Geometric Optimization of Li-O2 Battery Cathodes with Pore Structure By Meso-Scale Modeling and Simulation Pursuing Practical Systems

Recent demands for post Li-ion batteries attracted enormous attentions to Li-air battery that is theoretically capable over 3000 Wh/kg specific energy [1].Since first introduction of the Li-air battery by Abraham and Jiang in 1996 [2], various experimental studies have been conducted to explore detailed physical and chemical properties as well as practical feasibilities of this energy storage system [3]. The most dominant reaction mechanism so far known is that lithium ion react with oxygen producing Li₂O₂ in a non-aqueous electrolyte and carbon composite cathode [3].The reaction product, Li₂O₂, is an insulator with a wide band gap that limits capacity of the cell when it deposits and passivates all the active surfaces with certain thickness, about 5 - 10 nm [4,5].

Couples of numerical models to assist in predicting the performance of the cell have also been developed by modifying the models developed for Li-ion batteries. However, detailed geometrical optimization of cathode components composed of an electrolyte, carbon particles and pores has not been reported. Among the Li-O₂battery research community, it has been the norm that reporting specific capacity versus only carbon weight but for practical and precise assessment, all the components comprising cathode including electrolytes should be considered as they also add mass to total cathode weight. Consequently, the optimization of the ratio of the components is absolutely necessary for practical system design.

Here, we modeled and simulated the Li-O₂ cell with an electrolyte and with an electrolyte and pores. Percolation limit for electric conduction was introduced to define minimum volume occupied by carbon particles. Dynamic electrodeposition of reaction products with the surface passivation effect that expresses exponentially increasing electric resistance as the film growing was introduced by using the Comsol multi-physics electrodeposition module. Random distribution of conductive particles was realized by using the Bruggeman equation for charge transport thorough a media. The simulated results were verified by experimental results. Li-O₂ cells with the cathode comprising of Printex carbon as the conductive media and PEO (polyethylene oxide) containing LiTFSI (lithium bis(trifluoromethanesulfonyl) imide) as the electrolyte were fabricated with various compositions and their charge discharge properties were characterized. Results from both simulation and experiments showed good agreement in trends for flooded type cells and suggests optimized the ratio of components. 

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