Abstract
The performance of lithium-oxygen batteries is strongly limited by the insulating properties of the discharge product lithium peroxide Li2O2. Controversial viewpoints have been put forward in the literature regarding the mechanisms of electronic transport across thin Li2O2 films. In this context, the ion transport properties of the films have been largly neglected. We have carried out a detailed study of ion and electron transport across dense Li2O2 films on top of glassy carbon electrodes in an acetonitrile-based electrolyte. To this end, we have combined electrochemical impedance measurements with AFM scratching experiments. The results show that the ionic resistance and the electronic resistance exhibit a very similar dependence on the thickness of the films. This similar thickness dependence can be explained in a straightforward fashion by nonlinear ion and electron hopping transport processes due to large electrical potential and chemical potential gradients in the thin Li2O2 layers. The impedance spectra of our film are at variance with hole tunneling models and with models describing a non-uniform film growth on the electrode.
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