‘Healing’ batteries with ultrasonics

High-capacity batteries have emerged as an essential building block of the clean energy future, but it likely won't be today's lithium-ion batteries alone that get us there. That's because, as powerful and ubiquitous as they are, lithium-ion batteries have some major limitations. For starters, they are very heavy - a fundamental quality that curbs the range of today's electric vehicles and basically rules them out as an option for powering commercial aircraft. The world's growing demand for the minerals needed to manufacture lithium-ion batteries also requires ever expanding mining operations, which have environmental, climate and geopolitical consequences. And on the safety front, the liquid electrolytes used in lithium-ion batteries are highly flammable, which is one reason why electric vehicle battery fires can be so catastrophic.

Because of these issues, researchers are developing new kinds of batteries, including solid-state batteries, which are lighter, can hold more energy for their weight and require fewer materials. They are also considered safer, since, as the name suggests, they don't require a liquid electrolyte. But mechanical engineering postdoc research fellow Yaohong Xiao, who works with Associate Professor Lei Chen, says that last quality is both a blessing and a curse. Inside a typical lithium-ion battery, the liquid electrolyte is responsible for transferring charged particles back and forth between the cathode and anode sides of the battery, which is necessary both for charging and discharging. Because this electrolyte is a liquid, Xiao says it creates a nearly uniform interface between the two sides of the battery, since liquids naturally fill tiny nooks and crannies that might exist between the anode and cathode materials. In a solid-state battery, however, the electrolyte separating the anode and cathode sides is a strong, stiff, solid piece of material, and Xiao says that makes it more difficult to get really good contact between the two sides. "Essentially, you have solid on solid on solid," Xiao explains. "Right after manufacturing, you might have no problem with this interface, but after a few cycles, or after the battery has experienced vibrations from being on the road, you start to get little voids in the interface so the electrolyte is no longer making perfect contact with both sides of the battery." As a result, Xiao says a battery can develop zones of higher than usual current, which reduces its overall effectiveness.

Typically, Xiao says researchers have tried to solve this solid-state battery interface problem in a couple of different ways. First, they use pressure to basically squish all the components back together. Or they can use an "interlayer" material, which, kind of like double-sided tape, keeps the anode and cathode materials snug up against the solid electrolyte. But each approach has its limitations. Xiao says introducing interlayers to a process is typically expensive. That might not be a huge deal for experiments in a lab, but it would increase the already high costs of solid-state batteries when they're being manufactured at scale. On the other hand, Xiao says using pressure to smooth out the interface is sort of like trying to squish two pieces of wood together. The rigidity of the lumber and any imperfections along the surfaces mean that pressure alone often isn't enough to create perfect contact.

A couple years ago, Xiao began kicking around a sort of unconventional solution for this interface challenge. Xiao's background is actually in metallurgy, not battery chemistry, and he saw this issue primarily as a question of how to get different metals to stick together. As it turns out, this is actually a pretty common problem in the metals universe, and one with a variety of well-established solutions. Specifically, Xiao was thinking that a technique called ultrasonic welding might be able to "heal" the voids in the interface, restoring uniform contact with the electrolyte. Ultrasonic healing, which is commonly deployed in a wide range of industries to weld pieces of plastic or thin pieces of metal together, uses high-frequency acoustic vibrations instead of high temperatures or solders to join materials together. The result is a highly uniform bond, but the process doesn't use a whole lot of energy and can be done very quickly. Xiao worked with his postdoc colleague in Chen's lab, XinXin Yao, who helped provide the theoretical feasibility for the welding concept, to set up an experiment, which revealed that ultrasonic welding could indeed nearly completely restore the uniformity of the electrolyte interface in as little as a minute. Moreover, the process required temperatures barely warmer than the water in your hot water heater and used about as much energy as an old-fashioned incandescent light bulb.

Xiao says the effectiveness of this metals-based approach came as a surprise to many of his colleagues, most of whom are experts in battery chemistry. Now, he's hoping the results, which were recently published in "Advanced Energy Materials,"will help inspire other researchers to further explore the technique's full potential. He says one possible application would be to use this as a tune-up strategy for eventual solid-state battery-powered EVs, which could come in for a quick ultrasonic healing treatment and get their optimal range back. And one other surprise finding from his experiment is that ultrasonic welding actually increased the conductivity of the electrolyte, albeit temporarily. That result was particularly intriguing to some colleagues at UM-Ann Arbor, who recently visited the newly established Battery Manufacturing and Testing Lab for a firsthand look. "They were very interested in this point," Xiao says. "The conductivity would slowly recover after treatment, but they were thinking, what if we can fix that? What if we can figure out a way to not let it recover? So it's very exciting to see this research already inspiring further work in this area."

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Story by Lou Blouin

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