Non-uniform lithium-ion migration on micrometre scale for garnet- and NASICON-type solid electrolytes studied by 7Li PGSE-NMR diffusion spectroscopy

Abstract

The migration behaviours of Li⁺ in three garnet- and one NASICON-type solid oxide electrolytes were studied on the micrometre scale by pulsed-gradient spin-echo (PGSE) ⁷Li NMR diffusion spectroscopy to clarify common and specific characteristics of each electrolyte. In these solid electrolytes, clear evidences of grain boundary effects in the diffusion of Li⁺ were not observed. The Li⁺ diffusion constants were dependent on parameters such as observation time (Δ) and pulsed field gradient (PFG) strength (g) for all the studied inorganic solid electrolytes. For low Δ values, Li⁺ ions underwent collisions and diffractions with diffraction distance R_diffraction [μm]. The apparent Li⁺ diffusion constants (D_apparent [m² s⁻¹]) exhibited distributions in a wide range. In this paper, we introduced the apparent diffusion radius, r_radius [μm], and compared it with R_diffraction and mean square displacement (MSD) [μm]; the lengths of these distances were of the micrometre order (10⁻⁶ m). The relations between the values of r_radius, R_diffraction and MSD suggested that the migration behaviours of Li⁺ on the micrometre scale were complicated. Using high Δ and high g values, we obtained an equilibrated value of D_Li. The temperature dependences of the number of carrier ions were estimated from the D_Li values and ionic conductivities in the four solid oxide electrolytes. For simple comparison and reference, the data of D_Li and ionic conductivity of LiPF₆ in 1 M solution of propylene carbonate were added.