Synthesis and characterization of novel Li-stuffed garnet-like Li_5+2xLa₃Ta_2−xGd_xO₁₂ (0 ≤ x ≤ 0.55): structure–property relationships

Abstract

In this article, we report the preparation and characterization of novel Li-stuffed garnets Li_5+2xLa₃Ta_2-xGd_xO₁₂ (0 ≤ x ≤ 0.55) for all-solid-state Li ion batteries. The conventional solid-state method was used to prepare Li₅La₃Ta₂O₁₂ in air at 1200 °C and Li_5+2xLa₃Ta_2-xGd_xO₁₂ at 1150 °C. Rietveld refinements for the powder X-ray diffraction (PXRD) patterns confirmed the formation of a cubic garnet-like structure (space group Ia3[combining macron]d) with cell constant increased from 12.8176(4) Å (x = 0) to 12.9372(2) Å (x = 0.55). However, small amounts of second phases were observed for higher Gd-doped samples. Scanning electron microscopy revealed that Li_5.7La₃Ta_1.65Gd_0.35O₁₂ exhibits the highest density among all investigated samples in this study. The AC impedance spectroscopy data of the samples have been analyzed in relation to ionic conductivity, dielectric constants, and loss tangent. Among the investigated electrolytes, the Li_5.7La₃Ta_1.65Gd_0.35O₁₂ composition demonstrated the highest bulk ionic conductivity of 8.18 × 10^-5 S cm^-1 at 25 °C, which is significantly higher than that of the parent garnet Li₅La₃Ta₂O₁₂ (1.65 × 10^-5 S cm^-1 at 25 °C). The appearance of a relaxation peak in the loss tangent plots for all samples seems to be due to the dipolar rotations of Li⁺ ions in Li-stuffed garnets.