Engineering the Site‐Disorder and Lithium Distribution in the Lithium Superionic Argyrodite Li₆PS₅Br

Abstract

Lithium argyrodite superionic conductors, of the form Li6PS5X (X = Cl, Br, and I), have shown great promise as electrolytes for all‐solid‐state batteries because of their high ionic conductivity and processability. The ionic conductivity of these materials is highly influenced by the structural disorder of S2−/X− anions; however, it is unclear if and how this affects the Li distribution and how it relates to transport, which is critical for improving conductivities. Here it is shown that the site‐disorder once thought to be inherent to given compositions can be carefully controlled in Li6PS5Br by tuning synthesis conditions. The site‐disorder increases with temperature and can be “frozen” in. Neutron diffraction shows this phenomenon to affect the Li+ substructure by decreasing the jump distance between so‐called “cages” of clustered Li+ ions; expansion of these cages makes a more interconnected pathway for Li+ diffusion, thereby increasing ionic conductivity. Additionally, ab initio molecular dynamics simulations provide Li+ diffusion coefficients and time‐averaged radial distribution functions as a function of the site‐disorder, corroborating the experimental results on Li+ distribution and transport. These approaches of modulating the Li+ substructure can be considered essential for the design and optimization of argyrodites and may be extended to other lithium superionic conductors.

On the Lithium Distribution in Halide Superionic Argyrodites by Halide Incorporation in Li7–xPS6–xClx

“…A higher value of Rmean describes an expansion of the Li + "cage" away from its center, and to a point, a higher level of interconnectivity of the cages. 29 The effect of increasing the chloride content x is to increase Rmean, from a value of 2.475 Å in xR = 0.235 to 2.525 Å in xR = 1.40. The T2 -T2 distance represents the shortest path between Li + positions of separate lithium cages; this has been invoked in the past as a critical point for improving transport, as the cages must be interconnected for three-dimensional Li + movement in the bulk.…”

“…This is expected as higher Rmean means a higher level of interconnectivity of the cages, easing Li movement through the structure. 26,29 Figure 5d show that the activation energy decreases as a function of Rmean. Interestingly, the increase in conductivity is not linear, instead exhibiting a drastic increase after x = 1.00.…”

Paradigms of frustration in superionic solid electrolytes

“…Finally, we point out that the disordered mixing of anions in solid solutions and alloys on an otherwise static lattice can also create an energy landscape that is intrinsically structurally frustrated and unable to accommodate cation coordination preferences. This phenomenon has been extensively cited in the context of the argyrodites [37,61,[71][72][73][74][75][76][77][78][79][80][81][82]. In these systems, structural disorder between the sulfur and halide anions can lead to disruption of the local cation coordination environment and hence increased diffusivity.…”

Influence of Br − /S 2− site-exchange on Li diffusion mechanism in Li 6 PS 5 Br: a computational study

“…Frequently, the mobile Li i is drawn close to the S Br defect and in some cases the T4 sites (see also §(e)) that do not belong to the original cages around the 4d sites become occupied. This is the onset of the cage shifting towards the 4a sites which becomes more pronounced at higher degrees of site-exchange [8,19]. Occasionally, the S Br defect is also able to strip an Li + from its regular neighbouring cages but the main source for mobile Li i still remains the Br S defect.…”

“…For Li 6 PS 5 Br, this was recently shown using quenching experiments from elevated temperatures. The quenching kinetically freezes a certain degree of Br − /S 2− site-exchange that is present at higher temperatures without the need for changing the composition [18,19]. The site-exchange influences the Li + substructure which in turn leads to enhanced transport properties.…”