Correlating Structural Disorder to Li⁺ Ion Transport in Li_4–xGe_1–xSb_xS₄ (0 ≤ x ≤ 0.2)

Strong compositional influences are known to affect the ionic transport within the thio-LISICON family; however, a deeper understanding of the resulting structure–transport correlations has up until now been lacking. Employing a combination of high-resolution neutron diffraction, impedance spectroscopy, and nuclear magnetic resonance spectroscopy, together with bond valence site energy calculations and the maximum entropy method for determining the underlying Li+ scattering density distribution of a crystal structure, this work assesses the impact of the Li+ substructure and charge carrier density on the ionic transport within the Li4–xGe1–xSbxS4 substitution series. By incorporating Sb5+ into Li4GeS4, an anisometric expansion of the unit cell is observed. An additional Li+ position is found as soon as (SbS4)3– polyhedra are present, leading to a better local polyhedral connectivity and a higher disorder in the Li+ substructure. Here, we are able to relate structural disorder to an increase in configurational entropy, together with a 2 order-of-magnitude increase in ionic conductivity. This result reinforces the typically believed paradigm that structural disorder leads to improvements in ionic transport.