Synthesis, Characterization, and Structural Modeling of High-Capacity, Dual Functioning MnO sub(2) Electrode/Electrocatalysts for Li-O sub(2) 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 [alpha]-MnO sub(2)/ramsdellite-Mn O sub(2) 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 (Li sub(2)O sub(2) and Li sub(2)O) appear to form and decompose on the air electrode surface, particularly at the MnO sub(2) surface, while Li sub(2)CO sub(3) is also formed. By contrast, cells without [alpha]-MnO sub(2)/ramsdellite-Mn O sub(2) 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 (Li sub(2)O sub(2)) and lithium oxide (Li sub(2)O) in the tunnels of [alpha]-MnO sub(2), and the reaction of lithium with ramsdellite-MnO sub(2), 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 Li sub(2)O-stabilized and partially-lithiated electrode component, 0.15Li sub(2)O.[alpha]-Li sub(x)MnO sub(2), that has Mn super(4+/3+) character may facilitate the Li sub(2)O sub(2)/Li sub(2)O discharge/charge chemistries providing dual electrode/electrocatalyst functionality. Lithium-oxygen batteries offer extremely attractive theoretical energy densities and thus represent a rapidly emerging area of research. Developing efficient, long-lasting Li-O sub(2) cells will require many technological improvements. Regarding the charge hysteresis, this approach develops transition metal oxide electrocatalysts that have a natural affinity to form compounds with Li sub(2)O, the ultimate discharge product in a non-aqueous Li-O sub(2) cell.