Structure and Properties of Lithium Ion Conductors in the Li2O-Y2O3-ZrO2 System

Abstract

<p> Ceramic samples of Li2+xYxZr1-xO3 with x=0 to 0. 1, Li8+xYxZr1-xO6 with x=0 to 0.2, Li1-xYxZr1-xO2 with x=0 to 0.5 and Li1+xZrxY1-xO2+x with x=0 to 0. 3 were prepared via conventional solid reactions. The solubilities, crystal structures and microstructures in these samples were studied by x-ray diffraction(XRD), infrared spectra, differential thermal analysis(DTA) and scanning electron microscopy(SEM). The results show that the solubilities are 0.05≤X<0.1 for Li2+xYxZr1-xO3 and Li8+xYxZr1-xO6, 0.1≤x<0.15 for Li1-xYxZr1-xO2, and x≥0.3 for Li1+xZrxY1-xO2+x, respectively. The crystal structures of the solid solutions of Li2+xYxZr1-xO3 and Li8+xYxZr1-xO6 are the same as Li2ZrO3 and Li8ZrO6, respectively but the cell constants change slightly with x, while the structure of Li1-xYxZr1-xO2 and Li1+xZrxY1-xO2+x changes from monoclinic for pure LiYO2 (x=0) to tetragonal (x≥0.005). The sinterability of Li2ZrO3 improves greatly with yttrium additions to Li2ZrO3. </p> <p> The conductivities of the samples were measured by complex impedance spectroscopy and dc polarization. The results show that lithium conductivity in Li2+xYxZr1-xO3 samples increases slightly from 3.9x10^-6 to 5.0x10^-6 S/cm at 400°C as x increases from 0 to 0.05 and the corresponding conduction activation energy decreases slightly from 0.99 to 0.92 eV. Based on the effective medium theory, the conductivity increase in the solid solution was estimated to be 3% for x=0.05 compared with pure Li2ZrO3 crystal.</p> <p>For Li8+xYxZr1-xO6 samples, a mixture of LiOH and Li2CO3, which melts at about 430°C, can be formed during the processing and measurements. The ionic conductivity depends to a large degree on the microstructure(the amount and distribution of the mixture) below 430°C. The lithium conductivity at 435°C increases from 1.0x10^-2 to 6.9x10^-2 S/cm as x increases from 0 to 0.05. The electronic contribution to total conductivity is lower than 1% below 435°C.</p> <p> The ionic conductivity in the tetragonal phase of Zr-doped LiYO2 is much lower than in pure monoclinic LiYO2. The conductivity values at 500°C are 1.3x10^-2 for pure LiYO2 and 1.2x10^-4 for Li1.3Zr0.3Y0.7O2.3. The ionic conduction activation energy in the tetragonal Zr-doped LiYO2 is much higher than pure LiYO2.</p> <p> The thermal stability and the hydrolysis tendency for Li2ZrO3, Li8ZrO6 and LiYO2 were examined by thermodynamic calculations and by experiments.</p>