Mechanistic Origin of Superionic Lithium Diffusion in Anion-Disordered Li6PS5X Argyrodites

Abstract

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.

On the Local Charge Inhomogeneity and Lithium Distribution in the Superionic Argyrodites Li6PS5X (X = Cl, Br, I)

“…The 132 tetrahedral interstices can be classified into five types (Figure 1), based on the number of corners or edges each tetrahedron shares with neighboring rigid PS4 3− units. 34,42 Previous neutron diffraction studies of Li6PS5X have assigned lithium ions as only occupying type 5 sites (Wyckoff 48h), with observations of a "smeared out" lithium density usually interpreted as some lithium occupying an additional "site", here denoted as type 5a (Wyckoff 24g), located at the shared face of adjacent type 5 tetrahedra pairs. 9,34 The type 5 and type 5a sites together form cage-like geometries, centered around the nominal S 2− positions on Wyckoff 4d (Figure 4b).…”

“…38,42 The role of non-type 5 tetrahedral sites in any contiguous lithium-diffusion pathways through the argyrodite lattice has previously been highlighted by analysis of bond valence calculations, 40,44,45 and molecular dynamics simulations. 42,46 Lithium occupation of non-type 5 sites has also been experimentally observed in "Li-excess" argyrodites with lithium stoichiometries x(Li) > 6, 33,47,48 such as Li6+xSb1-xSixS5I together with the halide-free compositions Li6.15Mʹ1.5S6 and the related oxysulfides Li6.15Mʹ1.5S5.4O0.6 (Mʹ = Al0.1Si0.9), and it has been proposed that occupation of non-type 5 sites is a direct consequence of these x(Li) > 6 stoichiometries. 33 Furthermore, Li-excess systems, such as Li6+xP1-xGexS5I and Li6+xSb1-xSixS5I, exhibit some of the highest reported roomtemperature ionic conductivities within the lithium-argyrodite family.…”

“…While this model is consistent with previous neutron diffraction analyses, 9,[24][25][26][27][32][33][34][35]40 recent first principles molecular dynamics simulations of Li6PS5X (X = I, Cl) have predicted that non-type 5 sites may be significantly occupied even for these x(Li) = 6 systems. 42 These simulations also predict that the proportion of lithium occupying non-type 5 sites increases with the degree of X − /S 2− site exchange, while the relationship between the proportion of lithium occupying non-type 5 sites and lithium diffusion is more complex: systems with 100 % X − /S 2− site-inversion exhibited the highest proportion of lithium in nontype 5 sites, but had poor lithium diffusion comparable to fully-ordered Li6PS5X. 42 Motivated by apparent contradiction between this theoretical study and existing experimental data, and to better understand the relationships between chemical composition, degree of site disorder, and lithium distribution, we have performed an experimental analysis of the lithium substructure in the Li6PS5X (X = Cl, Br, I) lithium argyrodites.…”

“…42 These simulations also predict that the proportion of lithium occupying non-type 5 sites increases with the degree of X − /S 2− site exchange, while the relationship between the proportion of lithium occupying non-type 5 sites and lithium diffusion is more complex: systems with 100 % X − /S 2− site-inversion exhibited the highest proportion of lithium in nontype 5 sites, but had poor lithium diffusion comparable to fully-ordered Li6PS5X. 42 Motivated by apparent contradiction between this theoretical study and existing experimental data, and to better understand the relationships between chemical composition, degree of site disorder, and lithium distribution, we have performed an experimental analysis of the lithium substructure in the Li6PS5X (X = Cl, Br, I) lithium argyrodites. To monitor the subtle changes of the lithium substructure with temperature, we have conducted high-resolution, temperaturedependent neutron powder diffraction measurements.…”

“…In anion-ordered Li6PS5I we assign lithium as exclusively occupying the type 5 and 5a sites, while in anion-disordered Li6PS5Br and Li6PS5Cl we identify a considerable fraction of lithium ions located at type 2 sites, in agreement with the recent theoretical predictions. 42 Analysis of the lithium radial distributions reveals that anion site-disorder, which can be considered as introducing anionic charge inhomogeneity, is correlated with a more spatially-diffuse lithium distribution. We note that this more delocalized lithium density is therefore correlated with the enhanced lithium transport properties observed for the anion-disordered Li6PS5Br and Li6PS5Cl, suggesting a possible mechanistic link between anion disorder and high lithium conductivities in these materials.…”