Production and Characterization of carbon-free bi-functional cathodes for the use in lithium-air batteries with an aqueous alkaline electrolyte

Kurzfassung

Lithium-Air Batteries (LAB) are one of the most promising upcoming energy storage devices. With a theoretical energy density of 11680 Wh/kg and a practical energy density around 1700 Wh/kg a LAB has similar energy densities to gasoline. Furthermore a practical energy density of 1700 Wh/kg would mean a 5-10 fold increase over todays Li-Ion Batteries (LIB) with 100-200 Wh/kg. Carbon materials are widely used in gas diffusion electrodes due to their high electronic conductivity, relatively low costs and catalytic activity towards oxygen reduction reaction (ORR), the cathodic reaction during discharging an air battery. During charging a lithium-air battery the cathode is operated in oxygen evolution reaction (OER) mode. Carbon materials corrode in OER mode, this leads to degradation and a power loss of the electrode. To improve long-term stability and reduce side reactions as H2 and CO2 evolution in OER operation mode carbon-free bi-functional cathodes for aqueous alkaline lithium-air batteries were prepared. Gas diffusion electrodes with silver catalysts show a high activity towards oxygen reduction reaction in alkaline media but a rather poor activity towards oxygen evolution reaction. For the use in future lithium-air batteries with an aqueous alkaline electrolyte the activity of such electrodes must be improved significantly. Combinations of silver with promising oxide catalysts show a significant improvement in OER performance and also long-term stability. Gas diffusion electrodes were electrochemically investigated in a half cell. In addition to cyclic voltammetry electrochemical impedance spectroscopy (EIS) was used for the electrochemical characterization of the porous gas diffusion electrodes. Impedance spectra were recorded in the frequency range from 100 mHz to 100 kHz at room temperature and at 50°C, different concentrations of LiOH, different potentials covering all the range of operation of the new developed electrodes. Beside the impedance elements attributed to the structure of the electrode and kinetic parameters of the ORR in the potential range of OER an additional impedance contribution attributed to the formation of an oxide layer can be found. To complete the electrochemical characterization SEM and XRD were used for the physical and morphological investigations particularly with regard to the composition and stability of formed silver oxide during OER in the anodic potential region.