A Li-O2 battery model coupled with LiO2 and Li2O2 reveals regulation mechanism of deposited product composition on mass transport and electron transfer

To elucidate the regulation mechanism of deposited product composition on the coupling process of mass transport and electron transfer in lithium‑oxygen (Li-O2) batteries, a novel multi-step discharge/charge model with solid lithium superoxide (LiO2(s)) and lithium peroxide (Li2O2) as hybrid precipitation is proposed. This model couples the dynamic competitive growth mechanisms between LiO2(s) and Li2O2, while incorporating the asymmetric deposition and decomposition behaviors of Li2O2. The electrode surface passivation caused by the sluggish kinetic rate of electron across the Li2O2 film is solely responsible for the deep discharge termination based on the reduced graphene oxide cathode. LiO2(s) dominates the precipitation products with a limited capacity and a continuous decline in the LiO2(s) percentage with discharge depth. Although promoting the LiO2(s) formation is conducive to alleviating the electrode surface passivation, it aggravates the O2 transport resistance due to occupying more electrode pores with the same charge contribution. Hence the discharge capacity demonstrates a three-stage variation with increasing lithium superoxide (LiO2) formation rate, which rapidly grows by more than two times in stage 2 benefiting from the increased LiO2(s) percentage and enhanced solution mechanism. Whereas a slow rise of 32 % in the discharge capacity in stage 3 is attributed to the conversion of LiO2(s) to Li2O2 toroid for the discharge process controlled by O2 transport. An increase in the thickness or specific surface area of the cathode improves the discharge capacity mainly by facilitating the production of Li2O2 toroid despite the decrease in the LiO2(s) percentage, which is different from the regulatory mechanism of electrode porosity for accelerating the LiO2(s) formation. Increasing LiO₂ solubility predominantly mitigates the electrode surface passivation through enhancement of the solution pathway, whereas the elevated O₂ solubility synergistically facilitates the co-formation of LiO₂(s) and Li2O2 toroid. In addition, the LiO2(s) percentage declines for the amorphous Li2O2 film with low resistance and the electrode surface passivation mainly originates from the coverage of reactive sites by Li2O2.