Conduction Mechanism of Li₁₀GeP₂S₁₂-type Lithium Superionic Conductors in a Li–Sn–Si–P–S System

Abstract

Crystal structures of Li 1 0 GeP 2 S 1 2 (LGPS)-type Li 10+δ [Sn y Si 1−y ] 1+δ P 2−δ S 12 (Li 4−x [Sn y Si 1−y ] 1−x P x S 4 ) solid electrolytes were analyzed by Rietveld refinement using neutron diffraction data. Maximum entropy method analysis was performed to visualize the distribution of lithium along the c-axis via the Li1−Li3 sites, which indicated one-dimensional (1D) lithium diff usion for all the ex amined compositions. The Li 10.35 Sn 0.27 Si 1.08 P 1.65 S 12 (Li 3.45 Sn 0.09 Si 0.36 P 0.55 S 4 ) (δ = 0.35, x = 0.55, y = 0.2) system, which had the highest ionic conductivity in Li−Sn−Si−P−S system, exhibited an additional lithium diffusion pathway in the ab-plane through the Li1 and Li4 sites. High ionic conductivity (>10 mS cm −1 ) was achieved in the Sn−Si derivatives owing to the formation of three-dimensional (3D) ion diffusion channels. Comparison of the conductivity and related crystal structural parameters revealed the requirements for fast lithium diffusion along the c-axis and 3D lithium diffusion in the LGPS-type crystal structure. Large atomic displacement of the Li1 site, a large S3−S3 distance, a large bottleneck size, and small differences in Li1−Li4 distances are important for 1D and 3D lithium diffusion, respectively.

Li10GeP2S12‐Type Superionic Conductors: Synthesis, Structure, and Ionic Transportation

“…[ 32 ] In fact, the widely spread lithium distribution has been visualized using MEM. [ 13,32,57,84,85 ]…”

“…highlighted changes in the conduction mechanism from 1D‐like to 3D‐like, and suggested that a 3D Li distribution at lower temperature may help improve the conductivity. [ 57 ] Although this has not been achieved for Li 9.81 Sn 0.81 P 2.19 S 12 (5 × 10 −3 S cm −1 at 298 K) shown in Figure 10d, the introduction of Si provides Li 10.35 [Sn 0.27 Si 1.08 ]P 1.65 S 12 [ 84 ] with 3D Li distribution and a higher conductivity of 1.1 × 10 −2 S cm −1 (Figure 10e). Moreover, a conductivity of 2.5 × 10 −2 S cm −1 has been achieved for the Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 system with 3D Li distribution (Figure 10f).…”

“…Li distribution for various LGPS‐type materials visualized by MEM. [ 13,57,84,85 ] For the purpose of comparison, the iso‐surface level is set at −0.06 fm Å −3 . Z‐MEM and Z‐3D software are used for the calculation and visualization of negative nuclear density distributions, respectively.…”

“…[ 57,85 ] A 3D connection of lithium nuclear densities was observed at 300 K for Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 , which has a very high ionic conductivity of 2.5 × 10 −2 S cm −1 at RT. [ 84 ] The recently reported Li 10.35 Sn 0.27 Si 1.08 P 1.65 S 12 also exhibited a high ionic conductivity above 1.0 × 10 −2 S cm −1 and a 3D diffusion path of lithium at RT. [ 84 ] This suggests that the formation of a 3D diffusion path of lithium through the octahedral interstitial site (4 c ) has an influence on the ionic conductivity of the material.…”

“…[ 84 ] The recently reported Li 10.35 Sn 0.27 Si 1.08 P 1.65 S 12 also exhibited a high ionic conductivity above 1.0 × 10 −2 S cm −1 and a 3D diffusion path of lithium at RT. [ 84 ] This suggests that the formation of a 3D diffusion path of lithium through the octahedral interstitial site (4 c ) has an influence on the ionic conductivity of the material.…”