Recently, the drive toward both safer and higher energy density storage has motivated an increasing focus on all-solid-state batteries, wherein the solid electrolyte is anticipated to preclude dendrite formation leading to electrical shorting and is furthermore non-flammable. If realized, these advantages could significantly improve battery safety and enable use of higher energy density electrodes.

Crystalline and amorphous sulfide electrolytes (e.g., Li₂S–P₂S₅ or LPS) have now been widely reported to have Li-ion conductivity near room temperature that is high enough (>10⁻⁴ S cm⁻¹) to warrant consideration as the basis for a new class of solid-state batteries. A key concern in these and other solid electrolytes, however, is their mechanical stability in the presence of strains in the adjacent electrode materials accompanying reversible Li storage (intercalation or alloying) that may vary from a few percent by volume up to a factor of three (e.g., in the case of silicon anodes).

Sulfide-based electrolytes have remarkably lower Young’s modulus (≈20 GPa) than many of these active materials (e.g., 100–200 GPa), as well as oxide-based solid electrolytes such as the garnets … which initially suggested to us that the sulfides might exhibit superior strain-accommodation characteristics in solid-state batteries. However, a detailed understanding of elastoplastic and fracture properties, which has heretofore been lacking, is required to draw clear conclusions of material design and selection for sulfide solid electrolytes.

—McGrogan et al.

Until now, the sulfide’s extreme sensitivity to normal lab air has posed a challenge to measuring mechanical properties including its fracture toughness. To circumvent this problem, members of the 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.

Previous researchers 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. But 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).

While other research groups have measured the elastic properties of the sulfide-based solid electrolytes, they had yet to measure the fracture properties, said Professor Krystyn Van Vliet, the corresponding author. The latter properties are crucial for predicting whether the material might crack or shatter when used in a battery application.

The researchers found that when subjected to stress, the material can deform easily—but at sufficiently high stress it can crack like a brittle piece of glass.

Knowing those properties in detail allows engineers to calculate how much stress the material can tolerate before it fractures, and design battery systems with that information in mind, Van Vliet said.

… we determined the Young’s modulus, hardness, and fracture toughness of glassy Li2_S–P₂S₅ solid electrolyte of 70:30 composition to be 18.5 ± 0.9 GPa, 1.9 ± 0.2 GPa, and 0.23 ± 0.04 MPa m^1/2, respectively, via indentation-based methods that maximized phase stability of the LPS sample. These results show that this LPS material—and by inference other solid electrolytes in the solid sulfide family—are distinguished as compliant yet significantly more brittle than crystalline oxide electrolytes considered for the same applications. Although the low stiffness of LPS suggests a capability of this solid electrolyte material to accommodate elastic mismatch with adjacent phases such as storage electrodes and current collectors in a solid-state battery, this capability is compromised by the low fracture toughness and corresponding high sensitivity to preexisting or cycling-generated flaws.

—McGrogan et al.

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

Resources

• F. P. McGrogan, T. Swamy, S. R. Bishop, E. Eggleton, L. Porz, X. Chen, Y.-M. Chiang, K. J. Van Vliet (2017) “Compliant Yet Brittle Mechanical Behavior of Li₂S–P₂S₅ Lithium-Ion-Conducting Solid Electrolyte” Adv. Energy Mater. 2017 doi: 10.1002/aenm.201602011