Probing the Lithium Substructure and Ionic Conductivity of the Solid Electrolyte Li₄PS₄I

In search of high-performance solid electrolytes, various materials have been discovered in the past, approaching or even exceeding the ionic conductivity of conventional liquid electrolytes. Among the reported classes of superionic electrolytes for solid-state battery applications, lithium thiophosphates appear to be the most promising owing to their high ionic conductivity and mechanical softness. A recent example is the Li4PS4I phase (P4/nmm). Surprisingly, this material shows a comparatively low ionic conductivity at room temperature ranging from 10-4 to 10-5 S cm-1 despite having favorable structural characteristics. Because of discrepancies between experiment and theory regarding the Li-ion conductivity and polymorphism in Li4PS4I, we herein examine the crystal structure over a broad temperature range using ex situ and in situ X-ray and neutron powder diffraction techniques. We demonstrate the absence of polymorphic transitions, with a lithium redistribution at low temperatures though, and confirm the relatively poor room-temperature ionic conductivity despite the presence of a three-dimensional (3D) percolation network for facile charge transport.