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 ≤ 0.75) was prepared via solid-state reaction in air and characterized using ex situ and in situ powder X-ray diffraction (PXRD), 7Li and 27Al 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. 7Li 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 × 10–4 S cm–1 at 25 °C, which is the highest value found in garnet-type compounds, and is only reported for Li7La3Zr2O12. The in situ PXRD measurements revealed structural stability up to 400–600 °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 27Al MAS NMR, does not affect the Li ionic conductivity and chemical stability of “Li5+2xLa3Nb2–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+.