Effect of lithium ion content on the lithium ion conductivity of the garnet-like structure Li5+xBaLa2Ta2O11.5+0.5x (x = 0–2)

We report systematic studies on the transport properties by varying the lithium oxide content of the garnet-based solid electrolyte Li5+xBaLa2Ta2O11.5+0.5x (x=0, 0.50, 0.75, 1.00, 1.25, 1.50, 1.75, 2.00) for understanding the ionic conductivity dependence on the crystal lattice parameter and carrier concentration. Powder X-ray diffraction data of Li5+xBaLa2Ta2O11.5+0.5x (x=0, 0.50, 0.75, 1.00, 1.25, 1.50, 1.75, 2.00) indicate the existence of the garnet-like structure for any of the compositions. The cubic lattice parameter was found to increase with increasing x and reaches a maximum at x=1.00, then decreases slightly with a further increase in x. Impedance measurements obtained at 50 °C indicate a maximum of the grain-boundary resistance (Rgb) contribution to the total resistance (Rb+Rgb) at x=0.0 and a considerable decrease with increase in lithium concentration. The total (bulk + grain-boundary) and bulk ionic conductivity increase with increasing lithium content and reach a maximum at x=1.00 and then decrease slightly with further increase in x. Among the investigated compounds, Li6BaLa2Ta2O12 exhibits the highest total (bulk + grain-boundary) and bulk ionic conductivity of 1.5×10-4 and 1.8×10-4 S/cm at 50 °C, respectively. The results obtained in the present investigation of the Li5+xBaLa2Ta2O11.5+0.5x (x=0–2) series clearly revealed that the lithium content plays a major role in decreasing the grain boundary resistance contribution to the total resistance and also in increasing the ionic conductivity of the garnet-like compound.