Studying the behavior of electrolyte materials is crucial for the development of next-generation batteries. (Photo: Adobe Stock)

The next generation of batteries promises higher performance, greater safety, and faster charging times, and ideally, a long service life. Yet, their development has thus far been hindered by a key problem: tiny lithium-metal structures known as dendrites can cause short circuits and shorten battery lifespan. Researchers from the Technical University of Munich (TUM) and the Massachusetts Institute of Technology (MIT) have now revealed the role played by previously underestimated regions within the material: grain boundaries. It was already known that these interfaces between grains inside a solid electrolyte could trigger dendrite formation; a recent study from Stanford showed that these structures form either on the surface or within the solid-state electrolyte, depending on the load.

The work by researchers from TUM and MIT goes a step further: in a study published in the journal Nature Nanotechnology, they characterize the electrical and chemical behavior of these grain boundaries, explain using a theoretical model why dendrites grow specifically at internal interfaces, and outline ways to control this process. “For the past 30 years, the battery world has been dominated by lithium-ion batteries. At the same time, there is a growing realization that different applications require other types of batteries, like those using solid electrolytes,” explains Jennifer L. M. Rupp, Professor of Electrochemical Materials at TUM and corresponding author of the study. “This work provides a fundamental understanding of space charges at grain boundaries. Building on this, we can derive technical concepts to improve ion transport, extend service life and, ultimately, develop better batteries.” Researchers from TUM and MIT demonstrate the validity of this approach using the promising solid electrolyte lithium-lanthanum-zirconium oxide (LLZO). They are able to modify the material to enhance ion transport and reduce electron leakage. The result: the critical current density increases by more than 300 percent compared to a non-modified sample. For solid-state batteries, this means faster charging and a longer operational lifespan. 

Focus on grain boundaries 

The team led by Rupp, who is also active in the e-conversion Cluster of Excellence and moved from MIT to TUM over the course of this research, has spent years studying the behavior of electrolyte materials for next-generation batteries. Solid electrolytes in next generation batteries consist of many densely packed, tiny crystals. “What we call a ‘grain’ — much like a grain of salt — is actually a single crystal, though it may measure only about one micrometer,” explains Harry Tuller, a professor in the Department of Materials Science and Engineering at MIT. “During high-temperature processing, the best materials densify to the point where they are virtually free of voids or pores, meaning they are almost completely densely packed. Nevertheless, each of these crystallites remains separated from its neighbors by a grain boundary.” Researchers suspect that grain boundaries, due to their unique chemical and electrical properties, promote the formation of lithium dendrites. They influence the transport of ions and electrons during charging and discharging. However, the exact nature of these processes had previously remained unclear. The team of TUM and MIT researchers has now moved a step closer to understanding this mechanism.

Researchers from TUM (from left to right): Prof. Johanna Eichhorn (Photo: Julia Baumann / TUM), Prof. David Egger (Photo: Andrea Heddergott / TUM) and Prof. Jennifer L.M. Rupp (Photo: Uli Benz / TUM).

Achieving success through a variety of methods 

In their jointly developed model, the researchers describe how local electrical imbalances at grain boundaries influence the movement of lithium ions and electrons; they validated this model using the solid electrolyte LLZO, employing a range of analytical techniques and simulations. "Using machine learning, we were able to model processes with near-quantum-mechanical accuracy while simultaneously covering larger length scales. We demonstrated that lithium vacancies cause negative space charge at grain boundaries and that zirconium-depleted regions trap these vacancies. This opens new possibilities for the targeted improvement of solid-state electrolytes," explains David Egger, Professor of Theory of Functional Energy Materials at TUM. Microscopy techniques also yielded valuable insights. "Atomic force microscopy allowed us to clearly detect local changes in material properties at the grain boundaries and better understand their function," adds Johanna Eichhorn, Professor of Nanoscale Microscopy and Spectroscopy of Energy Materials at TUM. The researchers demonstrated that local electrical imbalances at grain boundaries play a crucial role in dendrite formation: they impede ion transport and promote the accumulation of electrons. 

Better battery materials 

The researchers then took the next step: based on their findings, they specifically modified the production process for the LLZO electrolyte to reduce disruptive charges at the interfaces. This facilitates easier lithium-ion movement and minimizes electron loss. "We demonstrated that we can control the formation of lithium dendrites to fully harness the high-performance potential of solid-state batteries," says lead author Dr. Hyunwon Chu, Rupp's former team member from MIT is now conducting research at the Lawrence Berkeley National Laboratory. "Starting with a theory on dendrite formation, we were able to experimentally confirm the mechanism and specifically improve the material." The result: a critical current density more than 300 percent higher. "Safe, rapid charging is the challenge facing the battery industry," says Rupp. "Our approach demonstrates how space charges at grain boundaries can be deliberately manipulated, thereby opening new possibilities for making solid-state batteries faster, safer, and more durable." The study thus offers a new approach to the material design and engineering of solid electrolytes: controlling processes at grain boundaries allows for the targeted regulation of ion and electron transport. 

Research funding 

The work was funded by Equinor ASA; the German Research Foundation (DFG) as part of the Excellence Strategy and the e-conversion Cluster of Excellence; the Federal Ministry for Research, Technology and Space; the U.S. National Science Foundation; and the U.S. Department of Homeland Security. Computing time was provided by the TUM-Oerlikon Advanced Manufacturing Institute and the Gauss Centre for Supercomputing e.V. (GCS) via the John von Neumann Institute for Computing on the GCS supercomputer JUWELS at the Jülich Supercomputing Centre. 

Publication:

Charged grain boundaries limit short-circuit endurance in garnet solid-state battery electrolytes; 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 & Jennifer L. M. Rupp. https://doi.org/10.1038/s41565-026-02206-0 

 

Contact:

Prof. Dr. Jennifer L. M. Rupp
Technical University of Munich
TUM School of Natural Sciences
Professorship for Electrochemical Materials
Website: https://ecm-tum.de/
E-Mail: jrupp@tum.de 

Link to original story: https://news.mit.edu/2026/discovery-helps-explain-why-solid-state-batteries-often-fail-0706