Effect of Y substitution for Nb in Li5La3Nb2O12 on Li ion conductivity of garnet-type solid electrolytes

We report the effect of Y substitution for Nb on Li ion conductivity in the well-known garnet-type Li₅La₃Nb₂O₁₂. Garnet-type Li₅La₃Nb_2−xY_xO_12−δ (0 ≤ x ≤ 1) was prepared by ceramic method using the high purity metal oxides and salts. Powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), ⁷Li nuclear magnetic resonance (Li NMR) and AC impedance spectroscopy were employed for characterization. PXRD showed formation of single-phase cubic garnet-like structure for x up to 0.25 and above x = 0.25 showed impurity in addition to the garnet-type phases. The cubic lattice constant increases with increasing Y content up to x = 0.25 in Li₅La₃Nb₂−_xY_xO12−δ and is consistent with expected ionic radius trend. ⁷Li MAS NMR showed single peak, which could be attributed to fast migration of ions between various sites in the garnet structure, close to chemical shift 0 ppm with respect to solid LiCl and which confirmed that Li ions are distributed at an average octahedral coordination in Li₅La₃Nb₂−_xY_xO₁₂−_δ. Y-doped compounds showed comparable electrical conductivity to that of the parent compound Li₅La₃Nb₂O₁₂. The x = 0.1 member of Li₅La₃Nb₂−_xY_xO₁₂−_δ showed total (bulk + grain-boundary) ionic conductivity of 1.44 × 10⁻⁵ Scm⁻¹ at 23 °C in air.