As electronic devices become smaller, engineers are challenged to design batteries that can fit into smaller and smaller spaces without compromising performance. One possible solution comes from a team of researchers who have fashioned a powerful 3D lithium ion battery with a footprint approximately the size of 100 grains of salt.
Talking about the design strategy, Bruce Dunn, a professor of materials science and engineering at the University of California, Los Angeles (UCLA), explained that the approach taken was akin to a “skyscraper in New York instead of a ranch house in California.” Dunn served as senior author for a paper on the research, which appears in a recent issue of the journal Joule.
Two-dimensional batteries, which are basically an anode slice and a cathode slice separated by an ion-conducting electrolyte, are limited in the shapes they can take. By contrast, 3D anodes and cathodes can be designed in innumerable ways to snap together like puzzle pieces, separated by a small amount of electrolyte.
While this may seem like a simple approach in theory, many previous researchers have managed to build only half of a 3D battery — creating anodes and cathodes that are stable on their own, but fail to work as a functional battery when assembled. Most of the 3D batteries that have worked, moreover, have not represented a significant improvement over ordinary 2D versions.
But Dunn and his team took methods normally reserved for semiconductors for their anode, carving silicon into a grid of precisely-shaped, evenly-spaced cylinders. Thin layers of a photo-patternable polymer electrolyte were then applied to the silicon structure and poured into a standard lithium-ion cathode material, using the anode as a mold to ensure that the two halves would fit together.
The resulting battery achieved an energy density of 5.2 milliwatt hours per square centimeter — among the highest reported for a 3D battery — while occupying a footprint of just 0.09 square centimeters and withstanding 100 cycles of charging and discharging.
Dunn says that the battery has not yet reached its full potential; he hopes that the team can boost energy density with further tuning. "Another challenge with batteries is always the packaging," he added. "You need to seal them up, keep them small, and make sure they function just as well in the real world as in the glove box."
