Origin of the enhanced Li+ ionic conductivity in Gd+3 substituted Li5+2xLa3Nb2−xGdxO12 lithium conducting garnets

Abstract

In the present study, we report the synthesis and the Li⁺-ion conductivity of new Gd⁺³ substituted Li_5+2xLa₃Nb_2−xGd_xO₁₂ (x = 0.0, 0.25, 0.4, 0.5, 0.6) garnets. The structural study by XRD showed that pure cubic garnet phases were obtained with upto x = 0.5 composition. With the further increase of the Gd⁺³ content to x ≥ 0.6, secondary phases are observed. The ionic conductivity was studied by impedance spectroscopy. We found that the Li⁺ ionic conductivity increased with increasing Gd⁺³ content with a maximum value of 1.12 × 10⁻⁴ S cm⁻¹ at RT, which was two orders of magnitude larger than the previously reported value of 10⁻⁶ S cm⁻¹ for pure Li₅La₃Nb₂O₁₂. A slight drop in the conductivity value to 6.25 × 10⁻⁵ S cm⁻¹ was observed for x = 0.6 composition. By a systematic analysis of the conductivity spectra at different temperatures of the investigated materials, we are able to estimate the true values of the concentration, n_c, and mobility, μ, of mobile Li⁺ that contribute to the conduction process; n_c was found to increase by a factor of only ∼2 with increasing Gd⁺³ content from x = 0.0 to x = 0.5, whereas the mobility/diffusivity of Li⁺ increased considerably with increasing Gd⁺³ content. Therefore, the enhanced conductivity of the current materials is mainly due to the enhanced mobility of Li⁺. Surprisingly, the fraction of mobile Li⁺ represents only 3.44–6.96% of the total Li⁺ density of the materials.