One of the problems with batteries is that they tend to be heavy. Another is that they take up space. An idea that has been floating around is to make the battery a part of the structure of the device that it powers. No more casings and housings formed into bulky battery packs. Instead, the cover of a laptop, the wings of a drone, or the fenders and bumpers of a car could be made from electrochemical materials that can also provide structural strength. That’s the concept behind a recent development described by the University of Michigan in a news release.
Battery as a Structure
“A battery that is also a structural component has to be light, strong, safe, and have high capacity. Unfortunately, these requirements are often mutually exclusive,” said Nicholas Kotov, the Joseph B. and Florence V. Cejka Professor of Engineering, who led the research at the U of M.
The team started out using zinc as an electrode—primarily because the metal has good structural properties. Zinc battery chemistries have been looked at before, but because needle-like dendrites of zinc crystals form during charging, it has found few champions. The spiky dendritic crystals can grow through liquid electrolytes, becoming large enough to reach across to the opposite electrode and short out the battery, potentially causing a fire.
A manganese oxide slurry is cast onto a sheet of aluminum foil to serve as the cathode of a prototype structural battery. (Image source: Evan Dougherty, Michigan Engineering)
Biology Holds a Possible Key
One way to reduce dendritic growth is to use a solid or semi-solid electrolyte. To create the battery that they wanted, the team turned to biomimicry. “Cartilage turned out to be a perfect prototype for an ion-transporting material in batteries,” Kotov said. “It has amazing mechanics, and it serves us for a very long time compared to how thin it is. The same qualities are needed from solid electrolytes separating cathodes and anodes in batteries.” So they developed an electrolyte from branched nanofibers that resemble the collagen fibers of cartilage. Aramid nanofibers—the material often used in bulletproof vests—stand in for collagen, with polyethylene oxide (a chain-like, carbon-based molecule) and a zinc salt replacing soft components of cartilage.
To make working battery cells, zinc electrodes were paired with manganese oxide—the same combination found in standard alkaline batteries. But in these rechargeable batteries, the cartilage-like membrane replaced the standard separator and alkaline electrolyte. The qualities of the cartilage are nearly identical to those of a good solid electrolyte, which has to resist damage from dendrites while also letting ions flow from one electrode to the other. The prototype cells can run for more than 100 cycles at 90 percent capacity.
Flexible and Robust
The cells made with the cartilage electrolyte are surprisingly flexible and robust enough to withstand hard impacts and even stabbing with a knife or cutting with scissors without losing voltage or starting a fire. With the cartilage-like solid electrolyte, there are no liquids that can leak out of the damaged battery.
At present, the use of zinc in the batteries is not optimum as zinc batteries can’t charge and discharge as quickly as batteries with lithium ion chemistries. Kotov’s team intends to explore whether there is a better partner electrode that could improve the charging speed and cycle-life longevity of zinc rechargeable batteries. Beyond that, however, the robust nature of the cartilage-like solid electrolyte show potential for similar materials in other types of battery chemistries.
Senior Editor Kevin Clemens has been writing about energy, automotive, and transportation topics for more than 30 years. He has masters degrees in Materials Engineering and Environmental Education and a doctorate degree in Mechanical Engineering, specializing in aerodynamics. He has set several world land speed records on electric motorcycles that he built in his workshop.
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