Insights into Li5.5PS4.5Cl1.5 Formation Based on Precursor Mixing: In Situ Monitoring via Neutron Powder Diffraction

Solid electrolytes are central to the development of solid-state batteries. Beyond the search for further high-conductivity compositions, the synthesis pathway can shape the structures and transport properties of the resulting material. Two scalable precursor-mixing strategies, planetary ball milling and cutting mill mixing, are compared for the formation of chloride-rich argyrodite Li5.5PS4.5Cl1.5. In situ neutron powder diffraction tracks the reaction progress, complemented by ex situ neutron and X-ray powder diffraction, differential scanning calorimetry, and impedance spectroscopy. The argyrodite phase in both products converges to the same final composition, but the two routes follow different reaction trajectories: ball-milled precursors react at lower temperatures and shorter dwell times than cutting mill precursors, which require higher temperatures and longer times to reach comparable chloride contents. The chloride content within the argyrodite phase is identified as the principal compositional descriptor of the ionic conductivity across the reaction conditions studied here, and extended annealing beyond complete reaction leads to chloride loss and reduced transport. These insights transfer to a reproducible large-scale synthesis under ambient-pressure gas-flow conditions, linking in situ mechanistic understanding to the scalable preparation of solid electrolytes.