Stressed ions cause failure of solid-state batteries

Solid-state batteries could play a key role in electric vehicles, promising faster charging, greater range and longer lifespan than conventional lithium-ion batteries. But current manufacturing and materials-processing techniques leave solid-state batteries prone to failure.

Now, researchers have uncovered a hidden flaw that is responsible for these failures. The next step is to design materials and techniques that account for these flaws and thus produce the next generation of solid-state batteries.

In a solid-state battery, charged particles called ions move through the battery within a solid material, whereas in traditional lithium-ion batteries, ions move in a liquid. Solid-state batteries offer several advantages, but local variations or tiny flaws in the solid electrolyte material can cause the battery to wear out or short, according to the new findings.

“A uniform material is important,” said lead researcher Kelsey Hatzell, assistant professor of mechanical and aerospace engineering at Princeton University. “You want ions moving at the same speed at every point in space.”

In a paper in Nature Materials, Hatzell and her co-authors explained how they used high-tech tools at Argonne National Laboratory to examine and track nano-scale material changes within a battery while it was charging and discharging. The research team, representing Princeton University, Vanderbilt University, and Argonne and Oak Ridge National Laboratories, examined crystal grains in the battery’s solid electrolyte, the core part of the battery through which electrical charge moves.

The researchers concluded that irregularities between the grains can accelerate battery failure by letting ions move faster to some regions of the battery than others. Adjusting material processing and manufacturing approaches could help to solve these problems.

Batteries store electrical energy in materials that make up the anode and cathode. When the battery discharges energy to power a car or a smartphone, ions move across the battery to the cathode. The electrolyte, solid or liquid, is the path the ions take between the anode and cathode.

In a solid-state battery, the electrolyte is typically either a ceramic or a dense glass. Solid-state batteries with a solid electrolyte may be able to utilize more energy-dense materials (e.g. lithium metal), and are often lighter and smaller. Weight, volume and charge capacity are key factors for transportation applications such as electric vehicles. Solid-state batteries should also be safer and less susceptible to fires than other types of battery.

Engineers have long known that the electrolyte in solid-state batteries is prone to failing, but the failures seemed to occur at random. Hatzell and her co-researchers suspected that the failures might not be random but actually caused by changes in the crystalline structure of the electrolyte.

To explore this hypothesis, they used the synchrotron at the Argonne National Laboratory to produce powerful X-rays that allowed them to look into a solid-state battery during operation. They combined X-ray imaging and high-energy diffraction techniques to study the crystalline structure of a garnet electrolyte at the angstrom scale, roughly the size of a single atom. This allowed them to study changes in the garnet at the crystal level.

A garnet electrolyte is comprised of an ensemble of building blocks known as grains. In a single electrolyte (1mm diameter), there are almost 30,000 different grains. The researchers found that across the 30,000 grains, there were two predominant structural arrangements. These two structures move ions at varying speeds. In addition, these differing forms or structures “can lead to stress gradients that lead to ions moving in different directions and ions avoiding parts of the cell,” Hatzell said.

She likened the movement of charged ions through the battery to water moving down a river and encountering a rock that redirects the water. Areas that have high amounts of ions moving through them tend to have higher stress levels.

“If you have all the ions going to one location, it is going to cause rapid failure,” Hatzell said. “We need to have control over where and how ions move in electrolytes in order to build batteries that will last for thousands of charging cycles.”

According to Hatzell, it should be possible to control the uniformity of grains through manufacturing techniques and by adding small amounts of different chemicals called dopants to stabilize the crystal forms in the electrolytes.

“We have a lot of hypotheses that are untested of how you would avoid these heterogeneities,” she said. “It is certainly going to be challenging, but not impossible.”

This story is adapted from material from Princeton University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.