The anodes, or negative electrodes, in most rechargeable batteries are currently made of graphite. But the next generation of high-capacity, fast-charging, low-cost batteries will use metal anodes instead, which will make them lighter and safer.
A new membrane developed by Xudong Wang, a Grainger Institute for Engineering Professor in the Department of Materials Science and Engineering at the University of Wisconsin-Madison, and his students, including recent PhD graduate Yutao Dong, overcomes a major hurdle in realizing this new approach: Over time, metal anodes tend to develop dendrites, or little metallic “branches,” that can reduce the lifespan of batteries or short them out completely. The membrane can stop dendrite formation, improve battery life and safety, and lead to more efficient manufacturing techniques for next-generation batteries. The team’s research appears in the April 15, 2024, issue of the journal Nano Letters.
Metal anodes work best when they are as smooth as possible. As energy flows through these anodes, however, bits of metal can start to pop up, making the surface rough. “This extruded surface area will concentrate the electric field there, cause ions to deposit at the tip and lead to the growth of the dendrite,” says Wang.
Researchers have tried various techniques to limit dendrite growth, including protective coatings, scaffoldings and alloying the metals. But no technique eliminates dendrite growth completely and none is universally applicable to different types of metal anodes.
Wang’s solution is a porous ferroelectric separator made from a polymer called P(VDF-TrFE). In ferroelectric materials, a small mechanical strain, such as bending or pressing, causes the material to show an electric polarization. When a protrusion develops on a metal anode during charging, it compresses the ferroelectric separator and generates a localized electric field at the protrusion front. This reverses the surface polarization and repels the ions that would typically collect at the protrusion tip. As the charging process continues, the protrusion is eventually reincorporated back into the flat surface. Wang’s team calls this process active dendrite suppression.
Wang and his team tested the membrane on a zinc-ion battery, finding that the battery with their ferroelectric separator lasted for more than 4,000 cycles, maintained a high voltage capacity, and the zinc anode surface remained smooth. (In contrast, three other membranes they tested developed dendrites and failed in less than a quarter of that time.) Beyond zinc-ion batteries, the team says its membrane should work on any battery system using metal anodes.
Wang says that while the team’s membrane could improve the longevity and safety of rechargeable batteries, it might be even more important when it comes to battery manufacturing. “Our membrane essentially enables a self-flattening of the anode during operation. Battery manufacturing requires a lot of effort to make the electrode surfaces flat,” says Wang.
“Using our membrane as a battery separator may largely improve the tolerance to surface roughness in initial battery assembly. That could save a lot of manufacturing costs and simplify the process” he says.
Wang has patented the membrane through the Wisconsin Alumni Research Foundation and is currently in the process of developing a company to scale up membrane fabrication.
Other UW-Madison authors include Wenjian Liu, Corey Carlos, Ziyi Zhang, Jun Li, Fengdan Pan and Jiajie Sui.
The authors acknowledge support from the National Heart, Lung, and Blood Institute of the National Institutes of Health under Award Number R01HL157077 and the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award # DE-SC0020283.
Featured image caption: Xudong Wang (left) and graduate student Wenjian Liu show off the new dendrite-inhibiting membrane that their lab designed. Credit: Joel Hallberg.