Caption:MIT and Technical University of Munich researchers uncovered tiny electrical imbalances between crystals of solid electrolyte material that hurt the performance of solid-state batteries.
Image Credit: MIT News; iStock
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Scientific Frontline: Extended "At a Glance" Summary: Dendrite Formation in Solid-State Batteries
The Core Concept: Solid-state batteries utilize solid electrolytes to achieve high energy densities, but they often fail prematurely due to the formation of lithium metal spikes, known as dendrites. Recent research reveals that hidden electrical imbalances at the microscopic boundaries between electrolyte grains drive the formation of these destructive structures.
Key Distinction/Mechanism: While previous research primarily focused on the interface between the electrolyte and the battery's electrodes, this discovery isolates the "grain boundaries"—the microscopic borders where individual crystals of the solid electrolyte meet. These boundary cores carry local electrical charges that create resistance for lithium ions while trapping leaked electrons, which subsequently reduce the lithium ions into solid metal dendrites that cause short circuits.
Major Frameworks/Components:
- Solid Electrolytes: Materials composed of microscopic, densely packed crystallites that conduct ions between battery electrodes.
- Lithium Lanthanum Zirconate (LLZO): A common solid electrolyte material utilized by the researchers to test their electrochemical models via electron microscopy and impedance spectroscopy.
- Grain Boundaries: The microscopic interfaces separating individual crystals within the electrolyte, which possess elevated levels of structural defects compared to the void-free crystal cores.
- Space Charge Interfaces: Localized electrical imbalances at the grain boundaries that impede ionic transit and allow electron leakage.
- Critical Current Density: A metric of electrical performance that researchers increased by more than 300 percent by adjusting the LLZO material processing conditions to minimize negative boundary charges.
Branch of Science: Materials Science, Engineering, Electrochemistry, Solid-State Chemistry, and Nanotechnology.
Future Application: This fundamental understanding allows engineers to adjust material processing techniques to mitigate space charges, enabling the development of solid-state batteries that can charge faster, discharge more efficiently, and resist short-circuiting over a longer lifespan.
Why It Matters: Solid-state batteries represent the next generation of high-capacity energy storage, but dendrite-induced short circuits present a severe efficiency limit and fire hazard. By successfully engineering these grain boundaries to suppress dendrite formation, researchers can overcome a primary barrier to safely commercializing advanced battery technology for real-world applications.
Next-generation batteries that use new electrolyte materials could achieve far higher energy density than today’s lithium-ion batteries, without many of the safety concerns. But advanced batteries, such as those that use solid or almost-solid electrolytes, have been plagued by the formation of tiny spikes of lithium metal called dendrites that cause the batteries to lose efficiency and fail.
Exactly how those dendrites form is still up for debate. While the interface between the battery’s electrolyte and electrodes has been the focus of most research, another culprit is the boundary where two grains of electrolyte in a solid material meet. Researchers know these boundaries can seed dendrites within electrolytes, although the effects have been difficult to study.
Now, researchers at MIT and the Technical University of Munich have uncovered why such boundaries can lead to dendrites: hidden electrical imbalances across the boundaries affect how the electrolyte conducts electrical charges, which influences how the ions and electrons move through the material during battery operation. In a paper published today in Nature Nanotechnology, the researchers characterized the electrical and chemical behavior of the boundaries and showed that adjusting how the electrolyte is processed enhances the movement of ions while reducing electron leakage. This adjustment can increase critical current density by more than 300 percent, which could enable solid-state batteries that charge faster and last longer.
“Grain boundaries are like the weather: Everyone talks about it, but nobody does anything about it,” says senior author Harry Tuller, a professor in MIT’s Department of Materials Science and Engineering. “In this paper, we’ve decided to do something about grain boundaries, and by doing something, we’ve shown improved performance and demonstrated the importance of grain boundaries more broadly.”
