Effect of V-doping on the structure and conductivity of garnet-type Li₅La₃Nb₂O₁₂

The effect of isovalent V substitution for Nb in Li₅La₃Nb₂O₁₂ on the electrical conductivity properties of the garnet-type structure was investigated by various physical methods, including powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and ⁷Li nuclear magnetic resonance (NMR) spectroscopy. Garnet-type metal oxides of the nominal chemical formula, Li₅La₃Nb_2−xV_xO₁₂ (x = 0 – 0.25) were prepared by conventional solid-state method at elevated temperature in air. PXRD results showed that the cubic garnet-type structure with space group (s.g.) of Ia-3d was maintained, but several peaks attributed to secondary phase LaVO₄ (s.g.P2₁/n) were also observed. SEM measurements indicate doping V for Nb improved particle-to-particle contact. The effect of V substitution for Nb in Li₅La₃Nb₂O₁₂ on ionic conductivity was also studied by AC impedance spectroscopy. The total (bulk + grain boundary) conductivity gradually increased with V content up until x = 0.15 and decreased thereafter. The x = 0.15 showed the highest total conductivity of 6 × 10⁻⁶ S cm⁻¹ at room temperature, with the lowest activation energy of 0.37 eV.