Catalytic properties of α-MnO₂ for Li-air battery cathodes: a density functional investigation

Details of the formation and dissociation of the first layer of Li₂O₂ on the α-MnO₂(100) surface as the cathode in Li-air batteries have been studied using first principles density functional theory. The bias dependence of the electrochemical steps of charge (Li₂O₂ dissociation) and discharge (Li₂O₂ formation) via two different mechanisms has been studied. Discharge potential is found to be 2.94 V for the mechanism in which O₂ adsorption is followed by lithiation. Charging potential for the reverse process is 3.37 V, giving an overpotential of 0.43 V, which is much lower than that on carbon electrodes. This is also in good agreement with experiments on α-MnO₂ cathodes. In Li₂O₂ formation via the disproportionation of two LiO₂ adsorbates, a maximum discharge potential of 2.61 V and a minimum charging potential of 3.48 V are obtained. The minimum energy pathway in this mechanism has a moderate kinetic barrier of 0.57 eV. Charging potentials of 3.37 V and 3.48 V imply that the typical charging potentials applied in the experiments (∼3.8 V) will dissociate the entire Li₂O₂ layer. These findings explain why α-MnO₂ performs so well as a catalyst in Li-air battery cathodes, and suggest that a larger area of α-MnO₂(100) can help reduce capacity loss.