Li₆ALa₂Nb₂O₁₂ (A = Ca, Sr, Ba): A new class of fast lithium ion conductors with garnet-like structure

Garnet-like structured metal oxides with the general formula Li ₆ALa₂Nb₂O₁₂ (A = Ca, Sr, Ba) have been prepared by solid-state reaction using appropriate amounts of corresponding metal oxides, nitrates, and hydroxides. The powder X-ray diffraction data reveal that Li₆ALa₂Nb₂O₁₂ compounds are isostructural with the parent compound Li₅La₃Nb ₂O₁₂. The cubic lattice parameter was found to increase with increasing size of the alkaline earth ions. The grain size decreases considerably with the substitution of La by the alkaline earth elements under the same preparation conditions. The Ca-substituted compound exhibits both bulk and major grain boundary contributions to the total resistance, while the Sr- and Ba-substituted compounds show mainly bulk resistance with a rather small grain boundary contribution (∼14% of the total resistance at 20°C) with further decrease with increasing temperature. In comparison, the ionic conductivity decreases with decreasing ionic radius of the alkaline earth elements. Among the investigated compounds, the Ba-compound Li ₆BaLa₂Nb₂O₁₂ shows the highest ionic conductivity of 6 × 10^-6 S/cm at room temperature (22°C) and lowest activation energy of 0.44 eV compared with 0.55 and 0.50 eV for the corresponding Ca- and Sr- compounds, respectively. The ionic conductivity is comparable with that of parent Li₅La₃Nb₂O ₁₂ and other fast lithium ion conductors known so far.