Fast Ion Conduction and Its Origin in Li_6–xPS_5–xBr_1+x

Abstract

High ionic conductivity of solid electrolytes is key to achieving high-power all-solid-state rechargeable batteries. The superionic argyrodite family is among the most conductive Li-ion conductors. However, their potential in ionic conductivity and stability is far from being reached, especially with Li 6 PS 5 Br. Here, we synthesized Li 6−x PS 5−x Br 1+x with increased site mixing of Br − /S 2− . An ionic conductivity of 11 mS cm −1 at 25 °C is achieved with a low activation energy of 0.18 eV for Li 5.3 PS 4.3 Br 1.7 . The influence of Br − /S 2− mixing on ion conduction is systematically investigated with multinuclear solid-state NMR coupled with X-ray diffraction and impedance spectroscopy. A statistically random distribution of Br − and S 2− at 4d sites is observed with 31 P NMR. The resulting local structures regulate the jump rates of their neighboring Li ions and Li redistribution. As a result, the increased Li + occupancy at 24g sites promotes fast ion conduction, and the role of Li (24g) in ion conduction has been elucidated with tracer-exchange NMR. Experimental evidence combined with density functional theory calculations has revealed that the particular arrangement of 1S3Br at 4d sites near Li maximizes overall Li + conduction. This insight applies to other argyrodites and will be useful to the design of new fast ion conductors.

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

“…(anion-ordered) is a poor Li-ion conductor compared to Li6PS5Cl and Li6PS5Br (both aniondisordered), which exhibit ×10 3 higher room-temperature ionic conductivities. 9,13,[38][39][40] While the correlation between anion disorder and fast lithium transport is well documented, 9,[24][25][26][27][28][29][30][31][32][33]36,41 the effect of anion site-disorder on the microscopic dynamics and local structure of the mobile lithium ions is less well understood. In Li6PS5X argyrodites, the anionic framework contains 132 tetrahedral voids per unit cell (4 formula units) that may accommodate the corresponding 24 lithium ions.…”

“…39 This three-jump model, in which the lithium diffusion mechanism is described entirely in terms of movement between type 5 sites, has been widely applied to explain experimental and theoretical trends in lithium transport in Li6PS5X and related lithium-argyrodites. 9,[24][25][26][27][28][29][30][31][32]41,43 This model, however, ignores the role of non-type 5 tetrahedra, and therefore gives an incomplete description of the lithium diffusion pathways in the argyrodite structure. The type 5 tetrahedra form face-sharing pairs, and pure 5 → 5 lithium motion is only possible for the doublet jump.…”

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

“…The occurrence of site-disorder and its influence on the ionic transport has been intensively investigated experimentally and computationally. 9,[24][25][26][27][28]32,41,43 The hereobtained X − /S 2− site-disorder is quantified in Figure 3b. In agreement with the literature, a higher degree of disorder can be found for the chloride compared to the bromide counterpart, which can be attributed to the similar ionic radii of S 2− and Cl − , 9,25,29 compared to the more dissimilar radii of S 2and Br − , 9,24,27,36 while no site-disorder is observed for Li6PS5I.…”

“…(anion-ordered) is a poor Li-ion conductor compared to Li6PS5Cl and Li6PS5Br (both aniondisordered), which exhibit ×10 3 higher room-temperature ionic conductivities. 9,13,[38][39][40] While the correlation between anion disorder and fast lithium transport is well documented, 9,[24][25][26][27][28][29][30][31][32][33]36,41 the effect of anion site-disorder on the microscopic dynamics and local structure of the mobile lithium ions is less well understood. In Li6PS5X argyrodites, the anionic framework contains 132 tetrahedral voids per unit cell (4 formula units) that may accommodate the corresponding 24 lithium ions.…”

“…39 This three-jump model, in which the lithium diffusion mechanism is described entirely in terms of movement between type 5 sites, has been widely applied to explain experimental and theoretical trends in lithium transport in Li6PS5X and related lithium-argyrodites. 9,[24][25][26][27][28][29][30][31][32]41,43 This model, however, ignores the role of non-type 5 tetrahedra, and therefore gives an incomplete description of the lithium diffusion pathways in the argyrodite structure. The type 5 tetrahedra form face-sharing pairs, and pure 5 → 5 lithium motion is only possible for the doublet jump.…”