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MIT Researchers Explore Lithium Sulfides to Create All-Solid Lithium Batteries

Majority of batteries are made up of two solid, electrochemically active layers known as electrodes, divided by a polymer membrane infused with a gel or liquid electrolyte. A recent study has investigated the possibility of having all-solid-state batteries, in which the liquid (which is potentially flammable) electrolyte could be substituted by a solid electrolyte, which in turn could improve the safety and energy density of the batteries.

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. (Courtesy of the researchers)

For the first time, a research team at MIT have investigated the mechanical properties of a sulfide-based solid electrolyte material, to establish its mechanical performance when added into batteries.

Their research findings were published in the journal Advanced Energy Materials, in a paper authored by Frank McGrogan and Tushar Swamy, both MIT graduate students; Krystyn Van Vliet, the Michael (1949) and Sonja Koerner Professor of Materials Science and Engineering; Yet-Ming Chiang, the Kyocera Professor of Materials Science and Engineering; and four others including an undergraduate participant in the National Science Foundation Research Experience for Undergraduate (REU) program administered by MIT’s Center for Materials Science and Engineering and its Materials Processing Center.

Lithium-ion batteries have offered a lightweight energy-storage option that has enabled many of today's high-tech devices, ranging from smartphones to electric cars. However, using a solid electrolyte instead of the conventional liquid electrolyte in such batteries could have major benefits. This all-solid-state lithium-ion batteries could offer even better energy storage ability at the battery pack level. Furthermore, they may nearly eliminate the hazard of miniature, fingerlike metallic projections known as dendrites that can grow through the electrolyte layer and cause short-circuits.

Batteries with components that are all solid are attractive options for performance and safety, but several challenges remain.

Van Vliet

In the lithium-ion batteries that dominate the market currently, lithium ions pass via a liquid electrolyte to travel from one electrode to the other while the battery is being charged, and then travel through in the opposite direction when it is being used. These batteries are highly efficient, but “the liquid electrolytes tend to be chemically unstable, and can even be flammable,” she says. “So if the electrolyte was solid, it could be safer, as well as smaller and lighter.”

However answers are needed for the big question - what kinds of mechanical stresses might arise within the electrolyte material in all-solid batteries as the electrodes charge and discharge frequently. This cycling results in the electrodes swelling and contracting as the lithium ions travel in and out of their crystal structure. In a solid electrolyte, those dimensional alterations can cause high stresses. In case the electrolyte is also fragile, that continuous changing of dimensions can cause cracks that swiftly degrade battery performance, and could even provide opportunities for damaging dendrites to develop, as seen in liquid-electrolyte batteries. However if the material is resistant to fracture, those stresses can be contained without rapid cracking.

So far, though, high sensitivity of the sulfide to normal lab air has created a hurdle to measuring mechanical properties such as its fracture toughness. To get around this problem, the MIT team performed the mechanical testing in a bath of mineral oil, shielding the sample from any chemical interactions with moisture or air. Applying that method, they were able to get comprehensive measurements of the mechanical properties of the lithium-conducting sulfide, which is considered a potential candidate for electrolytes in all-solid-state batteries.

There are a lot of different candidates for solid electrolytes out there.

McGrogan

Other teams have explored the mechanical properties of lithium-ion conducting oxides, but only little work has been carried out with regard to sulfides, in spite of them being especially promising due to their ability to conduct lithium ions quickly and easily.

Earlier researchers used acoustic measurement methods, passing sound waves via the material to investigate its mechanical behavior, but that technique does not quantify the resistance to fracture. But the recent study, which used a fine-tipped probe to poke into the material and observe its reactions, offers a more comprehensive picture of the key properties, including fracture toughness, hardness, and Young’s modulus (a measure of a material’s ability to stretch reversibly when exposed to applied stress).

Research groups have measured the elastic properties of the sulfide-based solid electrolytes, but not fracture properties.

Van Vliet

The latter are vital for estimating if the material might crack or break when applied in a battery application. The researchers discovered that the material has a combination of properties fairly similar to salt water taffy or silly putty: When exposed to stress, it can deform easily, but at adequately high stress it can crack like a fragile piece of glass.

By understanding those properties in detail, “you can calculate how much stress the material can tolerate before it fractures,” and engineer battery systems keeping in mind that information, Van Vliet says.

The material looks more brittle than would be suitable for battery use, but given that its properties are identified and systems designed in view of that, it could still have potential for such uses, McGrogan says. “You have to design around that knowledge.”

“The cycle life of state-of-the-art Li-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, who was not involved in this research. “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,” he says.

Lithium metal anodes exhibit a significant increase in capacity compared to state-of-the-art graphite anodes. This could translate into about a 100 percent increase in energy density compared to [conventional] Li-ion technology.

Sakamoto

The research team also included MIT researchers Sean Bishop, Erica Eggleton, Lukas Porz, and Xinwei Chen. The work was supported by the U.S. Department of Energy’s Office of Basic Energy Science for Chemomechanics of Far-From-Equilibrium Interfaces.

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