Changing the Static and Dynamic Lattice Effects for the Improvement of the Ionic Transport Properties within the Argyrodite Li₆PS_5–xSe_xI

The lithium argyrodites Li₆PS₅X (X = Cl, Br, and I) have been gaining momentum as candidates for electrolytes in all-solid-state batteries. While these materials have been well-characterized structurally, the influences of the static and dynamic lattice properties are not fully understood. Recent improvements to the ionic conductivity of Li₆PS₅I (which as a parent compound is a poor ionic conductor) via elemental substitutions have shown that a multitude of influences affect the ionic transport in the lithium argyrodites and that even poor conductors in this class have room left for improvement. Here we explore the influence of isoelectronic substitution of sulfur with selenium in Li₆PS_5–xSe_xI. Using a combination of X-ray diffraction, impedance spectroscopy, Raman spectroscopy, and pulse–echo speed of sound measurements, we explore the influence of the static and dynamic lattice on the ionic transport. The substitution of S^2– with Se^2– broadens the diffusion pathways and structural bottlenecks, as well as leading to a softer more polarizable lattice, all of which lower the activation barrier and lead to an increase in the ionic conductivity. This work sheds light on ways to systematically understand and improve the functional properties of this exciting material family.