Phase Field Modeling of the Li2O2 Growth in a Porous Cathode in Li-Air Batteries with Organic Electrolytes

The electrochemistry of the Li-air batteries is based on a reversible reaction of 2Li⁺+2e^- +O₂ = Li₂O₂ with forward direction describing the discharge of the cell. As the discharge proceeds, the pores of air cathodes become increasingly filled with Li₂O₂ precipitates, which eventually blocks oxygen from diffusing to the reaction sites. Therefore, the materials and architecture of air electrode influence its performance significantly. The optimization of the cathode microstructure by changing its specific surface area, thickness, and pore size distribution is an important aspect for developing efficient batteries. Beside, different ratios of carbon and electrolyte, wettablility of electrolytes, the effective electrochemical interface, result in variable electrochemical performance. To this end, a phase field model was developed to simulate and predict the Li₂O₂ crystalline growth in a three-dimensional electrolyte-filled porous carbon cathode in Li-air batteries. The model is able to capture the three-phase (carbon, electrolyte, and Li₂O₂) morphologies and their microstructural evolutions during the cell operation. The nucleation and growth of Li₂O₂ on the carbon surface is observed. The effects of oxygen pressure, interfacial energies, carbon substrate properties such as pore size and its distribution, thickness and volume fraction on the cathode performance will be discussed. The model provides useful information to optimize the electrode microstructure to enhance the battery performance.