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. Here we reveal that in Li1.17–xNi0.21Co0.08Mn0.54O2, these properties arise from a strong coupling between anion redox and cation migration. We combine various X-ray spectroscopic, microscopic, and structural probes to show that partially reversible transition metal migration decreases the potential of the bulk oxygen redox couple by > 1 V, leading to a reordering in the anionic and cationic redox potentials during cycling. First principles calculations show that this is due to the drastic change in the local oxygen coordination environments associated with the transition metal migration. We propose that this mechanism is involved in stabilizing the oxygen redox couple, which we observe spectroscopically to persist for 500 charge/discharge cycles.

Reversible high voltage redox chemistry is an essential component of many electrochemical technologies, from (electro)catalysts to lithium-ion batteries. Oxygen anion redox has garnered intense interest for such applications, particularly lithium ion batteries, as it offers substantial redox capacity at > 4 V vs. Li/Li + in a variety of oxide materials. However, oxidation of oxygen is almost universally correlated with irreversible local structural transformations, voltage hysteresis, and voltage fade, which currently preclude its widespread use. By comprehensively studying the Li 2-x Ir 1-y Sn y O 3 model system, which exhibits tunable oxidation state and structural evolution with y upon cycling, we reveal that this structure-redox coupling arises from the local stabilization of short ~ 1.8 Å metal-oxygen π bonds and ~ 1.4 Å O-O dimers during oxygen 42 redox, which occurs in Li 2-x Ir 1-y Sn y O 3 through ligand-to-metal charge transfer. Crucially, formation of these oxidized oxygen species necessitates the decoordination of oxygen to a single covalent bonding partner through formation of vacancies at neighboring cation sites, driving cation disorder. These insights establish a point defect explanation for why anion redox often occurs alongside local structural disordering and voltage hysteresis during cycling. Our findings offer an explanation for the unique electrochemical properties of lithium-rich layered oxides, with implications generally for the design of materials employing oxygen redox chemistry. 3 Main Text: Reversible redox chemistry in solids under highly oxidizing conditions (e.g. vs H 2 /H + , Li/Li + , or 52 O) is a powerful tool in (electro)chemical systems, increasing the catalytic activity of oxygenevolution and methane-functionalization (electro)catalysts as well as the energy and power densities of lithium-ion batteries (LIBs). 1 In LIBs in particular, employing high-voltage redox has been identified as a promising avenue to meeting the energy density demands of nextgeneration technologies such as plug-in electric vehicles. Recently, anionic oxygen redox has been shown to offer access to substantial high-voltage (de)intercalation capacity in a range of electrode materials, 2-7 spurring an intense research effort to understand this phenomenon. While many oxygen-redox-active materials have been developed, they almost universally exhibit a host of irreversible electrochemical behaviors such as voltage hysteresis and voltage fade. 8 This is most notable in the anion-redox-active Li-rich 62 layered oxides, Li 1+x M 1-x O 2 (M = a transition metal (TM) or non-transition metal such as Al, Sn, Mg, etc.), which exhibit capacities approaching 300 mAh g-1 but have yet to achieve commercial success due to such electrochemical behaviors. 5, 9 It has been shown both experimentally 10-12 and 65 from first-principles thermodynamics 13 that the migration of M into empty Li sites 9-creating structural disorder in the form of M Li /V M antisite/cation vacancy point defect pairs-is at the root of voltage profile...

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