Stable lithium-ion conducting perovskite lithium–strontium–tantalum–zirconium–oxide system

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 ABO3 in which A=La, Li and B=Ti exhibit high lithium ion conductivities, with a bulk conductivity of 1.2 mS/cm at 30 °C and an apparent grain-boundary conductivity of 0.03 mS/cm; this solid electrolyte is only stable above 1.6 V (vs. Li°) 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 °C 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, Li3/8Sr7/16Ta3/4Zr1/4O3, exhibited a bulk lithium-ion conductivity of 0.2 mS/cm at 30 °C 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 Li3/8Sr7/16Ta3/4Zr1/4O3.