Mass transport in gas diffusion electrodes for aqueous Li-O2 batteries: Modeling and experiments

Kurzfassung

The lithium-oxygen (Li-O2) battery is an interesting candidate to substitute conventional combustion technologies in the mobility sector. However, the development of Li-O2 cells is still at an early stage and a list of challenges has to be addressed. A relevant aspect is the need to deliver high power densities during discharge. Therefore, good oxygen supply in the cathode is a crucial factor in the design of this battery concept. Gas Diffusion Electrodes (GDEs) offer fast oxygen transport, a high specific surface area, and therefore high discharge rates. Although GDEs are already used in various applications like electrolysis or fuel cells, they have not been studied in detail in the context of lithium-air cells. We develop a continuum model of Li-O2 batteries describing the coexistence of solid, liquid, and gaseous phases in the GDE. Oxygen dissolution in the aqueous electrolyte, oxygen reduction and lithium oxidation at the two electrodes are taken into account, as well as precipitation of solid LiOH∙H2O that occurs at concentrations above the solubility limit. Kinetic parameters for the oxygen reduction reaction have been extracted from experimentally obtained polarization curves in one molar solution. For the validation of the proposed model half-cell experiments using silver electrodes have been carried out for various temperatures and concentrations. Experiments include cyclic voltammetry and electrochemical impedance spectroscopy. The combination of experimental techniques and our model approach allows detailed insight into the transport and saturation behavior of the GDE. The validated model agrees reasonably well with experiments and can be used to improve the design of future electrodes.