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, α-MnO2 materials have shown unique electrochemical properties in rechargeable lithium oxygen batteries. Although extensive research has been performed to investigate the structure of α-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 α-MnO2 nanowires (i.e., potassium and ammonia versions) together with bulk α-MnO2 in oxygen cathodes through environmental in situ X-ray absorption spectroscopy. The results show that the α-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 α-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 α-MnO2 materials. During cell recharge along with oxygen evolution, the materials were found to return to their initial states at low overpotential.