Contact: Yan-Yan Hu
TALLAHASSEE, Fla. — A team at the Florida State University-headquartered National High Magnetic Field Laboratory is offering new insight into the cause of failures in solid-state lithium-ion batteries.
Solid-state batteries, which use a solid electrolyte rather than liquid or gel, are considered a next-generation technology with the potential to revolutionize energy storage for electric vehicles, consumer electronics, and renewable energy systems. They provide more energy density without the safety issues of conventional liquid lithium-ion batteries, which are prone to overheating and fire.
However, developing reliable solid-state lithium batteries faces a challenge of its own: the buildup of what are called dendrites. As the battery is used, tiny needles of metallic lithium form and branch through the material like growing trees, connecting to each other and short-circuiting the battery.
Now, after more than five years of research using one-of-a-kind magnets and custom techniques, a team at the MagLab has more clearly pinpointed where and how dendrites form.
"If you don't understand the problem, it's hard to address it," said Yan-Yan Hu, an FSU chemistry and biochemistry professor who led the research. "We're trying to understand the mechanisms of dendrite formation in solids."
The work, published in Nature Materials, offers an accurate look at what happens inside a solid-state lithium battery as it's depleted and recharged repeatedly. Researchers obtained the unprecedented images by developing a custom probe allowing them to see inside the battery during those cycles using the MagLab's world-record magnetic resonance imaging system.
The specialized MRI coil for lithium-ion batteries attached to a specialized probe, ready to be placed into the main magnet for scanning.
"We can discharge and recharge batteries inside the nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) probe in the magnetic field. Meanwhile, we can look at dendrite formation inside the batteries," Hu explained.
"Our high field magnets at the Maglab are ideal for analyzing normally hard to detect elements such as lithium, opening up the periodic table to imaging elements not accessible at lower magnetic fields," said Sam Grant, an FSU chemical and biomedical engineering professor and director of the Maglab’s MRI program who is a senior author on the study.
The group also developed a unique process to mark the source of dendrite formation chemically —determining whether it was coming from lithium at the edge or middle of the battery. The battery was cycled many times while researchers monitored dendrite buildup.
"One of the unique aspects of this study is high-field MRI coupled with NMR.
MRI provides a picture of the distribution and growth of dendrite formation, while NMR provides insights into the chemistry and origin of the lithium deposited as dendrites," said Grant.
Their work untangled the complex interplay between two mechanisms that cause dendrites. The lithium needles first build up at the interface between the battery's electrode and electrolyte. The electrode connects the battery terminal to the electrolyte, which moves charged particles through the battery. The researchers also found that, as the battery is used and recharged, other dendrites form in the middle of the solid electrolyte as well. The dendrites at the edge and in the middle then branch out and can link up, leading to short circuits and battery failure.
An MRI image of a cycled and shorted electrolyte pellet from a test battery. The blue regions are the lithium dendrites.
"We now have a comprehensive understanding of how these dendrites can form, grow, and evolve," said Florida State University graduate student Yudan Chen, one of the lead authors of the paper.
The test battery consisted of two electrodes made of solid lithium, sandwiching a solid electrolyte compound made of what's called LLZO: lithium lanthanum and zirconium oxide.
With the new understanding of what causes dendrite buildup and battery breakdown, the challenge will be tweaking the battery's ingredients and design to mitigate dendrites. That could include using a different combination of materials, re-engineering the interface where the electrode meets the electrolyte, and adjusting the microstructure of the solid electrolyte.
"We have ideas, or possible methods to engineer, to design, to modify our battery cell," said Chen, "and after that, we can use our magnetic resonance techniques here to verify whether our engineering methods can work, really mitigate dendrite formation. We can use it as an evaluation toolkit."
(Left to right) Professor Yan-Yan Hu, grad student Yudan Chen, and professor Sam Grant stand in front of the MagLab’s 21 tesla MRI magnet with the custom probe used for their battery dendrite research.
That will be the next phase of the research. The study adds to the growing body of work by researchers around the world, from universities to industry, looking to design the ideal solid-state battery that boosts performance, improves safety, and can be quickly, cheaply manufactured at scale. This will benefit all of us in more efficiently powering the newest cell phones, earbuds, and laptops.
Scientists are also interested in how this research can help advance alternative and affordable energy from natural sources.
"We have many ways to generate energy," Chen said. "The key problem is how are we going to store that generated energy to let us use it when needed."
