In recent years, body-centered-cubic (bcc)-type anion framework has been considered as a deft structural design descriptor for identifying unexplored potential sulfide lithium-ion conductors (SICs). However, the bcc guidance must be completed, and other factors impacting lithium diffusion must be considered. Therefore, they are not always useful in the design of new lithium-ion conductors. In this study, based on previous works, we investigated the lithium-ion hopping and diffusion mechanisms in sulfides. We discussed how the local structural environment impacts lithium hopping from site to site in a micro-manner and how the chemical composition impacts diffusion in a macro-manner. In conclusion, we proved that the zero-transition-metal (0-TM) coordinated channel is essential for the facile hopping of lithium ions, and macro-diffusion can be expected in the materials only if the contents of 0-TM channels exceed the penetrating thresholds. Our study provides a more thorough interpretation of the sulfides' lithium diffusion mechanism and directly connects the structural and functional features of the material.
Keywords：lithium ionconductors
;
ion framework
;
lithium ion diffusion
;
diffusion coefficient
Fig. 1
(a), (b) The unit cell of LZPS viewed from different directions. The LiS4, ZnS4, PS4 and interstitial site are colored by dark green, blue, cyan and pastel orange. The sulfur is labeled with yellow spheres. (c) The local environment involving the lithium hopping in the LZPS structure which is extracted from the red box in (b). (d) The diffusion path in the octahedral cage determined by NEB calculation
Fig. 2
(a) The calculated energy landscape of diffusion path from the Li site to: the interstitial in the stoichiometry LZPS (blue color). The interstitial in a Li-Zn substitution local environment (green color). The Zn site in the same layer (orange color). (All the migration paths are scaled to the same length.) (b) The calculated energy landscape of diffusion path from the Li site to the interstitial site in a perfect bcc sulfur framework
Fig. 4
Periodic wrapping probabilities (red color line) and percolation thresholds (indicated by vertical bars). Accessible amount of lithium connecting to the percolating network (blue color line)
Fig. 5
(a) The calculated phonon band structures for ε-Li3PS4; (b), (c), (d) The calculated Gibbs free energy of ε-Li3PS4 and γ-Li3PS4 at different pressures
Fig. 7
The unit cell of LZGS (a) and LZSS (b). The LiS4, ZnS4, GeS, and LiSi4 are colored by dark green, blue, pastel cyan and dark cyan. (c), (d), (e) The local environment involving the lithium hopping in the LZGS structure marked as path 1, path 2 and path 3 in (a)
Fig. 8
Ideal body-centered-tetragonal (bct) lattice (red) overlaid on the sulfur framework (yellow) of β-Li2ZnGeS4 (a) and Li2ZnSiS4 (b). The root-mean-square (rms) distances from the S atoms in the β-Li2ZnGeS4 and 0.56 Å in the Li2ZnSiS4 to the idealized bcc position are 0.53 Å and 0.56 Å respectively
Fig. 9
The calculated energy landscape of diffusion path in the LZGS from the Li site to the adjacent vacancy through the Oct(2LiS4-PS4) cage (orange color). The Oct(2LiS4-ZnS4) cage (blue color). The Tet(2LiS4-ZnS4-PS4) cage (green color). The Oct(2LiS4-VacS4) cage (black color). The Tet(2LiS4-VacS4-PS4) cage (red color)
Fig. 10
The mean square displacement (MSD) of lithium ions in the of β-Li2ZnGeS4 (left) and Li2ZnSiS4 (right) calculated from the AIMD simulation at 600 K. After the composition engineering, the diffusion is significantly enhanced, proving the success of our designing strategy
Fig. 11
The calculated energy landscape of diffusion path in the LZSS from the Li site to the adjacent vacancy through the Oct(2LiS4-SiS4) cage (orange color). The Oct(2LiS4-ZnS4) cage (blue color). The Tet(2LiS4-ZnS4-PS4) cage (green color). The Oct(2LiS4-VacS4) cage (black color)
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