Enhancing Li Ion Conductivity of Garnet-Type Li5La3Nb2O12 by Y- and Li-Codoping: Synthesis, Structure, Chemical Stability, and Transport Properties

Novel Li-stuffed garnet-like Li5+2xLa3Nb2-xYxO12 (0.05 <= x 5 <= 0.75) was prepared via solid-state reaction in air and characterized using ex situ and in situ powder X-ray diffraction (PXRD), Li-7 and Al-27 magic angle spinning nuclear magnetic resonance (MAS NMR), scanning electron microscopy (SEM), thermo-gravimetric analysis (TGA), and AC impedance spectroscopy. Rietveld refinement with the PXRD data confirmed the formation of a cubic, garnet-like Ia-3d structure. Li-7 MAS NMR showed a single sharp peak close to 0 ppm as the usual trend for fast Li ion conducting garnets. Among the materials studied, Li6.5La3Nb1.25Y0.75O12 showed a very high bulk ionic conductivity of 2.7 X 10(-4) S cm(-1) at 25 C, which is the highest value found in garnet-type compounds, and is only reported for Li2La3Zr2O12. The in situ PXRD measurements revealed structural stability up to 400-600 degrees C after water treatment as well as chemical compatibility with high voltage Li cathodes Li2MMn3O8 (M = Co, Fe). The current work demonstrates that slight Al contamination from the Al2O3 crucible, which is commonly observed in this class of materials and was detected by Al-27 MAS NMR, does not affect the Li ionic conductivity and chemical stability of Li5+2La3Nb2-xYxO12 garnets. It also shows that Li stuffing is critical to improve further the ionic conductivity of parent compound Li5La3Nb2O12.Y3+ seems to be a very efficient dopant to improve the ionic conductivity of garnets compared to other investigated dopants that include M2+. (M = alkaline earth metals), In3+, and Zr4+.