Towards an Understanding of Li 2 O 2 Evolution in Li–O 2 Batteries: An In Operando Synchrotron X‐ray Diffraction Study

One of the major challenges in developing high‐performance Li–O 2 batteries is to understand the Li 2 O 2 formation and decomposition during battery cycling. In this study, this issue was investigated by synchrotron radiation powder X‐ray diffraction. The evolution of Li 2 O 2 morphology and structure was observed under actual electrochemical conditions of battery operation. By quantitatively tracking Li 2 O 2 during discharge and charge, a two‐step process was suggested for both growth and oxidation of Li 2 O 2 owing to different mechanisms during two stages of both oxygen reduction reaction and oxygen evolution reaction. From an observation of the anisotropic broadening of Li 2 O 2 in XRD patterns, it was inferred that disc‐like Li 2 O 2 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 Li 2 O 2 particles with a LiO 2 ‐like surface, which could cause parasitic reactions and hinder the formation of Li 2 O 2 . During the charge process, Li 2 O 2 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–O 2 batteries.