Illuminating the Barrier to Next-Generation Battery That Charges Very Quickly

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.

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.

Theory into Practice
A real-world solid-state battery is made of layers upon layers of cathode-electrolyte-anode sheets stacked one atop 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, as by a tunnel of metallic lithium, a short circuit occurs.

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

“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 Chueh’s lab and now works in industry. “When that happens, the battery fails.”

The 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 little 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 Chueh’s lab.

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.

“These improvements all start with a single question: Why?,” said Cui, a postdoctoral scholar 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.”

Andrew Myers is writer, editor, and wrangler for a host of national and international media. The article was originally posted to the Stanford University News Service website, and is published courtesy of Stanford University.