Modeling and experimental investigation of transport processes in the porous cathode of aqueous Li-air batteries

Kurzfassung

Lithium-air batteries are promising candidates as power sources for next-generation battery electric vehicles. Due to their high theoretical capacity they might allow driving ranges which are impossible to realize with state-of-the-art Li-ion technology. Mainly two types of Li-O2 batteries are in the focus of intensive research at the moment, namely, batteries employing aprotic or aqueous electrolytes. Most of the publications are reported on the aprotic Li-O2 battery, however, the stability of the solvent proved to be a major issue of this system. Moreover, the formation of solid, insulating discharge products limits the capacity of the battery. The aqueous Li-O2 battery is an interesting alternative. Offering a similar theoretical capacity it has a stable electrolyte and better rate-capability due to the applicability of so-called gas diffusion electrodes (GDE). In this type of electrode hydrophobic binders ensure the coexistence of gas and liquid phase, thereby, offering fast transport of O2 in the gas phase. Similar to the aprotic system the formation of solid LiOH.H2O limits the capacity of the battery. The interplay of transport, electrochemistry, and precipitation is very complex and hard to study experimentally. In this work a multiscale modeling framework for the investigation of transport processes in porous GDEs for application in Li-O2 batteries was developed. First, electrochemical measurements in a half-cell setup were conducted to access the electrochemical properties of Ag/Ag2O model electrodes. In a following step 3D reconstructions of the electrode were used for Lattice-Boltzmann multiphase simulations on the micro-structure of the GDE. The simulations allow the determination of pore space saturation and corresponding effective transport parameters. The results of experiments and micro-structure simulations were used for the parametrization of a detailed continuum model. Simulations of the half-cell setup are in good agreement to the experimental data over a large range of conditions and demonstrate the validity of the model. The validated model allows the determination and optimization of important design parameters and is able to guide the development of future electrodes for application in Li-O2 batteries. Moreover, the GDE model was inserted into a virtual battery cell. This allows conclusive insights in the operation of aqueous Li-O2 batteries and reveals the limitations of different battery designs. The model gives the unique opportunity for the assessment of battery power and energy which already in this early stage of research allows a comparison to other battery chemistries.