Investigating Grain Boundaries
Rupp’s research group, which moved from MIT to TUM during this research, has spent years studying the behavior of next-generation electrolyte materials. Electrolytes in solid-state batteries are made of many tiny crystals of material packed together.
“What we call a grain, like a grain of salt, is actually a single crystal, but it might only be on the order of 1 micron in size,” explains Tuller. “Under high-temperature processes, the best materials essentially consolidate to be void- or pore-free and can be nearly 100 percent dense, but each of those crystallites is separated from its neighbor by a grain boundary.”
Solid-state battery researchers have increasingly focused on grain boundaries as the source of the lithium metal dendrites that cause them to short-circuit. It has been suspected that grain boundaries have different chemical and electrical properties from the grains, which interact with the ions and electrons shuttling between electrodes during battery charging and discharging. However, the exact mechanisms by which the boundaries slowed the ions down, leaked electrons, and led to dendrites were unknown.
“Grain boundaries are like defects,” Tuller says. “The boundaries have a higher level of defects than in the grains themselves, and generally, that means as carriers of charge approach the boundary, whether electrons or ions, there’s some kind of blockage to overcome.”
To better understand that interference, the researchers developed a model to explain how local electrical imbalances at grain boundaries change the movement of lithium ions and electronic charge carriers. They tested the model in a common solid electrolyte material called lithium lanthanum zirconate, or LLZO, using techniques including electron microscopy, machine-learning modeling, and electrochemical impedance spectroscopy, which measures how easily a charge moves through a material.
They found the cores of the boundaries carry a local electrical charge, building up local electric fields that lead to enhanced ionic resistance while causing a buildup of electrons in the boundary region, where they can reduce lithium ions, leading to lithium metal dendrite formation.
“For the last 30 years, the world has been dominated by lithium-ion batteries, but there is a growing recognition that other battery types are needed for batteries used in a variety of uses,” Rupp explains. “This work gives us the fundamental understanding of the space-charge interface at the grain boundary. If understood properly, we can come up with engineering concepts to increase cycle life, transference of ions over electrons at these interfaces, and ultimately a better battery.”
Better Battery Materials
The researchers used their observations to adjust the material processing conditions of the LLZO electrolyte material and minimize the negative charges at the boundaries, finding that they could ease the movement of lithium ions and reduce the leakage of electrons.
The modifications allowed them to create an electrolyte that had a critical current density more than 300 percent higher than a baseline sample. Higher current density allows for faster charging and discharging. It should also delay short-circuiting to extend the life of batteries.
“Fires are currently a huge issue in the battery industry,” Rupp says. “By showing how to engineer these space charges in a controlled way, which is new in the field, we can have a strong impact on safety. It’s a new way to turn up the notch and get these batteries to charge faster and last longer before they break.”
The findings, along with the researchers’ engineering work, present a roadmap for battery researchers to accelerate the development of high-performance, longer-lasting solid-state batteries.
“We showed we can control the initiation of these dendrites to maximize solid-state batteries’ high performance,” Chu says. “In this paper, we started with a theory for how these dendrites form, then we did the material characterization to support that theory, then we did the engineering to apply the findings and actually improve battery performance.”
Funding: The work was supported, in part, by the National Science Foundation and the US Department of Homeland Security.
Published in journal: Nature Nanotechnology
Title: Charged grain boundaries limit short-circuit endurance in garnet solid-state battery electrolytes
Authors: Hyunwon Chu, Thomas Defferriere, Proloy Nandi, Waldemar Kaiser, Lukas M. Wolz, Fran Kurnia, Kun Joong Kim, Willis O’Leary, Thomas Altantzis, Johan Verbeeck, David A. Egger, Sara Bals, Johanna Eichhorn, Harry L. Tuller, AND Jennifer L. M. Rupp
Source/Credit: Massachusetts Institute of Technology | Zach Winn
Edited by: Scientific Frontline
Reference Number: MS070626_03