Opening Diffusion Pathways through Site Disorder: The Interplay of Local Structure and Ion Dynamics in the Solid Electrolyte Li_6+xP_1–xGe_xS₅I as Probed by Neutron Diffraction and NMR

Abstract

Solid electrolytes are at the heart of future energy storage systems. Li-bearing argyrodites are frontrunners in terms of Li + ion conductivity. Although many studies have investigated the effect of elemental substitution on ionic conductivity, we still do not fully understand the various origins leading to improved ion dynamics. Here, Li 6+ x P 1– x Ge x S 5 I served as an application-oriented model system to study the effect of cation substitution (P 5+ vs Ge 4+ ) on Li + ion dynamics. While Li 6 PS 5 I is a rather poor ionic conductor (10 –6 S cm –1 , 298 K), the Ge-containing samples show specific conductivities on the order of 10 –2 S cm –1 (330 K). Replacing P 5+ with Ge 4+ not only causes S 2– /I – anion site disorder but also reveals via neutron diffraction that the Li + ions do occupy several originally empty sites between the Li rich cages in the argyrodite framework. Here, we used 7 Li and 31 P NMR to show that this Li + site disorder has a tremendous effect on both local ion dynamics and long-range Li + transport. For the Ge-rich samples, NMR revealed several new Li + exchange processes, which are to be characterized by rather low activation barriers (0.1–0.3 eV). Consequently, in samples with high Ge-contents, the Li + ions have access to an interconnected network of pathways allowing for rapid exchange processes between the Li cages. By (i) relating the changes of the crystal structure and (ii) measuring the dynamic features as a function of length scale, we were able to rationalize the microscopic origins of fast, long-range ion transport in this class of electrolytes.

Ionic Conductivity of Nanocrystalline and Amorphous Li10GeP2S12: The Detrimental Impact of Local Disorder on Ion Transport

“…Judging from the chemical shifts of both the amorphous and the crystalline phase, which do not shift upon milling, we assume that disordered LGPS is characterized by the same stoichiometry as ordered LGPS; see also below. As shown by recent studies, 67 changes in the Ge content would, for example, sensitively affect the 31 P NMR chemical shift, which is not observed here. Therefore, we exclude any enrichment or depletion of Ge in either of the two phases.…”

Cation Disorder and Large Tetragonal Supercell Ordering in the Li-Rich Argyrodite Li7Zn0.5SiS6

“…As such, all four faces of the 48 h (T5) tetrahedral environment in the average high-temperature structure of Li 7 Zn 0.5 SiS 6 are shared with other tetrahedral environments (one 48 h T5, two 48 h T2, one 16 e T4), yielding a significantly delocalized distribution of Li + , such as those observed recently in Li 6 PS 5 X (X = Cl, Br), Li 6.15 Al 0.15 Si 1.35 S 5.4 O 0.6 , Li 6.15 Al 0.15 Si 1.35 S 5.734 O 0.266 , and Li 6.6 P 0.4 Ge 0.6 S 5 I. 31 − 34 The T2 site is located within the Li cages, which are formed by the T5 and T5a Li + positions centered around the 4 d S 2– /X – site, whereas the T4 site is located between these cages; both the T2 and T4 sites are known to play an important role in the formation of ionic diffusion pathways throughout the argyrodite structure, which will be discussed in Section 3.4 .…”

“…Though Li 7 Zn 0.5 SiS 6 and Li 6.6 P 0.4 Ge 0.6 S 5 I both exhibit simultaneous occupation of the T5, T5a, T2, and T4 sites, specific site occupancies and distances between these sites that form part of the extended conduction pathway are considerably different. 34 In F 3 m Li 7 Zn 0.5 SiS 6 , the T5 sites are occupied by 0.405(8) Li and 0.0414(5) Zn with small Li occupancy of 0.099(7) on the T5a site, whereas the local Li distribution in Li 6.6 P 0.4 Ge 0.6 S 5 I is more homogeneous with the T5 site occupied by 0.318 Li and T5a site, by 0.358 Li. The additional T2 and T4 sites are occupied with 0.037 and 0.042 Li in Li 6.6 P 0.4 Ge 0.6 S 5 I and 0.048(5) and 0.243(6) Li in Li 7 Zn 0.5 SiS 6 .…”

Deciphering Fast Ion Transport in Glasses: A Case Study of Sodium and Silver Vitreous Sulfides

“…Fast Li + and Na + ion transport in superionic solid conductors as Li 10 GeP 2 S 12 or Na 3 SbS 4 is often associated with 3D diffusion pathways representing either a combination of one-dimensional channels crossing diffusion planes or orthogonal channels, parallel to crystallographic axes . Nevertheless, superior ionic mobility requires both disorder, as tetragonal remnants in a cubic lattice, and defects (vacancies, interstitials) in the cationic sublattice. − Aliovalent substitution, when Ge replaces phosphorus in Li-bearing argyrodites, Li 6+ x P 1– x Ge x S 5 I, causes both anionic (S/I) and cationic site disorder, leading to a significant increase in ionic conductivity up to four orders of magnitude. A paddle-wheel mechanism, representing a coupling of tetrahedral rotation and cation translation, is also considered as a favorable factor and a research strategy, enabling to achieve even higher ionic conductivity, especially in glassy electrolytes with lower atomic number density .…”