Next-generation batteries utilizing new electrolyte materials could achieve significantly higher energy densities than today's lithium-ion batteries, while alleviating many safety concerns. However, advanced batteries that employ solid or nearly solid electrolytes have been plagued by the formation of tiny lithium metal spikes known as dendrites, which degrade battery efficiency and lead to failure. The exact formation mechanisms of these dendrites are still up for debate. While most research has focused on the interface between the battery's electrolyte and electrodes, another factor is the boundary where two grains of electrolyte in a solid material meet.
Researchers understand that these boundaries can initiate dendrites within electrolytes, although studying their effects has been challenging. Now, researchers from MIT and the Technical University of Munich have uncovered why such boundaries can lead to dendrites: hidden electrical imbalances across the boundaries affect the electrolyte's ability to conduct electrical charges, influencing how 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, demonstrating that adjusting the processing of the electrolyte enhances ion movement while reducing electron leakage. This adjustment can increase critical current density by over 300%, potentially enabling solid-state batteries that charge faster and last longer.
Senior author Harry Tuller, a professor in MIT's Department of Materials Science and Engineering, stated, "Grain boundaries are like the weather: Everyone talks about it, but nobody does anything about it. 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."
Rupp's research group has spent years studying the behavior of next-generation electrolyte materials. Solid-state battery electrolytes consist of numerous 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," Tuller explains.
Under high-temperature processes, the best materials consolidate to be void or pore-free and can be nearly 100% 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 lithium metal dendrites that cause short circuits. It has been suspected that grain boundaries possess different chemical and electrical properties than the grains, which interact with the ions and electrons shuttling between electrodes during charging and discharging.
However, the precise mechanisms by which the boundaries slow down ions, leak electrons, and lead to dendrites remain unclear. "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 this interference, the researchers developed a model to explain how local electrical imbalances at grain boundaries alter the movement of lithium ions and electronic charge carriers. They tested this model in a common solid electrolyte material called lithium lanthanum zirconate (LLZO), using techniques such as electron microscopy, machine learning modeling, and electrochemical impedance spectroscopy, which measures how easily a charge moves through a material.
They discovered that the cores of the boundaries carry a local electrical charge, generating local electric fields that enhance ionic resistance while causing a buildup of electrons in the boundary region, where they can reduce lithium ions, leading to lithium metal dendrite formation.
Rupp explains, "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 various applications. 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."
The researchers utilized their observations to adjust the material processing conditions of the LLZO electrolyte, minimizing negative charges at the boundaries, which facilitated lithium ion movement and reduced electron leakage. These modifications enabled them to create an electrolyte with a critical current density over 300% higher than a baseline sample.
Higher current density allows for faster charging and discharging and should also delay short-circuiting to extend battery life. Rupp notes, "Fires are currently a huge issue in the battery industry. By demonstrating 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 enhance battery charging speed and longevity before they fail."
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 states. "In this paper, we started with a theory for how these dendrites form, then we did the material characterization to support that theory, and finally we did the engineering to apply the findings and actually improve battery performance." This work was partially supported by the National Science Foundation and the U.S. Department of Homeland Security.
Blogger's Review: The research on solid-state batteries is accelerating, particularly in addressing the lithium metal dendrite issue. This study not only elucidates the electrical behavior of grain boundaries but also offers new insights for improving battery performance. With a deeper understanding of electrolyte materials, the future applications of solid-state batteries look promising.