Revealing of the Activation Pathway and Cathode Electrolyte Interphase Evolution of Li-Rich 0.5LiMnO·0.5LiNiCoMnO Cathode by in Situ Electrochemical Quartz Crystal Microbalance.

The first-cycle behavior of layered Li-rich oxides, including LiMnO activation and cathode electrolyte interphase (CEI) formation, significantly influences their electrochemical performance. However, the LiMnO activation pathway and the CEI formation process are still controversial. Here, the first-cycle properties of xLiMnO·(1- x) LiNiCoMnO ( 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 LiMnO and LiNiCoMnO structures can significantly affect the activation pathway of LiNiCoMnO, leading to an extra-high capacity. It is demonstrated that LiMnO activation in Li-rich materials is dominated by electrochemical decomposition (oxygen redox), which is different from the activation process of pure LiMnO governed by chemical decomposition (LiO 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.