7Li Diffusion in Thin Disks of Single-Crystal Garnet LLZO-Ta Studied by PFG-NMR Spectroscopy

Abstract

Pulsed field-gradient (PFG) NMR spectroscopy was applied to thin disk samples (0.5, 1 and 2 mm thick, 4 mm diameter) of LLZO-Ta (Li₆La₃Zr_1.5Ta_0.5O₁₂) single crystal. We have measured the diffusion of ⁷Li in thin pellets (0.5 to 3 mm thick) of NASICON-type LAGP powder and found that the Li⁺ diffuses preferentially in the thicker direction. In this study, we placed 1 mm thick, 4 mm diameter disk-shaped single crystals vertically, i.e., parallel to the PFG direction. We found that ⁷Li ions diffuse similarly as in rod-shaped single crystals. The ⁷Li diffusion is slow (diffusion coefficient; ~2 × 10^-13 m²s^-1) at Δ ≥ 100 milliseconds and gradually becomes faster to ~1 × 10^-11 m²s^-1 at Δ ≤ 10 milliseconds at 28°C. A diffraction pattern was observed in the echo decay plots, which was not observed in well-prepared single crystal rod samples. The diffraction patterns are often observed in powder inorganic electrolytes (LLZOs, LLTO, LAGP, and (Li₂S)_x(P₂S₅)_y). In this study, edge effects are assumed for the diffraction pattern of the vertically placed thin disk of the single crystal. For horizontally placed disks, at long observation times (Δ ≥ 100 milliseconds), the diffusion coefficients of ⁷Li agreed with those of the rod-shaped samples. However, as Δ became shorter, contrary to the vertically placed disk, the diffusion of ⁷Li decreased and the plots became less linear. Surprisingly, as Δ became shorter (Δ ≤ 15 milliseconds), sinusoidal patterns were observed in the real and imaginary elements of the echo attenuation plots,. The pattern is reproducible and the first example of sinusoidal real and imaginary components of the echo attenuation plot was observed, but an adequate explanation has not been obtained.