The bane of battery scientists are little things called dendrites — little spikes of lithium that can pierce battery cells and lead to fires or explosions. Like the stalagmites and stalactites found in underground caves, those dendrites grow larger and stronger over time, particularly during charging cycles.
Dendrites are one of the primary reasons battery researchers are struggling to invent solid state batteries, which are less likely to be adversely affected by them. But there may be a better, easier way to deal with the dendrite problem. Researchers at the Lawrence Berkeley National Laboratory, in collaboration with colleagues at Carnegie Mellon University in Pittsburgh, claim the way to conquer the dendrites is a new class of soft yet solid electrolytes made from both polymers and ceramics. These electrolytes suppress dendrites before they can propagate and cause the battery to fail. Their work was published recently in the journal Nature Materials.
“Our dendrite-suppressing technology has exciting implications for the battery industry,” says co-author Brett Helms, a staff scientist in Berkeley Lab’s Molecular Foundry. “With it, battery manufacturers can produce safer lithium metal batteries with both high energy density and a long cycle life.” He adds that lithium metal batteries manufactured with the new electrolyte could also be used to power electric aircraft.
According to Berkeley Lab, Helms says the key to the design of these new soft, solid electrolytes is the use of soft polymers of intrinsic microporosity, or PIMs, whose pores were filled with nano-sized ceramic particles. Here’s the important part. Because the electrolyte remains a flexible, soft yet solid material, battery manufacturers will be able to manufacture rolls of lithium foils with the electrolyte as a laminate between the anode and the battery separator.
These lithium-electrode sub-assemblies, or LESAs, can be drop-in replacements for conventional graphite anodes, allowing battery manufacturers to use their existing assembly lines. That’s a huge advantage when it comes to moving scientific breakthroughs out of the lab and into commercial production.
To demonstrate the dendrite-suppressing features of the new PIM composite electrolyte, the Helms team used X-rays at Berkeley Lab’s Advanced Light Source to create 3D images of the interface between lithium metal and the electrolyte, and to visualize lithium plating and stripping for up to 16 hours at high current. Continuously smooth growth of lithium was observed when the new PIM composite electrolyte was present, while in its absence the interface showed telltale signs of the early stages of dendritic growth.
These and other data confirmed predictions from a new physical model for electro-deposition of lithium metal which takes into account both chemical and mechanical characteristics of the solid electrolytes.
“In 2017, when the conventional wisdom was that you need a hard electrolyte, we proposed that a new dendrite suppression mechanism is possible with a soft solid electrolyte,” said co-author Venkat Viswanathan, an associate professor of mechanical engineering and faculty fellow at Scott Institute for Energy Innovation at Carnegie Mellon University who led the theoretical studies for the work. “It is amazing to find a material realization of this approach with PIM composites.”
Pursuant to the ARPA-E IONICS program, 24M Technologies has already integrated these soft but solid materials into larger format batteries that could be used in EVs or electric vertical takeoff and landing aircraft. “While there are unique power requirements for EVs and eVTOLs, the PIM composite solid electrolyte technology appears to be versatile and enabling at high power,” Helms says. 24M has been deeply involved in semi-solid state battery research and production for a number of years.
At the end of last year, we reported on researchers at the University of Illinois at Urbana-Champagne who also focused on the dendrite problem. Our story said, “Many (researchers) are focusing their attention on solid materials such as ceramics or polymers. The drawback is that many of those solids are rigid and brittle, resulting in poor electrolyte-to-electrode contact and reduced conductivity.” The Lawrence Berkeley research solves that brittleness problem and, unlike the solution the University of Illinois scientists came up with, it is production-ready, or nearly so.
Electrically powered VTOL aircraft are seen as the next step forward in emissions-free urban transportation with such industry giants as Toyota making significant investments in companies like Joby Aviation. Toyota CEO Akio Toyoda said in a statement earlier this year, “Air transportation has been a long-term goal for Toyota, and while we continue our work in the automobile business, this agreement sets our sights to the sky. As we take up the challenge of air transportation together with Joby, an innovator in the emerging eVTOL space, we tap the potential to revolutionize future transportation and life. Through this new and exciting endeavor, we hope to deliver freedom of movement and enjoyment to customers everywhere, on land, and now, in the sky.”
The soft but solid electrolyte developed by Lawrence Berkeley National Laboratory could go a long way toward making that dream a reality.
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