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

Li6PS5I 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 Li6PS5X (X = Br, Cl) are known as fast ion conductors because structural disorder facilities intercage dynamics. As yet, the two extremely distinct jump processes in Li6PS5I have not been visualized separately. Here, we used a combination of P-31 and Li-7 NMR relaxation measurements to probe this bimodal dynamic behavior, that is, ultrafast intracage Li+ hopping and the much slower Li+ intercage exchange process. While the first is to be characterized by an activation energy of ca. 0.2 eV as directly measured by Li-7 NMR, the latter is best observed by P-31 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 Ds obtained from conductivity spectroscopy agrees very well with that from P-31 NMR (D-NMR approximate to 4.6 x 10(-15) cm(2) s(-1)). D-NMR was directly extracted from the pronounced diffusion-controlled 31P NMR spin-lock spin-lattice relaxation peak appearing at 366 K.