A new LGPS structure ordering and Li ion dynamics unveiled in Li₄GeS₄-Li₃PS₄ superionic conductors-a solid-state NMR study.

Superionic conductors of Li₁₀GeP₂S<sub>12</sub >(LGPS) structure presented extraordinary high ionic conductivities attribute to its fast Li ion pathways in crystal. Composition tuning is expected to improve the conductivity. Phase behavior, microstructure and ion dynamics of a series of solid solutions of xLi₄GeS₄-yLi₃PS₄ (4/1 ≥ x/y ≥ 1/2) were studied by multiple ⁷Li and ³¹P solid-state NMR methods. Li₁₀GeP₂S<sub>12</sub > (Ge/P = x/y = 1/2) is the smallest x/y of disordered LGPS structure. With the Ge/P ratio rising, room-temperature Li ion conductivity first increases to a maximal at x/y = 1/1.2 and then decreases. Meanwhile, a disordered LGPS phase transforms into a new ordered LGPS' phase synchronously with the reduction in ion conductivity. The disorder of local structure profits fast ionic conductivity. Li₄GeS₄-Li₃PS₄ phase diagram was reconstructed accordingly. Both ordered LGPS' and disordered LGPS exhibit similar 2D and 1D Li diffusion paths. But the disordered LGPS structure profits fast ionic conductivity, rooting in its fast 2D Li⁺ diffusion in ab plane rather than 1D diffusion along c- axis. Two high-temperature relaxation processes are observed in LGPS' structure, suggesting heterogeneous 2D jumps of rapid and slow rates. Whereas only single homogeneous 2D jump process in LGPS phase. Our findings provide insight into understanding the structure-conductivity relationship of superionic materials, and offer guidelines to optimize the ionic conductivity for extensive solid electrolyte materials more than LGPS materials.