Further Evidence for Energy Landscape Flattening in the Superionic Argyrodites Li_{6+ x}P_{1- x}M_xS₅I (M = Si, Ge, Sn)

All-solid-state batteries are promising candidates for next-generation energy-storage devices. Although the list of candidate materials for solid electrolytes has grown in the past decade, there are still many open questions concerning the mechanisms behind ionic migration in materials. In particular, the lithium thiophosphate family of materials has shown very promising properties for solid-state battery applications. Recently, the Ge-substituted Li₆PS₅I argyrodite was shown to be a very fast Li-ion conductor, despite the poor ionic conductivity of the unsubstituted Li₆PS₅I. Therein, the conductivity was enhanced by more than 3 orders of magnitude due to the emergence of I^-/S^2- exchange, i.e., site disorder, which led to a sudden decrease of the activation barrier with a concurrent flattening of the energy landscapes. Inspired by this work, two series of elemental substitutions in Li_6+xP_1-xM_xS₅I (M = Si and Sn) were investigated in this study and compared to the Ge analogue. A sharp reduction in the activation energy was observed at the same M⁴⁺/P⁵⁺ composition as previously found in the Ge analogue, suggesting a more general mechanism at play. Furthermore, structural analyses with X-ray and neutron diffraction indicate that similar changes in the Li sublattice occur despite a significant variation in the size of the substituents, suggesting that in the argyrodites the lithium substructure is most likely influenced by the occurring Li⁺-Li⁺ interactions. This work provides further evidence that the energy landscape of ionic conductors can be tailored by inducing local disorder.