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Elucidating Effect of Transition Metal on Anionic Oxygen Activity in High-Energy Li-Rich Layered Oxides

Monday, 2 October 2017: 09:00
Maryland C (Gaylord National Resort and Convention Center)
J. Xu, M. Sun, R. Qiao (Lawrence Berkeley National Laboratory), S. E. Renfrew (University of California, Berkeley), L. Ma, J. Lu (Argonne National Laboratory), S. Hwang (Brookhaven National Lab), D. Su (Brookhaven National Laboratory), D. Nordlund, A. Mehta (SLAC National Accelerator Laboratory), K. Amine (Argonne National Laboratory), B. D. McCloskey, W. Yang, and W. Tong (Lawrence Berkeley National Laboratory)
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 oxides7, 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 (Ni2+/4+) can only account for a theoretical capacity of 127 mAh/g in a Co-free compound, Li1.2Ni0.2Mn0.6O2. 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, Li2-x-yNixTmyO2 (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 O2- 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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