Lithium-rich Mn-based oxides (LRMOs) with high specific capacity have been accepted as key cathode materials for the development of high energy density secondary batteries. The oxygen-involved redox reaction contributes extra capacity. However, the resulting oxygen vacancies (Vo) hinder Li+ diffusion, accelerate transition metal ion (TM) migration and cause irreversible structural transformation, leading to structural deterioration, voltage hysteresis and continuous discharge potential drop. Nevertheless, the practice shows that the coordination environment of oxygen atoms and its effect can be actively modified, instead of just passively accepted. This improves the reversibility of the anion redox reaction, simultaneously mitigates the redox oxygen-induced negative influence, and enhances the integrated electrochemical properties of the materials. This review will focus on oxygen as the key point for the close-packed framework of the Li-rich Mn-based cathode material. Specifically, the formation mechanism of Vo in Li-rich cathode materials and its influences, especially the Vo actively introduced strategy and the corresponding characterization methods, are reviewed herein in detail. These studies shed light on the development of LRMOs in view of decoupling the extra capacity and irreversible structural transformation originating from the anionic redox reaction.
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