Using specialized equipment, the MIT team did tests in which they used a pyramidal-tipped probe to indent the surface of a piece of the sulfide-based material. Surrounding the resulting indentation (seen at center), cracks were seen forming in the material (indicated by arrows), revealing details of its mechanical properties. Credit: The Massachusetts Institute of Technology

All-solid-state batteries could eventually replace the common lithium-ion battery made with liquid electrolytes, enhancing the batteries’ energy density and safety.

Researchers from the Massachusetts Institute of Technology have examined the mechanical properties of a sulfide-based solid electrolyte material to determine its mechanical performance when incorporated into batteries. Their findings were published this week in the journal Advanced Energy Materials.

The majority of batteries are comprised of two solid, electrochemically active layers called electrodes, separated by a polymer membrane infused with a liquid or gel electrolyte.

While lithium-ion batteries are a lightweight energy-storage solution that has enabled many high-tech devices, substituting the conventional liquid electrolyte with a solid electrolyte could provide an even greater energy storage ability pound for pound at the battery pack level. It may also virtually eliminate the risk of tiny, fingerlike metallic projections called dendrites that can grow through the electrolyte layer and lead to short-circuits.

More advancements are still needed before the battery can be implemented on an industry level, said study author Krystyn J. Van Vliet, Ph.D, professor of Material Science and Engineering and Biological Engineering at MIT.

“Batteries with components that are all solid are attractive options for performance and safety but several challenges remain,” Van Vliet said in a statement.

In the current lithium-ion batteries, the lithium ions pass through a liquid electrolyte to get from one electrode to the other while the battery is being charged, and then flow through in the opposite direction as it is being used.

While the batteries are efficient, the liquid electrolytes tend to be chemically unstable and can even become flammable, while the solid electrolyte could be safer as well as smaller and lighter.

There are questions as to what kind of mechanical stresses may occur in the all-solid batteries within the electrolyte material as the electrodes charge and discharge repeatedly.

The cycling causes the electrodes to swell and contract as the lithium ions pass in and out of their crystal structure.

In a stiff electrolyte the dimensional charges can lead to high stress and if the electrolyte is also brittle the constant changing of dimensions can lead to cracks that rapidly degrade battery performance and could even provide channels for damaging dendrites to form as they do in liquid-electrolyte batteries.

However, if the material is resistant to fracture then those stresses could be accommodated without rapid cracking.

Until now the sulfide’s extreme sensitivity to normal lab air made it challenging to measure mechanical properties, including its fracture toughness.

The researchers at MIT were able to conduct the mechanical testing in a bath of mineral oil, which protected the sample from any chemical interactions with air or moisture and enabled the researchers to obtain detailed measurements of the mechanical properties of the lithium-conducting sulfide.

“There are a lot of different candidates for solid electrolytes out there,” MIT graduate student Frank McGrogan and study author said in a statement.

Previous researchers have used acoustic measurement techniques, passing sound waves through the material to probe its mechanical behavior, but the method does not quantify the resistance to fracture.

However, in the new study, which used a fine-tipped probe to poke into the material and monitor its responses, gives a more complete picture of the important properties including hardness, fracture toughness and Young’s modulus—a measure of a material’s capacity to stretch reversibly under an applied stress.

“Research groups have measured the elastic properties of the sulfide-based solid electrolytes, but not fracture properties,” Van Vliet said.