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

Abstract

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 ⁷Li 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.