Activation of Oxygen Redox by Inhibited Dynamic Phase Transition for High-Energy Li-Rich Layered Oxide Cathode

Li-rich layered oxides have garnered tremendous research interest as a promising high-energy cathode by harnessing combined cationic and anionic redox. Despite the great promise of oxygen redox, however, the distinct activity of oxygen redox in Li-rich materials has inhibited the precise understanding of the oxygen redox activation mechanism. Herein, we unravel the interplay between de/activation of oxygen redox and the phase transition behaviors of Li-rich cathodes through a comprehensive investigation of two Ru-based model Li-rich layered oxides, Li_1.2Ni_0.2Mn_0.3Ru_0.3O₂ and Li_1.2Ni_0.2Ru_0.6O₂. It is corroborated by synchrotron-based X-ray absorption spectroscopy analyses that the reversible oxygen redox occurs in Li_1.2Ni_0.2Mn_0.3Ru_0.3O₂ but the reaction is not triggered in Li_1.2Ni_0.2Ru_0.6O₂ despite high relevance in structure and stoichiometry. Operando X-ray diffraction elucidates that the O3-type Li_1.2Ni_0.2Ru_0.6O₂ undergoes a phase transition into the T3-type phase during charging while there is negligible structural change in O3-type Li_1.2Ni_0.2Mn_0.3Ru_0.3O_2. First-principles studies reveal that the theoretical redox potential to extract Li⁺ from T3-type phase is significantly higher than that of O3-type Li_1.2Ni_0.2Ru_0.6O₂, inhibiting the activation of the oxygen redox. Our findings indicate the significance of the dynamic structural evolution on governing the activation of oxygen redox, thereby offering guidance for further design of Li-rich layered oxides toward the full utilization of the oxygen redox.