The discharging and charging reactions at the cathode of the Li–O2battery were discussed in detail, and the x values in Li2–xO2were obtained, which is a major product during discharging, based on the operando/in situ XRD measurements along with the Rietveld analyses. It was clarified that the P63/mmcspace group-type LiO2was formed during the initial stage of discharge, and then Li2–xO2was deposited accompanied by an increase in lithium occupancy with an anisotropic change in the c-lattice parameter and domain size along the a-axis direction. The charging process was also clarified such that the x value in the undissolved Li2–xO2decreased with slight changes in the lattice parameters and domain shapes. Based on the dependences of the discharging current density, discharging amount, and cycle number, the overall reaction was proposed as follows. During discharging, the electrochemical formation of amorphous Li2O2, crystalline Li2–xO2, and side-reaction products (most likely Li2CO3) simultaneously occurred. During charging, amorphous Li2O2is oxidatively dissolved first, followed by crystalline Li2–xO2, and finally Li2CO3dissolved and/or decomposed. Since part of the last Li2CO3remained undissolved on the cathode surface, the proportion of the crystalline Li2–xO2generated during the next discharging process decreased, leading to poor cyclability.