The discharge and charge mechanisms of rechargeable Li-O₂ batteries have been the subject of extensive investigation recently. However, they are not fully understood yet. Here we report a systematic study of the morphological transition of Li₂O₂ from a single crystalline structure to a toroid like particle during the discharge–charge cycle, with the help of a theoretical model to explain the evolution of the Li₂O₂ at different stages of this process. The model suggests that the transition starts in the first monolayer of Li₂O₂, and is subsequently followed by a transition from particle growth to film growth if the applied current exceeds the exchange current for the oxygen reduction reaction in a Li-O₂ cell. Furthermore, a sustainable mass transport of the diffusive active species (e.g., O₂ and Li⁺) and evolution of the underlying interfaces are critical to dictate desirable oxygen reduction (discharge) and evolution (charge) reactions in the porous carbon electrode of a Li-O₂ cell.