Stanford University researchers have revealed that the problem with lithium metal batteries with solid electrolytes is mechanical stress.
New lithium metal batteries with solid electrolytes have many advantages – they charge quickly, pack a lot of energy, and are lightweight and inflammable. However, development on these batteries has been slow due to unexplained short-circuiting and failure. Now, researchers at Stanford University and SLAC National Accelerator Laboratory believe they have solved this mystery, identifying the problem as mechanical stress, which especially occurs during potent recharging.
“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,” explained senior author William Chueh, 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,” said Chueh.
Failing solid electrolytes is not a new phenomenon, with many theories over the years trying to pinpoint the exact cause. Some have argued that the intended flow of electrons is to blame for the failure, some identify chemistry as the problem, and others theorise that different forces are at play.
In the study, ‘Mechanical regulation of lithium intrusion probability in garnet solid electrolytes,’ led by co-lead authors Geoff McConohy, Xin Xu, and Teng Cui, it is explained how nanoscale defects and mechanical stress cause solid electrolytes to fail.
Because of this research, scientists around the world can now design solid electrolyte rechargeable batteries around the problem. Lithium metal batteries with newly designed solid electrolytes have the potential to overcome the main barriers to the widespread use of electric vehicles.
The problem with ceramic solid electrolytes
Today, many battery technologies use ceramic solid electrolytes that enable fast transport of lithium ions, and crucially, are fireproof. However, ceramic solid electrolytes have a tendency to develop tiny cracks on their surface.
The researchers conducted over 60 experiments which revealed that ceramics are often imbued with nanoscopic cracks, dents, and fissures, many less than 20 nanometres wide. During fast charging, these inherent fractures open, allowing lithium to intrude.
In each experiment, an electrical probe was applied to a solid electrolyte to create a miniature battery. The researchers then used an electron microscope to observe fast charging in real-time.
To understand why the lithium collects on the surface of the ceramic in some locations, but burrows deeper until it bridges across the solid electrolyte – causing a short circuit – in other locations, the team used an ion beam as a scalpel.
The team found that the difference is pressure – when the probe touches the surface of the electrolyte, lithium gathers on top of the electrolyte, even when the lithium metal battery is charged in less than one minute. However, when the probe presses into the ceramic electrolyte, mimicking mechanical stresses, it is more likely that the battery short circuits.
The team aims to improve lithium metal batteries during manufacturing
Solid-state lithium metal batteries are made of layers upon layers of cathode-electrolyte-anode sheets stacked one atop another. The electrolyte should separate the cathode from the anode whilst allowing lithium ions to travel between the two. If the cathode and anode touch or are connected electrically, a short circuit occurs.
The team demonstrated that 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. “When that happens, the battery fails.”
The researchers demonstrated the new understanding repeatedly and recorded video of the process using scanning electron microscopes.
“Lithium is actually a soft material, but all it takes is pressure to widen the gap and cause a failure,” said Xu, a postdoctoral scholar in Chueh’s lab.
Now, the team is looking at ways to use these same mechanical forces 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?” concluded 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.”