Examining Electrochemical and Mechanical Behavior of Amorphous and Crystalline Sulfide-Based Solid-State Electrolytes

Abstract

Traditional liquid electrolytes used in lithium-metal batteries have severe safety issues, poor power density, as well as thermal and electrochemical instability that prevent scaling to newer applications, such as electric vehicles. Sulfide-based solid-state electrolytes (SSEs) are viable for addressing many of the issues seen with liquid electrolytes. However, one of the key hindrances to the use of sulfide-based SSEs is the understanding of repeatable processing of highly conductive, stable material. There are limitations in producing electrolyte with known defect types and density, as well as degree of crystallinity. The degree of crystallinity greatly effects electrochemical properties, such as ionic conductivity, due to the change in ionic pathways through the electrolyte. However, other hindrances to SSE capabilities, such as interfacial stability and mechanical integrity, are not noted as a function of crystallinity. Here, multiple forms of characterization were utilized to confirm structural information on dry ball milled Li7P3S11 (LPS) SSEs. A range of crystallinity of pelletized electrolytes were then analyzed with different electrochemical and mechanical tests to evaluate the viability of SSEs in operation. There is particular interest in the near-amorphous regime of crystallinity, where minimal differences are seen in electrochemical capabilities, though there is uncertainty in the possible benefits observed for interfacial compatibility and in mechanical properties. Electrochemical impedance spectroscopy (EIS) and symmetric cell cycling were utilized to examine the bulk and interfacial capabilities of electrolytes, all operated under various loads with a specialty designed load cell. Mechanical properties were analyzed with mechanical testing on different scales, from bulk loading down to atomic force microscopy mapping and nanoindentation testing. With this wide array of characterizations, a wholistic understanding of slightly varied crystallinity of sulfide-based SSE is accomplished.