Effect of transition metal oxide cathodes on the oxygen evolution reaction in Li-O₂ batteries

Abstract

Li-oxygen batteries could provide energy density that is up to five times greater than that of state-of-the-art Li-ion batteries. However, Li-oxygen cell rechargeability is limited by cathode passivation due to nonconductive discharge products. Despite efforts to efficiently oxidize these products, oxygen recovery remains poor at potentials where cell constituents are stable. Transition metal oxide (TMO) cathodes have shown low charging potentials, but a correspondence to improved oxygen evolution efficiency is debated. This is because the deposition of electrically insulating Li2O2 during the battery discharge could passivate the TMO surfaces and render them inactive for catalyzing oxygen evolution during charge. Contrary to this, we show that TMOs enable charging at low overpotentials without compromising the oxygen evolution efficiency. Charge potentials are lowered by 130-400 mV in batteries employing TMO-based cathodes compared to carbon-only cathodes. By a combination of current-sensing atomic force microscopy and differential electrochemical mass spectrometry, we show that Li2O2 has a greater propensity for deposition on carbon surfaces and only sparingly covers the RuO2 surfaces. Our results suggest that chemically heterogeneous cathodes containing (1) surfaces that favor Li2O2 growth and (2) surfaces that are catalytically active but do not promote Li2O2 deposition can decrease charge overpotentials without decreasing oxygen evolution efficiency. (Graph Presented).