Lithium ion conductivity in the solid electrolytes (Li0.25La0.25)1-xM0.5xNbO3 (M=Sr, Ba, Ca, x=0.125) with perovskite-type structure

Perovskite-type structured solid electrolytes with the general formula (Li0.25La0.25)(1-x)M0.5xNbO3 (M=Sr, Ba, Ca, x=0.125) have been prepared by solid-state reaction. Their crystal structure and ionic conductivity were examined by means of X-ray diffraction analysis (XRD), scanning electron microscope (SEM), and alternating current (AC) impedance technique. All sintered compounds are isostructural with the parent compound Li0.5La0.5Nb2O6. Some impurity phase is detected at the grain boundary in the Ba- and Ca-substituted compounds. The substitution of partial Li+ by alkaline earth metal ions has responsibility for the cell volume expansion as determined by the XRD data. The densification is accelerated, with the overall porosity and grain boundary minimized as Sr2+ ions are doped. Among the investigated compounds, the perovskite (Li0.25La0.25)(0.875)Sr0.0625NbO3 shows a remarkable ionic conductivity of 1.02x10(-5) S/cm at room temperature (20 degrees C) and the lowest activation energy of 0.34 eV in comparison with 0.38 eV and 0.44 eV for the corresponding Ba- and Ca-doped samples, respectively. It is identified that the enhancement of ionic conductivity is attributed to a reduction in activation energy for ionic conduction which is related to an increase in the cell volume.