Several companies have created partially transparent gadgets like digital photo frame or cell phones. “If you want to make everything transparent, what about the battery?” said Yi Cui, an associate professor of materials science and engineering and of photon science at Stanford University, renowned for his work with batteries.
Together with Yuan Yang, graduate student in Materials Science and the first author of the paper “Transparent lithium-ion batteries” in the July 25 edition of 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,” said Professor Cui, who praised Yang for coming up with this unusual research idea.
Usually active materials in batteries cannot be made by transparent materials or replaced with other transparent ones. Yang and Cui realized that they had to build a battery from smaller nontransparent components almost invisible for human eye. According to Yang, because the maximum resolving power of the human eye is between 50 to 100 microns, “if something is smaller than 50 microns, your eyes will feel like it is transparent.”
These two researchers 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.
The whole process of getting transparent materials is held in three stages and use low-cost commonly available substances. First, regular materials such aluminum or copper were replaced with polydimethylsiloxane (PDMS) settled upon a transparent, slightly rubbery. As Yang said: “PDMS is pretty cheap, and already being used in plastic surgery and contact lenses” and also added that: “But it is not conductive, so we had to deposit metals onto it to make it conductive.”
Next step in the process, 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 transparent battery.
Transparency is maintained as long as the gridlines are matched accurately. The light transmittance tests showed a 62 percent transparency in visible light and approximately 60 % transparency even with three full cells stacked on top of each other. Best of all, the transparent battery is less expensive than one might expect. “Its cost could be similar to those of regular batteries,” said 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. According to Yang, “the energy density is currently lower than lithium-ion batteries,” and “it is comparable to nickel-cadmium batteries right now.”
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, said Cui, who has filed a patent for the transparent battery.
Somebody will ask why is necessary to have a transparent gadget or a transparent battery? “It’s very exciting for doing fundamental scientific research,” said Cui. “You can study what is happening inside batteries since they are transparent now.”