Environmental In Situ X-ray Absorption Spectroscopy Evaluation of Electrode Materials for Rechargeable Lithium-Oxygen Batteries

Lithium-oxygen batteries have attracted much recent attention due their high theoretical capacities, which exceeds that of Li-ion batteries. Among all the metal oxides that have been investigated in oxygen cathodes, alpha-MnO2 materials have shown unique electrochemical properties in rechargeable lithium oxygen batteries. Although extensive research has been performed to investigate the structure of alpha-MnO2 upon lithium intercalation, its behavior upon reacting with lithium under an oxygen environment remains to be fully explored. Here, we performed a systematic study on the behavior of two forms of alpha-MnO2 nanowires (i.e., potassium and ammonia versions) together with bulk alpha-MnO2 in oxygen cathodes through environmental in situ X-ray absorption spectroscopy. The results show that the alpha-MnO2 materials undergo lithium insertion/removal and lithium peroxide formation/decomposition simultaneously. The former causes a self-switching of the oxidation state of Mn during cycling. Additionally, we found that potassium-containing alpha-MnO2 nanowires exhibit a suppression of Mn reduction until late in cell discharge under oxygen, retaining a higher degree of Mn4+. character for enhanced oxygen reduction activity than other, similar alpha-MnO2 materials. During cell recharge along with oxygen evolution, the materials were found to return to their initial states at low overpotential.