New LiGePS Structure Ordering and Li-Ion Dynamics Unveiled in LiGeS-LiPS Superionic Conductors: A Solid-State Nuclear Magnetic Resonance Study.

The fast Li-ion pathways in crystals contribute to superionic conductivity-extraordinarily high ionic conductivity-of the LiGePS (LGPS) structure. Composition tuning is expected to improve the conductivity. The phase behavior, microstructure, and ion dynamics of a series of solid solutions of LiGeS-LiPS (4/1 ≥ / ≥ 1/2) were studied by multiple Li and P solid-state NMR methods. LiGePS (Ge/P = / = 1/2) is the smallest / of the disordered LGPS structure. When the Ge/P ratio increases, the room-temperature Li ionic conductivity first increases to a maximum around / = 1/1.2 and then decreases. Meanwhile, a disordered LGPS structure transforms into an ordered LGPS' structure synchronously with conductivity reduction. The LiGeS-LiPS phase diagram with the order-disorder structure transition was reconstructed accordingly. Both ordered LGPS' and disordered LGPS exhibit similar two-dimensional (2D) and one-dimensional (1D) Li diffusion paths. But the disordered LGPS structure is conducive to fast ionic conductivity, rooted in its fast 2D Li diffusion in the -plane rather than 1D diffusion along the axis. Two high-temperature relaxation processes are observed in the LGPS' structure, suggesting heterogeneous 2D jumps of rapid and slow rates, whereas only a single homogeneous 2D jump process was found in the LGPS structure. Our findings provide insight into understanding the relationship between structure order (or disorder) and ionic conductivity of superionic materials, offering guidelines for optimizing ionic conductivity for extensive solid electrolyte materials rather than LGPS materials.