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 a -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 Li ₂O) appear to form and decompose on the air electrode surface, particularly at the MnO₂ surface, while Li₂O₃ is also formed. By contrast, cells without a -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 a -MnO₂ , and the reaction of lithium with ramsdellite-MnO₂ , as determined by fi rst 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 · a - Li x MnO₂ , that has Mn 4 ± /3 ± character may facilitate the Li₂O₂ /Li ₂O discharge/ charge chemistries providing dual electrode/ electrocatalyst functionality. © 2013 WILEY-VCH Verlag GmbH & Co.