Hidden flaw – unlocking better batteries for electric vehicles

Fixing a hidden defect could lead to improved batteries for electric cars.

Compared to traditional lithium-ion batteries, solid-state batteries provide faster charging, greater range and longer life, and can play a key role in electric vehicles. However, solid-state batteries are prone to failure due to existing methods of manufacturing and processing materials. Now researchers have discovered a hidden flaw that causes the failures. The next stage is to develop materials and manufacturing processes that take these shortcomings into account and create next-generation batteries.

Unlike traditional lithium-ion batteries, which have charged particles called ions moving in a liquid, solid-state batteries have ions that travel through the battery inside a solid material. The new research shows that while solid-state cells have advantages, local variations or small defects in the solid material can cause the battery to short out or wear out.

“The single material is important,” said lead researcher Kelsey Hatzel, assistant professor of mechanical and aerospace engineering and the Andlinger Center for Energy and the Environment. “You want ions to move at the same speed at every point in space.”

Hatzell and coauthors described how they used high-tech instruments at Argonne National Laboratory to inspect and track nanoscale changes in the battery’s material as they charge and discharge it in a paper recently published in Natural materials. The team of researchers from Princeton University, Vanderbilt University, Argonne National Laboratory and Oak Ridge National Laboratory analyzed crystal grains in the battery’s solid electrolyte, the main part of the battery through which electrical charge flows. By moving ions faster in one area of ​​the battery than in another, the researchers concluded that the irregularities between the grains could accelerate the battery’s failure. Changing materials handling and manufacturing methods can help solve battery reliability issues.

Batteries store electrical energy in materials that make up their electrodes: the anode (the end of the battery, marked with a minus sign) and the cathode (the end of the battery, marked with a plus sign). When a battery discharges energy to power a car or smartphone, charged particles (called ions) move through the battery to the cathode (+ end). The electrolyte, solid or liquid, is the path through which ions travel between the anode and the cathode. Without an electrolyte, ions cannot move and store energy at the anode and cathode.

In a solid-state battery, the electrolyte is usually either ceramic or solid glass. Solid-state batteries with solid electrolytes can enable the use of materials with higher energy density (eg lithium metal) and make batteries lighter and smaller. Weight, volume and charging capacity are key factors for transportation applications such as electric vehicles. Solid-state batteries are also supposed to be safer and less prone to fires than other forms.

Engineers have known that solid-state batteries tend to fail in the electrolyte, but the failures seem to occur randomly. Hatzell and co-researchers suspected that the failures might not be random, but actually caused by changes in the crystalline structure of the electrolyte. To investigate this hypothesis, the researchers used the synchrotron at Argonne National Laboratory to produce powerful X-rays that allowed them to look inside the battery as it worked. They combined X-ray imaging and high-energy diffraction techniques to probe the crystal structure of a garnet electrolyte at the angstrom scale, roughly the size of a single[{” attribute=””>atom. This allowed the researchers 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 different 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 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.”

Hatzell said 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.”

Reference: “Polymorphism of garnet solid electrolytes and its implications for grain-level chemo-mechanics” by Marm B. Dixit, Bairav S. Vishugopi, Wahid Zaman, Peter Kenesei, Jun-Sang Park, Jonathan Almer, Partha P. Mukherjee, and Kelsey B. Hatzell, 1 September 2022, Nature Materials.
DOI: 10.1038/s41563-022-01333-y

The study was funded by the National Science Foundation and the US Department of Energy.