Highly Li-Stuffed Garnet-Type Li7+xLa3Zr2-xYxO12

Abstract

We report the synthesis, microstructure and Li ion conductivity of Li-stuffed garnet-type metal oxides, with nominal chemical formulae Li7.06La3Zr1.94Y0.06O12 and Li7.16La3Zr1.84Y0.16O12. Powder X-ray diffraction (PXRD) shows the formation of cubic garnet-type structure at 950°C (12 h in air) with cell constant of 12.974(3) Å and 12.995(2) Å for Li7.06La3Zr1.94Y0.06O12 and Li7.16La3Zr1.84Y0.16O12, respectively. The slight increase in cubic cell constant is consistent with Y-substitution for Zr in Li7La3Zr2O12. For samples sintered at 1100–1150°C, 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°C. Electrochemical AC impedance spectra exhibit mainly bulk contribution at temperatures higher than 150°C, 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 ∼10−6 Scm−1 at 23°C 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.