For the first time, a team of researchers at the Massachusetts Institute of Technology (MIT) (Cambridge, Massachusetts) has probed the mechanical properties of a sulfide-based solid electrolyte material—seeking to determine its mechanical performance when incorporated into batteries.
Most batteries are composed of two solid, electrochemically active layers called electrodes, separated by a polymer membrane infused with a liquid or gel electrolyte. But recent research has explored the possibility of all-solid batteries, in which the liquid (and potentially flammable) electrolyte would be replaced by a solid electrolyte. That could enhance the energy density and safety of a battery.
Lithium-ion batteries have provided a lightweight energy-storage solution that has enabled many modern devices, from smartphones to electric cars. But substituting the conventional liquid electrolyte with a solid electrolyte could have significant advantages. For example, all-solid lithium-ion batteries could provide greater energy storage ability at the battery pack level, the researchers explain. They could also eliminate the risk of tiny, finger-like metallic projections called dendrites that can grow through the electrolyte layer and lead to short-circuits.
“Batteries with components that are all solid are attractive options for performance and safety, but several challenges remain,” says Krystyn Van Vliet, a professor leading the research.
In the lithium-ion batteries on the market today, lithium ions pass through a liquid electrolyte to get from one electrode to the other while the battery is being charged. They then flow through in the opposite direction while being used.
These batteries are very efficient, but “the liquid electrolytes tend to be chemically unstable, and can even be flammable,” Van Vliet says. “So if the electrolyte was solid, it could be safer, as well as smaller and lighter.”
But the key question regarding the use of all-solid batteries is what kinds of mechanical stresses might occur within the electrolyte material as the electrodes charge and discharge repeatedly. This cycling causes the electrodes to swell and contract as lithium ions pass in and out of their crystal structure.
In a stiff electrolyte, those dimensional changes can lead to high stresses. If the electrolyte is also brittle, that 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.
But if the material is resistant to fracture, those stresses could be accommodated without rapid cracking.
In the past, the sulfide’s extreme sensitivity to normal lab air has posed a challenge to measuring its mechanical properties, including fracture toughness. To circumvent this problem, members of the MIT research team conducted the mechanical testing in a bath of mineral oil, protecting the sample from any chemical interactions with air or moisture. Using that technique, they were able to obtain detailed measurements of the mechanical properties of the lithium-conducting sulfide, considered a promising candidate for electrolytes in all-solid batteries.
“There are a lot of different candidates for solid electrolytes out there,” says Frank McGrogan, an MIT graduate student involved with the research.
Other groups have studied the mechanical properties of lithium-ion conducting oxides, but there had been little work so far on sulfides, the researchers say. However, sulfides are among the most promising, the researchers note, because of their ability to conduct lithium ions easily and quickly.
Other researchers have used acoustic measurement techniques, passing sound waves through the material to probe its mechanical behavior. But that method does not quantify the resistance to fracture, the MIT researchers explain.
On the other hand, 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. These properties include 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 says. The latter are crucial for predicting whether the material might crack or shatter when used in a battery application, the MIT team explains.
In their research, the team found that the material has a combination of properties somewhat similar to silly putty or salt water taffy. When subjected to stress, it can deform easily. But at sufficiently high stress, it can crack like a brittle piece of glass.
By knowing those properties in detail, “you can calculate how much stress the material can tolerate before it fractures,” Van Vliet says, adding that developers could then design battery systems with that information in mind.
The material is more brittle than would be ideal for battery use, the researchers note. But as long as its properties are known and systems designed accordingly, it could still have potential for such uses, McGrogan says. “You have to design around that knowledge.”
“The cycle life of lithium-ion batteries is primarily limited by the chemical/electrochemical stability of the liquid electrolyte and how it interacts with the electrodes,” says Jeff Sakamoto, a professor of mechanical engineering at the University of Michigan (Ann Arbor, Michigan), who was not involved in this work. “However, in solid-state batteries, mechanical degradation will likely govern stability or durability. Thus, understanding the mechanical properties of solid-state electrolytes is very important.”
“Lithium metal anodes exhibit a significant increase in capacity compared to state-of-the-art graphite anodes,” Sakamoto adds. “This could translate into about a 100% increase in energy density compared to [conventional] lithium-ion technology.”
The research work was supported by the U.S. Department of Energy (Washington, DC). For more information,
visit MIT’s web site.