High lithium-ion conductivity, especially at room temperature, and chemical stability against reducing lithiated negative electrodes are two main requirements for a solid electrolyte in an all solid-state lithium battery. Perovskite-type materials ABO(3) in which A = La, Li and B = Ti exhibit high lithium ion conductivities, with a bulk conductivity of 1.2 mS/cm at 30 degreesC and an apparent grain-boundary conductivity of 0.03 mS/cm; this solid electrolyte is only stable above 1.6 V (vs. Lidegrees) because Ti(IV) can be reduced to Ti(III) below this voltage. In this study, we investigated a highly conducting perovskite-type Li-Sr-Ta-Zr-O structure, in which the A and B cations of SrZrO3 are partially substituted by Li and Ta, respectively. Four compositions were selected and synthesized to find an optimal composition. Sintering temperatures between 1200 and 1400 degreesC were used to determine the optimum synthesis conditions. X-ray diffraction (XRD) was used to identify the crystalline phases within the sintered products. The lithium-ion conductivity of these materials was measured by AC impedance spectroscopy. The sample with optimized composition, Li(3/8)Sr(7/16)Ta(3/4)ZT(1/4)O(3), exhibited a bulk lithium-ion conductivity of 0.2 mS/cm at 30 degreesC and an apparent grain-boundary conductivity of 0.13 mS/cm. This solid electrolyte was found to be stable above 1.0 V against metallic lithium. Below 1.0 V, about 0.08 mol lithium can be inserted irreversibly per mol of Li3/8Sr7/16Ta3/4Zr3/4O3. (C) 2004 Elsevier B.V. All rights reserved.