Synthesis,Characterization, and Structural Modeling of High‐Capacity,Dual Functioning MnO2 Electrode/Electrocatalysts for Li‐O2 Cells

It has become clear that cycling lithium‐oxygen cells in carbonate electrolytes is impractical, as electrolyte decomposition, triggered by oxygen reduction products, dominates the cell chemistry. This research shows that employing an α‐MnO₂/ramsdellite‐MnO₂ electrode/electrocatalyst results in the formation of lithium‐oxide‐like discharge products in propylene carbonate, which has been reported to be extremely susceptible to decomposition. X‐ray photoelectron data have shown that what are likely lithium oxides (Li₂O₂ and Li₂O) appear to form and decompose on the air electrode surface, particularly at the MnO₂ surface, while Li₂CO₃ is also formed. By contrast, cells without α‐MnO₂/ramsdellite‐MnO₂ fail rapidly in electrochemical cycling, likely due to the differences in the discharge product. Relatively high electrode capacities, up to 5000 mAh/g (carbon + electrode/electrocatalyst), have been achieved with non‐optimized air electrodes. Insights into reversible insertion reactions of lithium, lithium peroxide (Li₂O₂) and lithium oxide (Li₂O) in the tunnels of α‐MnO₂, and the reaction of lithium with ramsdellite‐MnO₂, as determined by first principles density functional theory calculations, are used to provide a possible explanation for some of the observed results. It is speculated that a Li₂O‐stabilized and partially‐lithiated electrode component, 0.15Li₂O·α‐Li_xMnO₂, that has Mn^4+/3+ character may facilitate the Li₂O₂/Li₂O discharge/charge chemistries providing dual electrode/electrocatalyst functionality.