Changing the Static and Dynamic Lattice Effects for the Improvement of the Ionic Transport Properties Within the Argyrodite Li6PS5-xSexI

Abstract

<p>The lithium argyrodites Li<sub>6</sub>PS<sub>5</sub>X (X = Cl, Br, I) have been gaining momentum as candidates for electrolytes in all-solid-state batteries. While these materials have been well-characterized structurally, the influences of the static and dynamic lattice properties are not fully understood. Recent improvements to the ionic conductivity of Li<sub>6</sub>PS<sub>5</sub>I (which as a parent compound is a poor ionic conductor) via elemental substitutions have shown that a multitude of influences affect the ionic transport in the lithium argyrodites, and that even poor conductors in this class have room left for improvement.</p><p>Here we explore the influence of isoelectronic substitution of sulfur with selenium in Li<sub>6</sub>PS<sub>5-<i>x</i></sub>Se<i><sub>x</sub></i>I. Using a combination of X-ray diffraction, impedance spectroscopy, Raman spectroscopy, and pulse-echo speed of sound measurements,we explore the influence of the static and dynamic lattice on the ionic transport. The substitution of S<sup>2-</sup>with Se<sup>2- </sup>broadens the diffusion pathways and structural bottlenecks, as well as leading to a softer more polarizable lattice, all of which lower the activation barrier and lead to an increase in the ionic conductivity. This work sheds light on ways to systematically understand and improve the functional properties of this exciting material family. </p>

Review on the lithium transport mechanism in solid‐state battery materials

“…Replacing the anions in SEs with higher‐atomic‐number isovalent ones usually receives lower activation energy because of the larger polarizability of the latter. Such rule has been demonstrated in Li 3 PX 4 (X = O and S), 235,285 LiTi 2 (PX 4 ) 3 (X = O and S), 195,286 Li 6 PX 5 I (X = S and Se), 287 and Li 3 ErX 6 (X = Cl and I) 288 . However, a few exceptions have also been reported, in which other factors more significantly affect the activation energy, for example, the inductive effect in Li 10 XP 2 S 12 (X = Ge and Sn) 207 and the anion disorder in Li 6 PS 5 X (X = Br and I) 289 .…”

Ion Hopping: Design Principles for Strategies to Improve Ionic Conductivity for Inorganic Solid Electrolytes

“…For instance, Se possesses the same formal charge and larger ionic radius comparing with S, making it more polarizable and suitable to construct a softer framework (Figure 7b). [88,89] In line with the above principle, the ionic conductivities of argyrodites Li 6 PS 5 X (X = Cl, Br) can be optimized by tailoring the anions. [90] Actually, there are usually nonmobile cation elements contained in ionic conductors beyond mobile cations and skeleton anions, which also playing an important role in lattice.…”

“…b) The influence of isoelectronic substitution of sulfur with selenium in Li 6 PS 5−x Se x I. Reproduced with permission. [88] Copyright 2020, American Chemical Society. c) Calculated NaS, Sb/WS, and NaNa radial distribution functions (RDF) for the orthorhombic Na 2.7 W 0.3 Sb 0.7 S 4 (Orth) and pseudo-cubic Na 2.895 W 0.3 Sb 0.7 S 4 (Na 2.895 ), compared to those of the cubic phase of Na 3 SbS 4 (Cub) which exhibits the exceptionally low activation energy.…”

“…Copyright 2017, American Chemical Society. b) The influence of isoelectronic substitution of sulfur with selenium in Li 6 PS 5−x Se x I. Reproduced with permission [88]. Copyright 2020, American Chemical Society.…”

Understanding Lithium-Ion Transport in Selenophosphate-Based Lithium Argyrodites and Their Limitations in Solid-State Batteries

“…35 Inspired by the previous success of introducing an excess of halide into the argyrodite structure, we have investigated the effect of introducing excess bromide into selenophosphatebased argyrodites of the Li 6−x PSe 5−x Br 1+x solid solution series. Based on reports on Li 6 PS 5−x Se x Br 40 and Li 6 PS 5−x Se x I, 41 the introduction of larger and more polarizable Se 2− is expected to lead to widening of diffusion pathways and lattice softening, which is expected to further improve ionic transport. Structural changes, including changes in the Li + substructure, have been investigated by high-resolution powder neutron diffraction and 31 P MAS NMR.…”