Synergistic effect of oxygen vacancy and dual electrocatalysts in activating anion redox in lithia-based cathodes

Abstract

Cathodes operated partially or totally by anion-redox reaction, such as lithium-rich oxides (LROs) or air cathodes of Li-O-2 batteries, are attractive owing to their high energy density. Among these, lithia (Li2O)-based cathode systems have drawn attention as an alternative to LROs or Li-O-2 battery systems because of their high energy density in safe, sealed environments. However, the practical realization of lithia cathodes is still hindered by poor cycling stability due to the oxygen emission and difficulties in activating the anion-redox reactions during charging. Herein, we employed catalysts that promote both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) and enhance the oxygen vacancies in the catalysts and lithia to achieve a reversible anion redox. The adopted OER and ORR catalysts were Co3O4 and Ru-doped CeO2, respectively. CeO2 was expected to mediate oxygen through its superior oxygen storage capacity and was thus employed as a complementary catalyst. Oxygen vacancy enhancement was achieved through the heat treatment of Li2O2 producing Li2O2-x, an oxygen-vacancy-rich intermediate phase, and Ru doping into CeO2. Through a comparative investigation, we identified that oxygen vacancies in the cathode considerably enhanced the cycling stability of the lithia cathode. In addition, the Co3O4 OER catalyst is essential for initiating anion redox in lithia; however, the ORR catalyst is required for the reversible operation of this cathode. The synergistic combination of oxygen vacancies, dual OER/ORR catalysts, and the complementary catalyst with oxygen-mediating capability in the Co3O4/Ru-CeO2/Li2O2-x electrodes resulted in superior electrochemical characteristics with cycling stability at a capacity limit of 450 mA h g(-1) over 100 cycles.