Substitutional disorder: structure and ion dynamics of the argyrodites Li6PS5Cl, Li6PS5Br and Li6PS5I

For the development of safe and long-lasting lithium-ion batteries we need electrolytes with excellent ionic transport properties. Argyrodite-type Li6PS5X (X: Cl, Br, I) belongs to a family of such a class of materials offering ionic conductivities, at least if Li6PS5Br and Li6PS5Cl 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 Li6PS5Br and Li6PS5Cl show an exceptionally high Li ion conductivity, that of Li6PS5I 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 Li-7 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 Li6PS5I is almost identical to that of its relatives. Considering the substitutional disorder effects in Li6PS5X (X = Br, Cl), we conclude that in structurally ordered Li6PS5I the important inter-cage jump processes are switched off, hindering the ions from taking part in long-range ion transport.