Highly Li-Stuffed Garnet-Type Li_7+xLa₃Zr_2-xY_xO₁₂

We report the synthesis, microstructure and Li ion conductivity of Li-stuffed garnet-type metal oxides, with nominal chemical formulae Li_7.06La₃Zr_1.94Y_0.06O₁₂ and Li_7.16La₃Zr_1.84Y_0.16O₁₂. Powder X-ray diffraction (PXRD) shows the formation of cubic garnet-type structure at 950^oC (12 h in air) with cell constant of 12.974(3) - and 12.995(2) - for Li_7.06La₃Zr_1.94Y_0.06O₁₂ and Li_7.16La₃Zr_1.84Y_0.16O₁₂, respectively. The slight increase in cubic cell constant is consistent with Y-substitution for Zr in Li₇La₃Zr₂O₁₂. For samples sintered at 1100-1150^oC, PXRD displays multi-phase mixtures, but the garnet-type structure remained the major phase. Scanning electron microscopy (SEM) studies show an average crystallite size in the range of 2-5 μm, and particle size increases with increasing sintering temperature, as anticipated. Energy dispersive spectroscopy (EDS) elemental mapping show single homogenous distribution of elements, for samples prepared at 950^oC. Electrochemical AC impedance spectra exhibit mainly bulk contribution at temperatures higher than 150^oC, and at low temperature, we see contributions from bulk and grain-boundary resistance. The bulk electrical conductivity of the sample was found to be comparable to those of the high temperature tetragonal Li₇La₃Zr₂O₁₂ phase. The electrical conductivity of Y-doped samples investigated in the present work was ~T0^-6 Scm^-1 at 23^oC with an activation energy of 0.47 eV. The lower conductivity of this Y-doped garnet was explained based on Raman spectroscopy and SEM analysis.