Li₆ALa₂Ta₂O₁₂ (A=Sr, Ba): Novel garnet-like oxides for fast lithium ion conduction

Oxides with the nominal chemical formula Li₆ALa ₂Ta₂O₁₂ (A = Sr, Ba) have been prepared via a solid-state reaction in air using high purity La₂O₃, LiOH·H₂O, Sr(NO₃)₂, Ba(NO ₃)₂, and Ta₂O₅ and are characterized by powder X-ray diffraction (XRD) in order to identify the phase formation and AC impedance to determine the lithium ion conductivity. The powder XRD data of Li₆ALa₂Ta₂O₁₂ show that they are isostructural with the parent garnet-like compound Li ₅La₃Ta₂O₁₂. The cubic lattice parameter was found to increase with increasing ionic size of the alkaline earth ions (Li₆SrLa₂Ta₂O₁₂:12.808(2) ̊; Li₆-BaLa₂Ta₂O₁₂:12.946(3) ̊). AC impedance results show that both the strontium and barium members exhibit mainly a bulk contribution with a rather small grain-boundary contribution. The ionic conductivity increases with increasing ionic radius of the alkaline earth elements. The barium compound, Li₆BaLa ₂Ta₂O₁₂, shows the highest ionic conductivity, 4.0×10^-5 S cm^-1 at 22 °C with an activation energy of 0.40 eV, which is comparable to other lithium ion conductors, especially with the presently employed solid electrolyte lithium phosphorus oxynitride (Lipon) for all-solid-state lithium ion batteries. DC electrical measurements using lithium-ion-blocking and reversible electrodes revealed that the electronic conductivity is very small, and a high electrochemical stability (> 6 V/Li) was exhibited at room temperature. Interestingly, Li ₆ALa₂Ta₂O₁₂ was found to be chemically stable with molten metallic lithium.