Hot Pressing Argyrodite Solid Electrolyte Powders Results in >2 mS cm^-1 Ionic Conductivity at 20 °C and <1 MPa Operating Pressure

The formation of Li₆PS₅Cl argyrodite solid electrolyte pellets typically involves compaction at ∼20 °C and hundreds of megapascal of pressure, and the resulting pellets usually need >10 MPa operating pressure to achieve ionic conductivities >1 mS cm^-1 at 25 °C and/or sputtered metal electrodes. This work demonstrates a key advance achieved with pellet fabrication at 150 °C and 300 MPa with foil electrodes: >2 mS cm^-1 ionic conductivity at 20 °C with <1 MPa operating pressure. Scanning electron microscopy reveals fused grains present in samples pressed at 150 °C but not in those at 20 °C. X-ray photoelectron spectroscopy and diffraction analysis show no significant difference in crystal structure or surface composition between 150 and 20 °C pressed samples, and the pellet densities are nearly identical. The ionic conductivity of 150 °C pressed samples is nearly invariant with operating pressure, while that at 20 °C has a strong operating pressure dependence. Nanoindentation on pellet surfaces shows a higher elastic modulus for the 150 vs 20 °C pellets. Overall, these results suggest that fabrication at 150 °C results in grain-grain fusion and motivate further study of the fabrication parameter space (e.g., pressure, temperature, time, and contacts) to find routes to <1 MPa operation of argyrodite structures.