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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, 0.15Li2α-LixMnO2, that has Mn4+/3+ character may facilitate the Li2O2/Li2O discharge/charge chemistries providing dual electrode/electrocatalyst functionality.