Influence of Ta doping on the structural stability and conductivity of Sr11Mo4O23 electrolyte

Abstract

Sr11Mo4O23(SMO) material has a double-layer perovskite superstructure, which exhibits unusual structural flexibility and oxygen ion mobility. However, the Sr11Mo4O23 cubic phase will transform into SrMoO4 tetragonal phase at 400 °C, which leads to a sharp decrease in conductivity. In order to solve this problem, Ta doped Sr11Mo4-xTaxO23-δ (SMTO, 0 ≤ x ≤ 1.25) electrolytes were synthesized by a route combining the Pechini method and solid-state reaction. The effects of acceptor-type Ta5+ doping on the structural stability, micro-scale structure and ionic conductivity of SMO are characterized by X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, scanning electron microscopy, and electrochemical impedance spectroscopy. All Sr11Mo4-xTaxO23-δ powders are single-phase and no secondary phase is detected. Moreover, the phase transition of Sr11Mo4O23 to SrMoO4 is highly inhibited by partially replacing Mo with Ta (x ≥ 0.50) during the heat treatment, and the Sr11Mo4O23 cubic phase with high conductivity may be maintained for a long time at 800 °C. The total ionic conductivity of Sr11Mo4-xTaxO23-δ samples increases with increasing the Ta concentration, and then declines at higher doping content (x = 1.25). The ionic conductivity of Sr11Mo3TaO23-δ (SMTO100) pellet is the highest, reaching 1.44 × 10−2 S/cm at 800 °C. More remarkably, the conductivity of STMO100 pellet remains at its peak during the 100 h annealing test at the temperature of 800 °C.