Structural Disorder in Li₆PS₅I Speeds ⁷Li Nuclear Spin Recovery and Slows Down ³¹P Relaxation-Implications for...

Lithium-thiophosphates have attracted great attention as they offer a rich playground to develop tailor-made solid electrolytes for clean energy storage systems. Here, we used poorly conducting Li₆PS₅I, which can be converted into a fast ion conductor by high-energy ball-milling to understand the fundamental guidelines that enable the Li⁺ ions to quickly diffuse through a polarizable but distorted matrix. In stark contrast to well-crystalline Li₆PS₅I (10^-6 S cm^-1), the ionic conductivity of its defect-rich nanostructured analog touches almost the mS cm^-1 regime. Most likely, this immense enhancement originates from site disorder and polyhedral distortions introduced during mechanical treatment. We used the spin probes ⁷Li and ³¹P to monitor nuclear spin relaxation that is directly induced by Li⁺ translational and/or PS₄ ^3- rotational motions. Compared to the ordered form, ⁷Li spin-lattice relaxation (SLR) in nano-Li₆PS₅I reveals an additional ultrafast process that is governed by activation energy as low as 160 meV. Presumably, this new relaxation peak, appearing at T _max = 281 K, reflects extremely rapid Li hopping processes with a jump rate in the order of 10⁹ s^-1 at T _max. Thus, the thiophosphate transforms from a poor electrolyte with island-like local diffusivity to a fast ion conductor with 3D cross-linked diffusion routes enabling long-range transport. On the other hand, the original ³¹P nuclear magnetic resonance (NMR) SLR rate peak, pointing to an effective ³¹P-³¹P spin relaxation source in ordered Li₆PS₅I, is either absent for the distorted form or shifts toward much higher temperatures. Assuming the ³¹P NMR peak as being a result of PS₄ ^3- rotational jump processes, NMR unveils that disorder significantly slows down anion dynamics. The latter finding might also have broader implications and sheds light on the vital question how rotational dynamics are to be manipulated to effectively enhance Li⁺ cation transport.