Mechanical Milling – Induced Microstructure Changes in Argyrodite Li₆PS₅Cl Solid-State Electrolyte and Influence on Electrochemical Performance

In the early stages of sulfide solid-state electrolyte (SSE) research, sulfides like Li₆PS₅Cl (LPSCl) were primarily synthesized in laboratories using fine precursors such as Li₂S, LiCl, and P₂S₅, employing meticulous synthetic procedures involving milling and sintering. This approach resulted in grams of as-synthesized SSE materials with a uniform particle size distribution and well-controlled morphology. Consequently, large-scale synthesis methods for LPSCl SSE have been developed, leading to the availability of commercial LPSCl SSE. Nonetheless, compared to lab-scale synthesis, commercial production of LPSCl SSE often yields a wide range of particle sizes and particle distributions. In-depth understanding is needed regarding how microstructural features such as the average particle size and distribution, and pore size and distribution, affect the compressed SSE's electrochemical performance.<br/><br/>In this presentation, we investigate mechanical milling – induced microstructure changes of LPSCl SSE and their influence on electrochemical performance. Planetary mechanical milling in wet media (m-xylene) is employed to alter commercial LPSCl powder. Quantitative stereology demonstrates how extended milling progressively refines grain and pore size/distribution, increases compact density, and geometrically smoothens the SSE-Li interface. Mechanical indentation demonstrates that these changes lead to reduced site-to-site variation in the compact's hardness. Microstructure, in turn, profoundly influences electrochemical behavior. For example, symmetric cells with 8 and 24-hour milled electrolytes remain stable after 190 cycles at 1 mA cm^-2/2 mAh cm^-2, while the unmilled baseline cells short-circuit during initial activation. The Li/Ni asymmetric half-cells with milled electrolytes allow for stable electrodeposition of approximately 20 mAh cm^-2 (100 μm Li thickness), while the baseline shorts at 1.2 mAh cm^-2. Combined cryogenic focused ion beam (cryo-FIB) and X-ray photoelectron spectroscopy (XPS) demonstrate that milled microstructures promote uniform early-stage electrodeposition on foil collectors and stabilize solid electrolyte interphase (SEI) reactivity. Mesoscale modeling reveals the relationship between Li-SSE interface morphology and the onset of electrochemical instability, based on underlying reaction current distribution.