In commercial Li-ion batteries, well-ordered close-packed oxides, particularly, layered lithium transition metal oxides, LiTMO₂ (TM = Ni, Mn, Co, Al), are widely used. Despite the high theoretical capacity of these layered oxides (> 270 mAh/g), they are typically operated to deliver a capacity of less than 200 mAh/g to attain good cycling and safety attributes.^1-6 Nowadays, strategies to push the capacity limit of such materials have led to the development of Li-rich layered oxides^{7, 8}, which can consistently deliver a reversible capacity approaching 300 mAh/g. This exceptionally high capacity is far beyond the theoretical capacity from Ni and Co redox, for example, Ni redox (Ni^2+/4+) can only account for a theoretical capacity of 127 mAh/g in a Co-free compound, Li_1.2Ni_0.2Mn_0.6O₂. This has been clearly directed to the participation of oxygen redox in the electrochemical reaction.^9-14

In our work, we aimed to probe the electrochemical activity of anionic oxygen in Li-rich layered oxides from material perspective via tackling the effect of transition metal species. We have successfully synthesized a series of Li-rich layered oxides, Li_2-x-yNi_xTm_yO₂ (TM is transition metal). Compounds with designed transition metals possess a similar crystal structure and enable a similar amount of Li removal and uptake during charge-discharge processes, but with a significantly different charge profile, characterized by the voltage plateau around 4.55 V. We performed a systematic study to capture the oxygen activity ranging from O^2- in the lattice to O⁰ in gaseous phase in the as-produced compounds by combining a suite of advanced characterization techniques with in-situ differential electrochemical spectrometry (DEMS). We have observed completely different oxygen behaviors in such compounds with varied transition metals. We will present our experimental evidence on a reversible participation of electrons from oxygen in Li-, Mn-rich layered oxide. We hope these findings will provide additional insights into the complex mechanism of oxygen redox and the development of advanced high-capacity Li-ion cathodes.

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