Metal–oxygen decoordination stabilizes anion redox in Li-rich oxides

Abstract

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...

Strategies to Break the Scaling Relation toward Enhanced Oxygen Electrocatalysis

“…132,133 O NB is a prerequisite for the formation of peroxo-or superoxolike O-O dimers without the risk of structural destabilization. [132][133][134][135][136][137] Considering Figure 9. Lattice Oxygen Oxidation Mechanism in Perovskite Oxygen Evolution Electrocatalysts (A and B) (A) Relations between charge-transfer energy, electrocatalytic activity, and the OER mechanism.…”

Inhibition of oxygen dimerization by local symmetry tuning in Li-rich layered oxides for improved stability

“…In this regard, Tarascon et al 35 reported that the d – sp hybridization associated with the reductive coupling mechanism results in good cycling behavior in Li 2 Ru 0.75 Sn 0.25 O 3 materials. Ceder et al 36 found that local structural defects can promote metal–oxygen decoordination, which stabilizes anionic redox reactions in the Li 2− x Ir 1− y Sn y O 3 model system. Zhou et al 13 demonstrated that a Li 2 Ni 1/3 Ru 2/3 O 3 cathode in the Fd-3m space group has more O–TM percolation networks and shows good cycling performance.…”

Reversible Oxygen Redox Chemistry in Aqueous Zinc‐Ion Batteries

“…0.8 V), [8] which severely limits their energy density.F urthermore,u nlike the well-established energystorage mechanisms in lithium/sodium-ion batteries,t he energy-storage mechanisms of vanadium-based aqueous ZIBs are under debate.B esides the traditional Zn 2+ ion insertion/extraction mechanism, an H + insertion/extraction process was also observed in aZ n/NaV 3 O 8 ·1.5 H 2 Os ystem. [9] Despite this difference,these energy-storage mechanisms are based on redox reactions of (vanadium) cations.H owever, until now,a nionic redox reactions,w hich have occurred in some cases of batteries based on organic electrolytes and could enhance the capacity and voltage of the corresponding batteries, [10] have not been observed in aqueous ZIBs.Ifredox reactions of (oxygen) anions could be realized in vanadiumbased aqueous ZIBs,their energy density would be enhanced significantly.…”