Mechanical Properties of Sulfide-Type Solid Electrolytes Analyzed by Indentation Methods

For the development of an all-solid-state battery, it is necessary to understand the mechanical properties of a solid electrolyte because they directly affect the all-solid-state battery performance, in terms of designing the electrode composites and a separator layer to prevent lithium dendrite formation. Sulfide-type solid electrolytes are promising candidates for the realization of such batteries due to their superior mechanical properties compared to oxide-type solid electrolytes. However, they are unstable in an ambient atmosphere. Hence, only a few studies have been reported on the mechanical properties of sulfide materials due to the difficulties encountered when conducting experiments. Herein, the mechanical properties of sulfide-type (75Li2S·25P2S5) and oxide-type (Li0.33La0.57TiO3, Li1+x+yAlx(Ti,Ge)2–xSiyP3–yO12) solid electrolytes were systematically studied through indentation methods for the first time. The Meyer hardness and the elastic modulus of hot-pressed pellets of the 75Li2S·25P2S5 solid electrolyte were higher than those of cold-pressed pellets due to their higher density. These results confirmed that the intrinsic mechanical properties of sulfide-type solid electrolytes could be analyzed by indentation methods with high-density pellets. In addition, the intrinsic Meyer hardness and the elastic modulus of 75Li2S·25P2S5 solid electrolytes were lower than those of Li0.33La0.57TiO3 and Li1+x+yAlx(Ti,Ge)2–xSiyP3–yO12. Therefore, this study provides a foundation for the fabrication of all-solid-state batteries from the viewpoint of the mechanical properties of solid electrolytes.