The NASICON-type lithium-ion conducting Li1+xAlxTi2-x(PO4)(3) solid electrolytes are attractive candidates as the separator for high energy density aqueous lithium-air batteries and all-solid-state batteries because they are stable in aqueous solutions with a high content of lithium ions. To improve the lithium-ion conductivity of 6.0 x 10(-4) S cm(-1) at 25 degrees C for Li1.4Al0.4Ti1.6(PO4)(3) (LATP), the additions of metal oxides of GeO2, TiO2, and Al2O3 were examined. The highest lithium-ion conductivity of 1.15 x 10(-3)S cm(-1) at 25 degrees C was observed for LATP-3 wt% GeO2 sintered at 950 degrees C. This enhancement is explained by enhancements in both bulk and grain boundary conductivity.