The first-cycle behavior of layered Li-rich oxides, including Li₂MnO₃ activation and cathode electrolyte interphase (CEI) formation, significantly influences their electrochemical performance. However, the Li₂MnO₃ activation pathway and the CEI formation process are still controversial. Here, the first-cycle properties of xLi₂MnO₃·(1- x) LiNi_0.3Co_0.3Mn_0.4O₂ ( x = 0, 0.5, 1) cathode materials were studied with an in situ electrochemical quartz crystal microbalance (EQCM). The results demonstrate that a synergistic effect between the layered Li₂MnO₃ and LiNi_0.3Co_0.3Mn_0.4O₂ structures can significantly affect the activation pathway of Li_1.2Ni_0.12Co_0.12Mn_0.56O₂, leading to an extra-high capacity. It is demonstrated that Li₂MnO₃ activation in Li-rich materials is dominated by electrochemical decomposition (oxygen redox), which is different from the activation process of pure Li₂MnO₃ governed by chemical decomposition (Li₂O evolution). CEI evolution is closely related to Li⁺ extraction/insertion. The valence state variation of the metal ions (Ni, Co, Mn) in Li-rich materials can promote CEI formation. This study is of significance for understanding and designing Li-rich cathode-based batteries.