William Gent3 1 Kipil Lim2 4 Yufeng Liang5 Qinghao Li1 Taylor Barnes5 Sung-Jin Ahn6 Kevin Stone4 Mitchell McIntire7 Jihyun Hong2 4 Jay Hyok Song8 Yiyang Li2 Apurva Mehta4 Stefano Ermon7 Tolek Tyliszczak1 David Kilcoyne1 David Vine1 Jin-Hwan Park6 Seok-Kwang Doo6 Michael Toney4 Wanli Yang1 David Prendergast5 William C. Chueh2

3, Stanford University, Stanford, California, United States
1, Lawrence Berkeley National Laboratory, Berkeley, California, United States
2, Stanford University, Stanford, California, United States
4, SLAC National Accelerator Laboratory, Menlo Park, California, United States
5, Lawrence Berkeley National Laboratory, Berkeley, California, United States
6, Samsung Advanced Institute of Technology, Suwon, , Korea (the Republic of)
7, Stanford University, Stanford, California, United States
8, Samsung SDI, Suwon, , Korea (the Republic of)

Lithium-rich layered transition metal oxide positive electrodes offer access to anion redox at high potentials, thereby promising high energy densities for lithium-ion batteries. However, anion redox is also associated with several unfavorable electrochemical properties, such as open-circuit voltage hysteresis and long-term voltage fade, that currently prevent the commercial application of these promising electrode materials. Mitigating these behaviors requires an understanding of the anion redox mechanism and its role in governing the unique electrochemistry of lithium-rich materials. Here we reveal that in Li1.17-xNi0.21Co0.08Mn0.54O2, these electrochemical properties arise from a strong coupling between anion redox and cation migration, which dynamically modulates the anion redox potential during cycling. We combine scanning transmission X-ray microscopy with resonant inelastic X-ray scattering to definitively show that a significant electronic state reshuffling occurs in the material bulk after states with predominantly O2p character are depopulated during the first charge voltage plateau, while oxygen evolution occurs only at the primary particle surfaces. In conjunction with local and average structure probes we show that this reshuffling is linked to transition metal migration during the high voltage plateau, which decreases the potential of the bulk oxygen redox couple by > 1 V during subsequent discharge, leading to a novel switch in the relative anionic and cationic redox potentials. First-principles calculations show that this is due to the drastic change in the local oxygen coordination environments associated with the transition metal migration, which shifts the depopulated O2p states to higher energy relative to the transition metal 3d states. We propose the following anion redox mechanism: {O2– + TM} → {O + TMmig} + e, where TM and TMmig indicate a transition metal before and after migration, respectively, which holistically explains the spectroscopic, structural, and electrochemical properties of anion redox in this material. We propose that this mechanism is involved in stabilizing the oxygen redox couple, which we observe spectroscopically to persist for 500 charge/discharge cycles. These insights provide opportunities to tune oxygen redox chemistry through control of the structural evolution of Li-rich materials.