With a Little Help from ³¹P NMR: The Complete Picture on Localized and Long-Range Li⁺ Diffusion in Li₆PS₅I

High Resolution Image Download MS PowerPoint Slide Li 6 PS 5 I acts as a perfect model substance to study length scale-dependent diffusion parameters in an ordered matrix. It provides Li-rich cages which offer rapid but localized Li + translational jump processes. As jumps between these cages are assumed to be much less frequent, long-range ion transport is sluggish, resulting in ionic conductivities in the order of 10 –6 S cm –1 at room temperature. In contrast, the site disordered analogues Li 6 PS 5 X (X = Br, Cl) are known as fast ion conductors because structural disorder facilities intercage dynamics. As yet, the two extremely distinct jump processes in Li 6 PS 5 I have not been visualized separately. Here, we used a combination of 31 P and 7 Li NMR relaxation measurements to probe this bimodal dynamic behavior, that is, ultrafast intra cage Li + hopping and the much slower Li + inter cage exchange process. While the first is to be characterized by an activation energy of ca. 0.2 eV as directly measured by 7 Li NMR, the latter is best observed by 31 P NMR and follows the Arrhenius law determined by 0.44 eV. This activation energy perfectly agrees with that seen by direct current conductivity spectroscopy being sensitive to long-range ion transport for which the intercage jumps are the rate limiting step. Moreover, quantitative agreement in terms of diffusion coefficients is also observed. The solid-state diffusion coefficient D σ obtained from conductivity spectroscopy agrees very well with that from 31 P NMR ( D NMR ≈ 4.6 × 10 –15 cm 2 s –1 ). D NMR was directly extracted from the pronounced diffusion-controlled 31 P NMR spin-lock spin–lattice relaxation peak appearing at 366 K.