Main content area

Electron and Molecule Transport across Thin Li₂O₂ Layers: How Can Dense Layers Be Distinguished from Porous Layers?

Müller, Sandra, Zhou, Wenjian, Ramanayagam, Asvitha, Roling, Bernhard
Journal of physical chemistry 2019 v.123 no.11 pp. 6388-6394
batteries, cobalt, electrochemistry, electrodes, electron transfer, mass transfer, oxygen, physical chemistry, scanning electron microscopy
In Li–O₂ batteries, charge and mass transport across the discharge product Li₂O₂ plays an important role for the kinetics. In general, it is distinguished between laterally homogeneous transport across dense Li₂O₂ layers and heterogeneous transport across porous layers. However, in many studies, the dense or porous nature was not verified. Here, we use a combination of scanning electron microscopy, atomic force microscopy-based scratching experiments, and electrochemical measurements on thin Li₂O₂ layers to demonstrate a simple method for verifying the dense nature of a layer. We show that dense layers with a fraction of the free electrode surface below 10–⁵ exhibit virtually the same charge-transfer resistance for oxygen reduction and for the redox reaction of Co(Cp)₂⁺/Co(Cp)₂ redox probe molecules, indicating that both charge-transfer resistances are determined by electron transport across the dense layers. In contrast, if this fraction exceeds 10–⁵, the charge-transfer resistance of the Co(Cp)₂⁺/Co(Cp)₂ redox reaction is much lower than that of the oxygen reduction. Our results lead to the conclusion that measuring the charge-transfer resistance of the oxygen reduction alone is not sufficient for characterizing charge-transport limitations, but additional information about the dense/porous nature of the Li₂O₂ layer is indispensable.