Oxygen Reduction Reaction Promotes Li+ Desorption from Cathode Surface in Li-O2 Batteries

Abstract

Li-O2 batteries are claimed to be one of the future energy storage technologies. Great number of scientific and technological challenges should be solved first to transform Li-O2 battery from a promise to real practical devices. Proposed mechanisms for oxygen reduction assume a reservoir of solved Li+ ions in the electrolyte. However, the role that adsorbed Li+ on the electrode surface might have on the overall oxygen reduction reaction (ORR) has not deserved much attention. Adsorbed Li+ consumption is monitored here using impedance measurements from extended electrochemical double layer capacitance, which depends on the carbon matrix surface area. The presence of O2 drastically reduces the amount of adsorbed Li+, signaling the kinetic competition between Li+ surface adsorption and its consumption, only for potentials corresponding to the oxygen reduction reaction. Noticeably double layer capacitance remains unaltered after cycling. This fact suggests that the ORR products (Li2O2 and Li2CO3) are not covering the internal electrode surface, but deposited on the outer electrode-contact interface, hindering thereby the subsequent reaction. Current results show new insights into the discharge mechanism of Li-O2 batteries and reveal the evidence of Li+ desorption from the C surface when the ORR starts.