What Triggers the Voltage Hysteresis Variation beyond the First Cycle in Li-Rich 3d Layered Oxides with Reversible Cation Migration?

Herein, the structure-electrochemistry relationship of O-2-Li-5/6(Li0.2Ni0.2Mn0.6)O-2 is deliberately studied by local-structure probes including site-sensitive Li-7 pj-MATPASS NMR, quantitative Li-6 magic-angle spinning NMR, and electron paramagnetic resonance (EPR). The extraction and reinsertion of Li-TM (Li in the transition metal layer) during the first cycle are only partially reversible, bringing about the formation of tetrahedral Li-Li (Li in the Li layer) that can be reversibly (de)intercalated after the activation cycle. The high-voltage oxygen redox process is preserved beyond the first cycle, further manifesting the structural superiority of O2 stacking over O3 stacking in bolstering oxygen redox. Moreover, the (de)lithiation process is highly reversible without pronounced structural hysteresis after the rearrangement of Li and transition metal upon the activation cycle, which can explain well the variation of voltage hysteresis from the first cycle to second cycle. These insights elucidate the imperfect structural stability of O2-type Li-rich layered oxides, which could be further improved by streamlining the returning path of Li-TM.