Gas Diffusion Electrodes for Li/O2 Batteries: Impact of Porosity, Wettabiliy and Electrode Design

Beyond lithium ion batteries the metal air technology is considered to be very promising for various energy storage applications in a long-term view. Due to its very high theoretical energy density the aprotic Li/O₂ system in particular has attracted broad attention during the recent years. Here molecular oxygen O₂ is reduced at a gas diffusion electrode (GDE) upon discharge forming Li₂O₂ as a solid discharge product which precipitates within the electrode to be reoxidized under the release of oxygen upon recharge. The performance of a Li/O₂cell is therefore critically influenced by the structural properties of the used GDE. Based on carbon, especially the porosity, the wetting behavior and the general design of the GDE has a huge impact on the capacity and efficiency of the cell.

Regarding the wettability of a GDE, a thin electrolyte film within the inner pore structure is considered to be ideal to provide efficient oxygen transport through open gas channels as well as improved diffusion properties towards the active surface. Using mesoporous carbon xerogels which exhibit defined open pore structures, large inner surfaces and pore volumes we were able to investigate the pore wettability with different solvents by using ¹H relaxation MRT techniques. To investigate them as active material in the Li/O₂ system they were synthesized directly into a woven carbon paper to create ready-to-use electrodes without any binder. Here, the discharge reaction was studied by electrical impedance spectroscopy (EIS) using 1M LiTFSI/DMSO as electrolyte and Li metal as reference electrode in a three electrode setup. A model for the interpretation of the limiting factors regarding the GDE performance due to the formation and deposition of the discharge product could be developed. For further investigation of these factors the chemical and morphological characterization of the discharge products in dependence on the GDE design was addressed by XPS analyses. Besides looking at the electrolyte facing side the deposition of discharge product in the bulk of the active layer in the GDE and at the oxygen facing side were investigated. For information about the bulk of the GDE cross-section measurements as well as depth profiling by argon ion sputtering were conducted.