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 alpha-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 O-2 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 alpha-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 (similar to 3.8 V) will dissociate the entire Li2O2 layer. These findings explain why alpha-MnO2 performs so well as a catalyst in Li-air battery cathodes, and suggest that a larger area of alpha-MnO2(100) can help reduce capacity loss.