Effects of Halogen and Sulfur Mixing on Lithium-Ion Conductivity in Li_7–x–y(PS₄)(S_2–x–yCl_xBr_y) Argyrodite and the Mechanism for Enhanced Lithium Conduction

Abstract

All-solid-state lithium-ion batteries containing phosphorus sulfides are promising next-generation batteries because they have higher energy densities than their liquid-electrolyte counterparts. Halogen-rich argyrodites, Li7−α(PS4)­(S2−αXα) (X = Cl, Br, I; α > 1), exhibit higher ionic conductivities than other phosphorus sulfides; varying the content of a single anion enables halogen substitution. However, there is no appropriate model for the ion-conducting path in these argyrodites. Herein, we discuss a range of composition maps to explain the lithium-ion conductivity of Li7–x–y (PS4)­(S2–x–y Cl x Br y ) (x + y > 1). The ionic conductivities (∼11.6 mS/cm) of Li5.4(PS4)­(S0.4Cl1.0Br0.6) and Li5.4(PS4)­(S0.4Cl0.6Br1.0) are among the highest reported. Bond valence sum mapping and maximum entropy methods for Li7–x–y (PS4)­(S2–x–y Cl x Br y ) (x + y > 1) were used. The results showed that these argyrodites have a lithium-cage structure surrounding the halide or sulfide ions at the 4d sites and a lithium-ion-conduction path between the cages via an interstitial site (16e). The degree of disorder of chloride, bromide, and sulfide ions on the 4a and 4d sites controls the conduction path length between the cages; a short lithium-ion-conduction path between the cages enhances lithium-ion conductivity. These results could inspire the development of halogen-rich compositions of Li7−α(PS4)­(S2−αXα) (α > 1) with enhanced ionic conductivities.

Exploring the Relationship Between Halide Substitution, Structural Disorder, and Lithium Distribution in Lithium Argyrodites (Li6–xPS5–xBr1+x)

“…However, recent reports have indicated that Li + can partially occupy the T4 site (Wyckoff 16 e , Figure b) in Li 6.15 Al 0.15 Si 1.35 S 5.4 O 0.6 and related Li 2 S-SnS 2 –SiS 2 –P 2 S 5 sulfides . Additionally, other argyrodites showed the Li + occupancy of T2 and T4 sites in “Li-deficient” materials with stoichiometries of <6.0 Li per formula unit . In order to study the lithium substructure in the Li 6– x PS 5– x Br 1+ x series, we conducted a detailed investigation using neutron powder diffraction, analyzed by Rietveld refinement in the cubic F 4̅3 m space group.…”

“…More charge is maintained on the Li atom as its electrons experience less interaction with the 4 d site, causing the 6 Li shift to move to lower ppm values . The increased Li cage radius is the origin of the reduced T2–T2 and T5–T4–T5 distances, effectively the result of the Br distribution over the 4 a and 4 d sites, which can be expected to result in better cage connectivity and thus higher overall conductivity. , …”

“…However, recent reports have indicated that Li + can partially occupy the T4 site 42 Additionally, other argyrodites showed the Li + occupancy of T2 and T4 sites in "Li-deficient" materials with stoichiometries of <6.0 Li per formula unit. 40 In order to study the lithium substructure in the Li 6−x PS 5−x Br 1+x series, we conducted a detailed investigation using neutron powder diffraction, analyzed by Rietveld refinement in the cubic F4̅ 3m space group. Figure S2 shows the occupancy of various Li sites in the argyrodite series by both quenching and slow cooling methods as a function of the bromide content.…”

“…61 The increased Li cage radius is the origin of the reduced T2−T2 and T5−T4−T5 distances, effectively the result of the Br distribution over the 4a and 4d sites, which can be expected to result in better cage connectivity and thus higher overall conductivity. 39,40 Ionic Transport. De Klerk et al 35 suggested that a higher site disorder up to 75% or a higher halide content in argyrodites can enhance ionic conductivity by facilitating intercage diffusion and by providing a more optimal Li-vacancy distribution.…”

Promoting Fast Ion Conduction in Li-Argyrodite through Lithium Sublattice Engineering

“…While these reports 20,22,24 highlight the function of the 48h′ site (type 2) and support the idea 4 that the cage-tocage Li + jump may be achieved by the 48h−48h′−48h′−48h migration pathway (type 5−type 2−type 2−type 5, Figure 2), relatively little is known about the potential contribution of the interstitial 16e sites (type 4) to the Li + migration (Figures 1 and 2), despite some very recent bond valence sum (BVS) analysis on this site. 25 Pioneering works, experimentally 10,17 and computationally, 22,26 proposed that the fast Li + conduction in Li-argyrodite is strongly correlated with the degree of anion site-disorder, i.e., mixing of S 2− and Cl − /Br − /I − on the Wyckoff site of 4a/ 4d. It is broadly believed that high degrees of anion sitedisorder can facilitate fast Li + migration 27 by altering the energy landscape for different Li + sites.…”