Understanding Li roles in chemical reversibility of O2-type Li-rich layered cathode materials

Traditional O3-type Li-rich layered materials are attractive with ultra-high specific capacities, but suffering from inherent problems of voltage hysteresis and poor cycle performance. As an alternative, O2-type materials show the potential to improve the oxygen redox reversibility and structural stability. However, their structure-performance relationship is still unclear. Here, we investigate the correlation between the Li component and dynamic chemical reversibility of O2-type Li-rich materials. By exploring the formation mechanism of a series of materials prepared by Na/Li exchange, we reveal that insufficient Li leads to an incomplete replacement, and the residual Na in the Li-layer would hinder the fast diffusion of Li+. Moreover, excessive Li induces the extraction of interlayer Li during the melting chemical reaction stage, resulting in a reduction in the valence of Mn, which leads to a severe Jahn-Teller effect. Structural detection confirms that the regulation of Li can improve the cycle stability of Li-rich materials and suppress the trend of voltage fading. The reversible phase evolution observed in in-situ X-ray diffraction confirms the excellent structural stability of the optimized material, which is conducive to capacity retention. This work highlights the significance of modulating dynamic electrochemical performance through the intrinsic structure.