Solid-state battery problem discovered

In the pursuit of improving portable power, new lithium metal batteries with solid electrolytes tick off every box in next-gen battery technology – lightweight, higher capacity, faster charging, and inflammable. This game-changing tech, however, has been slow to develop due to a mysterious failure that causes it to short-circuit. Now, researchers at Stanford University and the school's SLAC National Accelerator Laboratory say they have discovered the culprit.

Turns out, solid electrolyte is just a tad too solid.

"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," said senior author William Cheuh, an associate professor of materials science and engineering in the school of engineering, and of energy sciences and engineering in the new Stanford Doerr School of Sustainability.

"Even dust or other impurities introduced in manufacturing can generate enough stress to cause failure," continued Cheuh, who directed the research with Wendy Gu, assistant professor of mechanical engineering.

The problem of failing solid electrolytes is not a new one, and many have studied the phenomena. Theories were often abounded as to what exactly was the cause of such a prospective liquid battery replacement being so unpredictable and unstable. Some said that the unintended flow of electrons is to blame, while others pointed at the chemistry.

Yet others theorized different forces are at play.

In a study published in the journal "Nature Energy," co-lead authors Geoff McConohy, Xin Xu, and Teng Cui explain in rigorous, statistically significant experiments how nanoscale defects and mechanical stress cause solid electrolytes to fail.

With scientists around the world trying to develop solid electrolyte rechargeable batteries around the problem or even to turn the weakness to their advantage, the Stanford team is researching it to formulate a foundational understanding.

Understanding the flaws

Energy-dense, fast-charging, non-flammable lithium metal batteries that last a long time are a quickly growing contender for the main barriers to widespread electric vehicle adoption, among other numerous benefits.

Many of today's leading solid electrolytes are ceramic. They enable fast transport of lithium ions and physically separate the two electrodes that store energy, and most importantly, they are fireproof.

But, like ceramics in everyday use, they can develop tiny cracks on their surface.

Demonstrating through more than 60 experiments, the scientists found that ceramics 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).

Cheuh and his team have found that 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 lithium collects on the surface of the ceramic in some locations, as desired, while in other spots, it would begin to burrow deeper and deeper until the lithium bridges across the solid electrolyte and creating a short circuit.

The difference? Pressure.

When the electrical probe merely touched the surface, lithium would gather 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.

Only "why" answers "how"

A real-world solid-state battery is made of layers upon layers of cathode-electrolyte-anode sheets stacked atop one another. The electrolyte's role is to physically separate the cathode from the anode yet allow lithium ions to travel freely between the two.

If cathode and anode touch or are connected electrically in any way, such as a tunnel of metallic lithium, a short circuit occurs.

As Cheuh and the team show, even a subtle bend, slight twist, or speck of dust caught between the electrolyte and the lithium anode will cause imperceptible imperfections.

"Given the opportunity to burrow into the electrolyte, the lithium will eventually snake its way through, connecting the cathode and anode," said McConohy, who completed his doctorate last year working in Cheuh's lab and now works in industry. "When that happens, the battery fails."

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

It's a bit like the way a pothole appears in otherwise perfect pavement, Xu 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," said Xu, a postdoctoral scholar in Cheuh's lab.

With their new understanding in hand, Cheuh and team are looking at ways to use these very same mechanical forces intentionally to toughen the material during manufacturing. They are also looking at ways to coat the electrolyte surface to prevent cracks or repair them if they emerge.

"These improvements all start with a single question: Why?" said Cui, a postdoctoral in Gu's lab. "We are engineers. The most important thing we can do is to find out why something is happening. Once we know that, we can improve things."