Chemical and Electrochemical Characterization of Hot‐Pressed Li6PS5Cl Solid State Electrolyte: Operating Pressure‐Invariant High Ionic Conductivity

Abstract Sulfide solid state electrolytes (SSE) are among the most promising materials in the effort to replace liquid electrolytes, largely due to their comparable ionic conductivities. Among the sulfide SSEs, Argyrodites (Li 6 PS 5 X, X=Cl, Br, I) further stand out due to their high theoretical ionic conductivity (~1×10 −2 S cm −1 ) and interfacial stability against reactive metal anodes such as lithium. Generally, solid state electrolyte pellets are pressed from powder feedstock at room temperature, however, pellets fabricated by cold pressing consistently result in low bulk density and high porosity, facilitating interfacial degradation reactions and allowing dendrites to propagate through the pores and grain boundaries. Here, we demonstrate the mechanical and electrochemical implications of hot‐pressing standalone LPSCl SSE pellets with near‐theoretical ionic conductivity, superior cycling performance, and enhanced mechanical stability. X‐ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and x‐ray diffraction spectroscopy (XRD) analysis reveal no chemical changes to the Argyrodite surface after hot pressing up to 250 °C. Moreover, we use electrochemical impedance spectroscopy (EIS) to understand mechanical stability of Argyrodite SSE pellets as a function of externally applied pressure, demonstrating for the first time pressed standalone Argyrodite pellets with near‐theoretical conductivities at external pressures below 14 MPa.