Mechanistic Evaluation of LixOy Formation on δ-MnO2 in Nonaqueous Li-Air Batteries

Transition metal oxides are usually used as catalysts in the air cathode of lithium-air (Li-air) batteries. This study elucidates the mechanistic origin of the oxygen reduction reaction catalyzed by delta-MnO2 monolayers and maps the conditions for Li2O2 growth using a combination of first principles calculations and mesoscale modeling. The MnO2 monolayer, in the absence of an applied potential, preferentially reacts with a Li atom instead of an O-2 molecule to initiate the formation of LiO2. The oxygen reduction products (LiO2, Li2O2, and Li2O molecules) strongly interact with the MnO2 monolayer via the stabilization of Li-O chemical bonds with lattice oxygen atoms. As compared to the disproportionation reaction, direct lithiation reactions are the primary contributors to the stabilization of Li2O2 on the MnO2 monolayer. The energy profiles of (Li2O2)(2) and (Li2O)(2) nucleation on delta-MnO2 monolayer during the discharge process demonstrate that Li2O2 is the predominant discharge product and that further reduction to Li2O is inhibited by the high overpotential of 1.21 V. Interface structures have been examined to study the interaction between the Li2O2 and MnO2 layers. This study demonstrates that a Li2O2 film can be homogeneously deposited onto delta-MnO2 and that the Li2O2/MnO2 interface acts as an electrical conductor. A mesoscale model, developed based on findings from the first-principles calculations, further shows that Li2O2 is the primary product of electrochemical reactions when the applied potential is smaller than 2.4 V.