As electronic devices are made ever smaller, there is increasing demand for similarly minuscule power sources. Now MIT researchers have reported an important advance toward building such microscopic batteries. They used a virus to assemble anodes on top of electrolyte layers–two of the three main components of a working battery–and connected them to current-collecting surfaces. The components, described this week in Proceedings of the National Academy of Sciences, are only four micrometers wide and could find application in labs on a chip or other small medical devices, the researchers say.
Building microscopic batteries has proved difficult in the past because the proportion of electrochemically active material inside a battery decreases as its size is reduced. Another trend in electronics is toward patterning devices onto flexible or curved surfaces, which power sources must be able to adapt to. The MIT work suggests that small, reliable batteries can be both made on the microscopic scale and embedded on a variety of surfaces.
“What’s new about this research is both the size [of the battery electrodes] and the process we used to position them,” says Angela Belcher, a professor of materials science at MIT, who collaborated with colleagues Yet-Ming Chiang and Paula Hammond on the work. They began by etching columns four micrometers wide and a few micrometers tall onto a silicon-based surface to effectively create a stamp. They then deposited alternating layers of two different polymers, which served as the solid electrolyte and battery separator, on top of these columns.
Next, a virus called M13, which the researchers have employed in earlier self-assembly studies, was used to make the anode. The virus is made of proteins, which can be genetically modified to react with particular substances. In this case, it generated structured arrays of cobalt oxide nanowires on top of the solid electrolyte. Finally, the assembled electrodes were flipped over and pressed onto thin bands of platinum, which were joined to a copper contact in order to collect current from the device.
The researchers tested the performance of the device using a layer of lithium foil and found that “the quality of the electrodes is exactly the same as before,” says Belcher, referring to the group’s earlier demonstrations of larger virus-assembled batteries. She adds that the cobalt oxide anode has a much higher charge storage capacity than the carbon-based electrodes typically used in lithium-ion batteries, and that it’s stable throughout charging and discharging. It also has a higher density of active material than do conventional batteries.
Other advantages of virus assembly include functioning at room temperature and precise control over the size and spacing of nanomaterials, leading to uniform and easily reproducible devices. The researchers’ next goal is to add a virus-assembled cathode to create a complete battery. As they have experimented with different materials and have fabricated cathodes on a larger scale, Belcher says that incorporating micro cathodes into the printing method is “definitely possible.” In the future, she adds, they will work toward devices with higher energy density and creating devices that are biocompatible.
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