Decoding Structural Disorder, Synthesis Methods, and Short- and Long-Range Lithium-Ion Transport in Lithium Argyrodites (Li_6–<i>x</i>PS_5–<i>x</i>Br_1+<i>x</i>)

By varying the bromine content and cooling method, we are able to induce site disorder in the Li_6-<i>x</i> PS_5-<i>x</i> Br_1+<i>x</i> (<i>x</i> = 0, 0.3, 0.5) system via two routes, allowing us to disentangle the impact of site disorder and chemical composition on conductivity. Through solid-state nuclear magnetic resonance (NMR), we can explore the chemical environment as well as short-range lithium-ion dynamics and compare these to results obtained from neutron diffraction and electrochemical impedance spectroscopy (EIS). We find that the cooling method has a profound effect on the ⁷Li and ³¹P environment that cannot be explained through 4<i>d</i> site disorder alone. The configurational entropy (<i>S</i> _conf) is used as a more complete descriptor of structural disorder and linked to distortions in both the phosphorus and lithium environment. These distortions are correlated to increased intercage movement through ⁷Li <i>T</i> ₁ spin-lattice relaxation (SLR) NMR. Further analysis of the prefactors obtained from SLR NMR and EIS allows us to obtain the migrational entropy (Δ<i>S</i> _m). For short-range SLR movement, the Δ<i>S</i> _m correlates well with <i>S</i> _conf, implying that increased intercage movement is related to distortion of the lithium cages as well as a decrease of the intercage distance. Comparison to EIS shows that an increase in short-range movement translates into increased long-range movement in a straightforward manner for slow-cooled samples. However, for quench-cooled samples, this correlation is lost. Lattice softness and phonon-ion interactions are suggested to play an important role in long-range conduction which only becomes apparent when chemical composition and disorder are disentangled. This work shows that by altering one synthesis step, the relationship between site-occupancy-based descriptors (site disorder or <i>S</i> _conf) and lithium dynamics is changed profoundly. Furthermore, it shows that chemical composition and descriptors of site disorder cannot be seen as one and the same, as both play a role that changes with the length scale probed. Finally, it challenges the implicit assumption that increased short-range diffusivity automatically results in increased long-range diffusivity.