Structural Disorder in Li₆PS₅I Speeds ⁷Li Nuclear Spin Recovery and Slows Down ³¹P Relaxation–Implications for Translational and Rotational Jumps as Seen by Nuclear Magnetic Resonance

Abstract

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.

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

“…For comparison, the independently measured dc conductivity at 373 K is 5 × 10 –6 S cm –1 ; at 353 K, it turned out to be 2 × 10 –6 S cm –1 . 22 , 33 These values are in very good agreement with the one probed by diffusion-induced 1/ T 1ρ 31 P NMR spin–lattice relaxation.…”

“…Turning to 31 P NMR spin–lattice relaxation (see Figure b), we recognize that the rate 1/ T 1 passes through two different maxima located at approximately 220 K (peak C) and 344 K (again labeled as peak A). As we have shown earlier, , peak C, most likely, reflects rotational motions of the polyanions as it is unique to 31 P and is not seen in 7 Li NMR. On the other hand, the 31 P NMR peak located at 330 K (peak A) mirrors the fast Li + translational intra cage jumps that also dominate the 1/ T 1 7 Li NMR response (see peak A in Figure a). , A comparison of the two peaks detected by 7 Li and 31 P NMR is given in Figure c.…”

“…As we have shown earlier, , peak C, most likely, reflects rotational motions of the polyanions as it is unique to 31 P and is not seen in 7 Li NMR. On the other hand, the 31 P NMR peak located at 330 K (peak A) mirrors the fast Li + translational intra cage jumps that also dominate the 1/ T 1 7 Li NMR response (see peak A in Figure a). , A comparison of the two peaks detected by 7 Li and 31 P NMR is given in Figure c. Note that the 7 Li and 31 P rates were recorded at almost the same Larmor frequencies (116 MHz vs 121 MHz), which is important for such a direct comparison, because to fulfil the condition ω 0 τ NMR ≈ 1 (see above), the position of the peaks will shift with increasing ω 0 toward higher temperatures.…”

“…On the other hand, the 31 P NMR peak located at 330 K (peak A) mirrors the fast Li + translational intra cage jumps that also dominate the 1/ T 1 7 Li NMR response (see peak A in Figure 2 a). 29 , 33 A comparison of the two peaks detected by 7 Li and 31 P NMR is given in Figure 2 c. Note that the 7 Li and 31 P rates were recorded at almost the same Larmor frequencies (116 MHz vs 121 MHz), which is important for such a direct comparison, because to fulfil the condition ω 0 τ NMR ≈ 1 (see above), the position of the peaks will shift with increasing ω 0 toward higher temperatures. Here, the 31 P nuclei are used as spies to indirectly sense the ( 31 P– 7(6) Li) spin fluctuations in their direct neighborhood, which are produced by Li + translational motions.…”

Opening Diffusion Pathways through Site Disorder: The Interplay of Local Structure and Ion Dynamics in the Solid Electrolyte Li6+xP1–xGexS5I as Probed by Neutron Diffraction and NMR

“… 38 , 39 Once again, structural disorder, possibly also including site disorder, slows down 31 P NMR spin–lattice relaxation in nano-Li 6 PS 5 I. 38 , 40 …”