A large potential hysteresis occurs between the charge and discharge reactions of Li1.2NixMn0.8-xO2 (x = 0.2). As the nickel content x was increased through x = 0.2, 0.3, and 0.4, the potential hysteresis was suppressed. The results obtained by X-ray absorption fine structure indicate that, for Li1.2NixMn0.8-xO2 (x = 0.2), the redox of the nickel, manganese, and oxygen contributed to the charge-discharge reactions. There was little difference in the redox potentials of nickel and manganese between the charge and discharge processes. In contrast, the redox potential of oxygen during the charge process was very different from that during the discharge process. This means that large potential hysteresis of Li1.2NixMn0.8-xO2 (x = 0.2) was mainly due to the progress of the reaction compensated by the redox of oxygen. As the nickel content x increased in Li1.2NixMn0.8-xO2 (x = 0.2, 0.3, and 0.4), the charge-discharge reactions compensated by the redox of nickel increased, but that of oxygen decreased. Therefore, for the lithium-rich layer-structured cathode material, the increased nickel/manganese ratio was effective in suppressing the potential hysteresis between the charge and discharge processes.