Dielectric characteristics of fast Li ion conducting garnet-type Li5+2xLa3Nb2−xYxO12 (x = 0.25, 0.5 and 0.75)

Abstract

Here, we report the dielectric characteristics of Li-stuffed garnet-type Li_5+2xLa₃Nb_2−xY_xO₁₂ (x = 0.25, 0.5 and 0.75) in the temperature range about −53 to 50 °C using AC impedance spectroscopy. All the investigated Li-stuffed garnet compounds were prepared, under the same condition, using conventional solid-state reaction at elevated temperature in air. The Nyquist plots show mainly bulk contribution to the total Li⁺ ion conductivity for Li_5.5La₃Nb_1.75Y_0.25O₁₂ (Li_5.5–Nb) and Li₆La₃Nb_1.5Y_0.5O₁₂ (Li₆–Nb), while both bulk and grain-boundary effects are visible in the case of Li_6.5La₃Nb_1.25Y_0.75O₁₂ (Li_6.5–Nb) phase at ∼−22 °C. Non-Debye relaxation process was observed in the modulus AC impedance plots. The dielectric loss tangent of Li_5+2xLa₃Nb_2−xY_xO₁₂ are compared with that of the corresponding Ta analogue, Li_5+2xLa₃Ta_2−xY_xO₁₂ and showed a decrease in peak intensity for the Nb-based garnet samples which may be attributed to a slight increase in their Li⁺ ion conductivity. The relative dielectric constant values were also found to be higher for the Ta member (>60 for Li_5+2xLa₃Ta_2−xY_xO₁₂) than that of the corresponding Nb analogue (∼50 for Li_5+2xLa₃Nb_2−xY_xO₁₂) at below room temperature. A long-range order Li⁺ ion migration pathway with relaxation time (τ₀) 10⁻¹⁸–10⁻¹⁵ s and an activation energy of 0.59–0.40 eV was observed for the investigated Li_5+2xLa₃Nb_2−xY_xO₁₂ garnets and is comparable to that of the corresponding Ta-based Li_5+2xLa₃Ta_2−xY_xO₁₂ garnets.