sized active electrode materials was dropped into the trenches.
Next, Yang developed a special transparent substance to be sandwiched between electrodes by modifying 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 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, the battery stays transparent.
Light transmittance tests showed a 62 percent transparency in visible light, and approximately 60 percent transparency even with three full cells stacked on top of each other.
The battery is less expensive that one might expect, says Cui. “Its cost could be similar to those of regular batteries. Especially if we use low cost metals as current collectors, there is no reason this cannot be cheap.”
Currently, the battery is only about 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.”
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. Advancements in materials science should improve of the energy density of the transparent battery, Cui says.
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