Modeling the multi-step discharge and charge reaction mechanisms of non-aqueous Li-O2 batteries

The modeling study plays a critical role in understanding the reaction mechanisms and predicting the performance of lithium-oxygen (Li-O2) batteries. Although several models have successfully captured the discharge behaviors of the Li-O2 batteries, modeling the charge behaviors of the Li-O2 batteries is a challenge due to lacking a thorough understanding of the related reaction processes. This work proposes a multi-step reversible discharge/charge model for the non-aqueous Li-O2 batteries. Both the surface and solution reaction pathways of lithium peroxide (Li2O2) are taken into account. The proposed model can predict the discharge behaviors well, and more importantly, can accurately capture the charge behaviors of the Li-O2 batteries by considering the following three aspects: the effect of Li2O2 morphology on its decomposition potential, the evolution of interfacial resistance by Li2O2 film collapse, and the different desorption rates of absorbed lithium superoxide (LiO*2) in different electrolytes. Based on this model, the discharge/charge processes of the Li-O2 batteries using dimethoxyethane(DME), tetraethylene glycol dimethyl ether (TEGDME) and dimethyl sulfoxide (DMSO) electrolytes, as well as a redox mediator TEMPO, are simulated and verified. The Li2O2 formation/decomposition through the surface and solution pathways are dominated by the specific surface area of the electrode and the potential, respectively. The Li2O2 generated by the solution pathway is more difficult to be decomposed during charge. The use of redox mediator TEMPO lowers the discharge capacity of the Li-O2 batteries due to the weakened solution reaction, while remarkably reduces the reversible capacity loss. The desorption time constant of LiO*2 in electrolyte is a critical parameter that determines the degree of the reaction through the solution pathway, thus determining the discharge/charge cycling performance of the Li-O2 batteries with various electrolytes.