First Principles Study of Lithium and Oxygen Adsorption on the β-MnO2 (110) Surface

The Li-air system is a novel battery technology which promises higher specific energy than Li-ion batteries. In this battery the cathode reaction is not the formation of an intercalation compound but the reduction of oxygen gas in the presence of Li+ ions forming lithium peroxide Li₂O₂ (or LiOH in the aqueous version).^1-5 Manganese oxides may also play an important role in this battery: it has been shown that nanostructured MnO₂ in different polymorphic states are able to catalyse the formation and decomposition of Li₂O₂ in the cathode. Understanding the behaviour of the cathode catalysts is the key for improving the function of Li-air batteries.⁶

The adsorption and co-adsorption of lithium and oxygen at the surface of rutile-like manganese dioxide (β-MnO₂), which are important in the context of Li-air batteries, are investigated using density functional theory. In the absence of lithium, the most stable surface of β-MnO₂, the (110), adsorbs oxygen in the form of peroxo groups bridging between two manganese cations. Conversely, in the absence of excess oxygen, lithium atoms adsorb on the (110) surface at two different sites, which are both tri-coordinated to surface oxygen anions, and the adsorption always involves the transfer of one electron from the adatom to one of the five-coordinated manganese cations at the surface, creating (formally) Li⁺ and Mn³⁺ species. The co-adsorption of lithium and oxygen leads to the formation of a surface oxide, involving the dissociation of the O₂ molecule, where the O adatoms saturate the coordination of surface Mn cations and also bind to the Li adatoms. This process is energetically more favourable than the formation of gas-phase lithium peroxide (Li₂O₂) monomers, but less favourable than the formation of Li₂O₂ bulk. These results suggest that the presence of β-MnO₂ in the cathode of a non-aqueous Li-O₂battery lowers the energy for the initial reduction of oxygen during cell discharge.

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