Mechanistic Origin of Superionic Lithium Diffusion in Anion-Disordered Li6PS5X Argyrodites
The rational development of fast–ion-conducting solid electrolytes for all-solid-state lithium-ion batteries requires understanding the key structural and chemical principles that give some materials their exceptional ionic conductivities. For the lithium argyrodites Li<sub>6</sub>PS<sub>5</sub><i>X</i> (<i>X</i> = Cl,Br,I), the choice of the halide, <i>X</i>, strongly affects the ionic conductivity, with room-temperature ionic conductivities for <i>X</i> = {Cl, Br} ×10<sup>3</sup> higher than for <i>X</i> = I. This variation has been attributed to differing degrees of S/<i>X</i> anion disorder. For <i>X</i> = {Cl, Br} the S/<i>X</i> anions are substitutionally disordered, while for <i>X</i> = I the anion sublattice is fully ordered. To better understand the role of substitutional anion disorder in enabling fast lithium-ion transport, we have performed a first-principles molecular dynamics study of Li<sub>6</sub>PS<sub>5</sub>I and Li<sub>6</sub>PS<sub>5</sub>Cl, with varying amounts of S/<i>X</i> anion-site disorder. Considering the S/<i>X</i> substructure as a tetrahedrally close-packed lattice, we identify three partially occupied lithium sites that form a contiguous three-dimensional network of face-sharing tetrahedra. The active lithium-ion diffusion pathways within this network, however, depend on the S/<i>X</i> anion configuration. For anion-disordered systems, the active site–site pathways give a percolating three-dimensional diffusion network; whereas for anion-ordered systems, critical site–site pathways are inactive, giving a disconnected diffusion network with lithium motion restricted to local orbits around S positions. Analysis of the lithium substructure and dynamics in terms of the lithium coordination around each sulfur site shows a mechanistic link between substitutional anion disorder and lithium disorder, which enables fast lithium diffusion. In anion-ordered systems the Li-ions are pseudo-ordered, with preferential 6-fold coordination of sulfur sites. Long-ranged lithium diffusion disrupts this SLi<sub>6</sub> pseudo-ordering, and is therefore disfavoured. In anion-disordered systems, a uniform 6-fold S–Li coordination is frustrated due to Li–Li Coulombic repulsion. Lithium positions become disordered, giving a range of S–Li coordination environments. Long-ranged Li diffusion is now possible with no net change in S–Li coordination numbers. This gives rise to superionic lithium transport in the anion-disordered systems, which is effected by a concerted string-like diffusion mechanism.
