Towards an Understanding of Li₂ O₂ Evolution in Li-O₂ Batteries: An In Operando Synchrotron X-ray Diffraction Study

One of the major challenges in developing high-performance Li-O₂ batteries is to understand the Li₂ O₂ formation and decomposition during battery cycling. In this study, this issue was investigated by synchrotron radiation powder X-ray diffraction. The evolution of Li₂ O₂ morphology and structure was observed under actual electrochemical conditions of battery operation. By quantitatively tracking Li₂ O₂ during discharge and charge, a two-step process was suggested for both growth and oxidation of Li₂ O₂ owing to different mechanisms during two stages of both oxygen reduction reaction and oxygen evolution reaction. From an observation of the anisotropic broadening of Li₂ O₂ in XRD patterns, it was inferred that disc-like Li₂ O₂ grains are formed rapidly in the first step of discharge. These grains can stack together so that they facilitate the nucleation and growth of toroidal Li₂ O₂ particles with a LiO₂ -like surface, which could cause parasitic reactions and hinder the formation of Li₂ O₂ . During the charge process, Li₂ O₂ is firstly oxidized from the surface, followed by a delithiation process with a faster oxidation of the bulk by stripping the interlayer Li atoms to form an off-stoichiometric intermediate. This fundamental insight brings new information on the working mechanism of Li-O₂ batteries.