Modeling and understanding of multi-step reactions and mass transfer coupling in Li-O2 batteries with mesoscale heterogeneous electrode structures

The Li-O2 batteries have the highest theoretical specific energy than other battery systems, while the practical value falls significantly short. The full utilization of porous cathodes is limited by the complex coupling of electrochemical reactions and mass transfer in Li-O2 batteries. In this study, the correlation between the limited mechanism and the coupling behavior is comprehensively investigated through a mesoscale heterogeneous model. By reconstructing the three-dimensional microstructure of the cathode, the model dynamically simulated the coupling behavior including multi-step electrochemical reactions and mass transfer within cathodes. The mass transfer and spatial distribution of reactants and products reveal that two key factors are limiting the full utilization of cathodes: the impeded mass transfer of O2 and LiO2 on the separator side, and inactive electrochemical reactions on the gas side as a result of Li2O2 deposition. Increasing the porosity of the cathode is found to significantly enhance discharge capacity by improving mass transfer efficiency and ensuring more uniform electrochemical reactions. Furthermore, compared to traditionally porous cathodes, a forward gradient cathode with higher gas-side porosity is proposed to improve discharge capacity by 83 %, while an ordered cathode with vertically cross-arranged structures achieves a 9 % enhancement. These findings not only provide fundamental insights into the improved mechanisms of capacity but also offer guidance for the rational design of advanced electrode structures in high-performance Li-O2 batteries.