Structure and Li+ dynamics of Sb-doped Li7La3Zr2O12 fast lithium ion conductors

Antimony-doped lithium stuffed garnets Li7−xLa3Zr2−xSbxO12 (x = 0.2–1.0) prepared using a conventional solid state reaction method are characterized using Powder X-ray Diffraction (PXRD), Scanning Electron Microscopy (SEM), Energy Dispersive Analysis by X-ray (EDAX), AC Impedance spectroscopy, Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) and Raman spectroscopic techniques. PXRD confirms the formation of a garnet-like structure with cubic symmetry for the entire selected compositional range. Among the investigated compounds, the compound with an Sb content corresponding to x = 0.4, i.e. Li6.6La3Zr1.6Sb0.4O12 exhibits the maximum total (bulk + grain boundary) ionic conductivity of 7.7 × 10−4 S cm−1 at 30 °C. The shape of the imaginary part of the modulus spectra suggests that the relaxation processes are non-Debye in nature. The full width at half maximum (FWHM) for the master modulus curve of Li6.6La3Zr1.6Sb0.4O12 is found to be the smallest among the investigated lithium garnets. The full width at half maximum (FWHM) of the 7Li MAS NMR spectrum for the composition Li6.6La3Zr1.6Sb0.4O12 is the smallest among the investigated compounds. Raman data collected for the compounds in this series indicates an increase of Li+ occupancy in the tetrahedrally coordinated site with an associated decrease of Li+ occupancy in the octahedrally coordinated site during an increase of x in Li7−xLa3Zr2−xSbxO12. The present investigation reveals that the optimal Li+ concentration required to achieve the maximum room-temperature Li+ conductivity in Li7−xLa3Zr2−xSbxO12 lithium stuffed garnet is around x = 0.4.