Substitutional disorder: Structure and ion dynamics of the argyrodites Li ₆ PS ₅ Cl, Li ₆ PS ₅ Br and Li ₆ PS ₅ I

For the development of safe and long-lasting lithium-ion batteries we need electrolytes with excellent ionic transport properties. Argyrodite-type Li ₆ PS ₅ X (X: Cl, Br, I) belongs to a family of such a class of materials offering ionic conductivities, at least if Li ₆ PS ₅ Br and Li ₆ PS ₅ Cl are considered, in the mS cm ^-1 range at room temperature. Although already tested as ceramic electrolytes in battery cells, a comprehensive picture about the ion dynamics is still missing. While Li ₆ PS ₅ Br and Li ₆ PS ₅ Cl show an exceptionally high Li ion conductivity, that of Li ₆ PS ₅ I with its polarizable I anions is by some orders of magnitude lower. This astonishing effect has not been satisfactorily understood so far. Studying the ion dynamics over a broad time and length scale is expected to help shed light on this aspect. Here, we used broadband impedance spectroscopy and ⁷ Li NMR relaxation measurements and show that very fast local Li ion exchange processes are taking place in all three compounds. Most importantly, the diffusion-induced NMR spin-lattice relaxation in Li ₆ PS ₅ I is almost identical to that of its relatives. Considering the substitutional disorder effects in Li ₆ PS ₅ X (X = Br, Cl), we conclude that in structurally ordered Li ₆ PS ₅ I the important inter-cage jump processes are switched off, hindering the ions from taking part in long-range ion transport.