Next-generation batteries utilizing new electrolyte materials could potentially offer much higher energy density compared to current lithium-ion batteries, while addressing many safety issues. However, advanced batteries with solid or nearly solid electrolytes often face challenges due to the formation of lithium metal spikes known as dendrites, which reduce efficiency and lead to failure. The precise mechanism of dendrite formation remains uncertain. While research has mainly focused on the interface between the battery’s electrolyte and electrodes, another key factor is the boundary where two electrolyte grains meet in a solid material. These boundaries can initiate dendrites, but their effects have been challenging to study.<\/p>\n
Researchers from MIT and the Technical University of Munich have discovered why these boundaries can lead to dendrites: hidden electrical imbalances across the boundaries influence how the electrolyte conducts electrical charges, which affects ion and electron movement during battery use. In a study published in Nature Nanotechnology, the researchers examined the electrical and chemical behavior of these boundaries. They demonstrated that altering the processing of the electrolyte can enhance ion movement while minimizing electron leakage, boosting critical current density by over 300 percent. This could result in solid-state batteries that charge more quickly and last longer.<\/p>\n
“Grain boundaries are like the weather: Everyone talks about it, but nobody does anything about it,” remarks 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.” The paper features contributions from first author Hyunwon Chu PhD \u201925, former MIT professor Jennifer Rupp, now at the Technical University of Munich, and other researchers from both institutions, as well as collaborators from the University of Antwerp.<\/p>\n
Rupp’s research group, which transitioned from MIT to TUM during the study, has long investigated the behavior of next-generation electrolyte materials. In solid-state batteries, electrolytes are composed of numerous small crystals 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.”<\/p>\n
Solid-state battery researchers are increasingly examining grain boundaries as the source of lithium metal dendrites causing short circuits. It’s suspected that these boundaries have different chemical and electrical properties compared to the grains, affecting the ions and electrons moving between electrodes during charging and discharging. However, the exact mechanisms by which boundaries slow ions, leak electrons, and cause dendrites were unclear. “Grain boundaries are like defects,” Tuller notes. “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.”<\/p>\n
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 using lithium lanthanum zirconate (LLZO), employing techniques such as electron microscopy, machine learning modeling, and electrochemical impedance spectroscopy, which measures charge movement through a material. They discovered that the cores of the boundaries carry local electrical charges, creating local electric fields that enhance ionic resistance and lead to electron build-up in the boundary region, where they can reduce lithium ions and form lithium metal dendrites.<\/p>\n
“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.”<\/p>\n
The researchers adjusted the material processing conditions of the LLZO electrolyte to minimize the negative charges at the boundaries, improving lithium ion movement and reducing electron leakage. These adjustments led to an electrolyte with a critical current density over 300 percent higher than a baseline sample, enabling faster charging and discharging while delaying short circuiting to extend battery life. “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\u2019s a new way to turn up the notch and get these batteries to charge faster and last longer before they break.”<\/p>\n
The findings, along with the engineering work, provide a roadmap for battery researchers to speed up 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\u2019 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, then we did the engineering to apply the findings and actually improve battery performance.” The work received support from the National Science Foundation and the U.S. Department of Homeland Security.<\/p>\n