Lithium metal batteries with solid electrolytes are lightweight, inflammable, pack a lot of energy, and can be recharged very quickly, but they have been slow to develop due to mysterious short circuiting and failure. Researchers at Stanford University and SLAC National Accelerator Laboratory now report that the root cause of lithium intrusion into the electrolyte is a combination of current focusing and the presence of nanoscale cracks, rather than electronic leakage or electrochemical reduction.

In a paper published in the journal Nature Energy, they suggest that these insights highlight the mechanical tunability of electrochemical plating reactions in brittle solid electrolytes.

Just modest indentation, bending or twisting of the batteries can cause nanoscopic fissures in the materials to open and lithium to intrude into the solid electrolyte causing it to short circuit. Even dust or other impurities introduced in manufacturing can generate enough stress to cause failure.senior author William Chueh

The problem of failing solid electrolytes is not new and many have studied the phenomenon. Theories abound as to what exactly is the cause. Some say the unintended flow of electrons is to blame, while others point to chemistry. Yet others theorize different forces are at play.

In this work, the researchers investigated statistically the effect of locally and globally applied stress on lithium penetration initiation in Li_6.6La₃Ta_0.4Zr_1.6O₁₂ (LLZO) via operando microprobe scanning electron microscopy.

The researchers demonstrated through more than 60 experiments that ceramic electrolytes are often imbued with nanoscopic cracks, dents, and fissures, many less than 20 nanometers wide. (A sheet of paper is about 100,000 nanometers thick.) During fast charging, Chueh and team say, these inherent fractures open, allowing lithium to intrude.

In each experiment, the researchers applied an electrical probe to a solid electrolyte, creating a miniature battery, and used an electron microscope to observe fast charging in real time. Subsequently, they used an ion beam as a scalpel to understand why the lithium collects on the surface of the ceramic in some locations, as desired, while in other spots it begins to burrow, deeper and deeper, until the lithium bridges across the solid electrolyte, creating a short circuit.

This artist’s rendition shows one probe bending from applied pressure, causing a fracture in the solid electrolyte, which is filling with lithium. On the right, the probe is not pressing against the electrolyte and the lithium plates on the ceramic surface, as desired. (Image credit: Cube3D)

They found that the difference is pressure. When the electrical probe merely touches the surface of the electrolyte, lithium gathers beautifully atop the electrolyte even when the battery is charged in less than one minute. However, when the probe presses into the ceramic electrolyte, mimicking the mechanical stresses of indentation, bending, and twisting, it is more probable that the battery short circuits.

Given the opportunity to burrow into the electrolyte, the lithium will eventually snake its way through, connecting the cathode and anode. When that happens, the battery fails.

—Geoff McConohy, first and co-corresponding author

The new understanding was demonstrated repeatedly, the researchers said. They recorded video of the process using scanning electron microscopes—the same microscopes that were unable to see the nascent fissures in the pure untested electrolyte.

A scanning electron microscopy video that shows lithium plating as it takes place on a solid electrolyte. (Image credit: Xin Xu, Geoff McConohy, and Wenfang Shi)

It’s a little like the way a pothole appears in otherwise perfect pavement, Xin Xu, a co-author on the paper, explained. Through rain and snow, car tires pound water into the tiny, pre-existing imperfections in the pavement producing ever-widening cracks that grow over time.

Lithium is actually a soft material, but, like the water in the pothole analogy, all it takes is pressure to widen the gap and cause a failure.

—Xin Xu

With their new understanding in hand, Chueh’s team is looking at ways to use these very same mechanical forces intentionally to toughen the material during manufacturing, much like a blacksmith anneals a blade during production. They are also looking at ways to coat the electrolyte surface to prevent cracks or repair them if they emerge.

Resources

• McConohy, G., Xu, X., Cui, T. et al. (2023) “Mechanical regulation of lithium intrusion probability in garnet solid electrolytes.” Nat Energy doi: 10.1038/s41560-022-01186-4