The reversibility and cyclability of anionic redox in battery electrodes hold the key to its practical employments. Here, through mapping of resonant inelastic X-ray scattering (mRIXS), we have independently quantified the evolving redox states of both cations and anions in Na2/3Mg1/3Mn2/3O2. The bulk-Mn redox emerges from initial discharge and is quantified by inverse-partial fluorescence yield (iPFY) from Mn-L mRIXS. Bulk and surface Mn activities likely lead to the voltage fade. O-K superpartial fluorescence yield (sPFY) analysis of mRIXS shows 79% lattice oxygen-redox reversibility during initial cycle, with 87% capacity sustained after 100 cycles. In Li1.17Ni0.21Co0.08Mn0.54O2, lattice-oxygen redox is 76% initial-cycle reversible but with only 44% capacity retention after 500 cycles. These results unambiguously show the high reversibility of lattice-oxygen redox in both Li-ion and Na-ion systems. The contrast between Na2/3Mg1/3Mn2/3O2 and Li1.17Ni0.21Co0.08Mn0.54O2 systems suggests the importance of distinguishing lattice-oxygen redox from other oxygen activities for clarifying its intrinsic properties.

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...

For P3-type Na 0.6 [Li 0.2 Mn 0.8 ]O 2 with relatively weak Mn-O covalent bonding, crystal structure factors play an important role in stabilizing the oxidized species, inhibiting the irreversible transformation of the oxidized species to O 2 gas. The finding is important for better design of layered oxide positive materials with higher reversible capacity via the introduction of a reversible anionic redox reaction.

Although single-atomically dispersed metal-Nx on carbon support (M-NC) has great potential in heterogeneous catalysis, the scalable synthesis of such single-atom catalysts (SACs) with high-loading metal-Nx is greatly challenging since the loading and single-atomic dispersion have to be balanced at high temperature for forming metal-Nx. Herein, we develop a general cascade anchoring strategy for the mass production of a series of M-NC SACs with a metal loading up to 12.1 wt%. Systematic investigation reveals that the chelation of metal ions, physical isolation of chelate complex upon high loading, and the binding with N-species at elevated temperature are essential to achieving high-loading M-NC SACs. As a demonstration, high-loading Fe-NC SAC shows superior electrocatalytic performance for O2 reduction and Ni-NC SAC exhibits high electrocatalytic activity for CO2 reduction. The strategy paves a universal way to produce stable M-NC SAC with high-density metal-Nx sites for diverse high-performance applications.

Voltage decay and redox asymmetry mitigation by reversible cation migration in lithiumrich layered oxide electrodes.

Rechargeable aqueous Zn-ion batteries (ZIBs) are very promising for large-scale grid energy storage applications owing to their low cost, environmentally benign constituents, excellent safety, and relatively high energy density. 1, 2 Their usage, however, is largely hampered by the fast capacity fade. The cycle stability seems to be highly rate-dependent, 3 which poses an additional challenge, but can also play a pivotal role in uncovering the reaction mechanisms. The complexity of the reactions has resulted in long-standing ambiguities of the chemical pathways of Zn/MnO2 system, and has led to many controversies with regard to their nature. In this report, we present a combined experimental and theoretical study of Zn/ MnO2 cells. We found that both H + /Zn 2+ intercalation and conversion reactions occur at different voltages, and that the rapid capacity fading can clearly be ascribed to the rate-limiting and irreversible conversion reactions at a lower voltage. By avoiding the irreversible conversion reactions at ~ 1.26 V, we successfully demonstrate ultra-high power and long-life Zn/MnO2 cells which, after 1000 cycles, maintain an energy density of ~ 231 Wh kg-1 and ~ 105 Wh kg-1 at a power density of ~ 4 kW kg-1 (9C, ~ 3.1 A g-1) and ~ 15 kW kg-1 (30C, ~ 10.3 A g-1), respectively. The excellent cycle stability and power capability are superior to most reported ZIBs or even some lithium-ion batteries. The results establish accurate electrochemical reaction mechanisms and kinetics for Zn/MnO2, and identify the interplay of the voltage window and rate as the determining factors for achieving excellent cycle life. Broader Context The increasing interest and importance in large-scale grid storage technology are attributed to multiple factors, including managing peak demands, improving the grid reliability, integrating most sustainable energy sources such as solar radiation, wind, wave power, geothermal energy, etc., and further powering the energy infrastructures. Rechargeable aqueous Zn-ion batteries (ZIBs) with mild electrolytes have the advantages of low cost materials (Zn/ MnO2), manufacturing (air-and water-inert Zn anode), and recycling (mild electrolytes); relatively high energy density; and excellent safety, making them prospective candidates for large-scale grid storage. Their low cyclability, however, has remained a grand challenge, hindering the widespread applications of these attractive ZIBs. A prerequisite for improving the cycle life and electrochemical performance of Zn/MnO2 batteries is to accurately determine the reaction mechanisms, especially under different rates, which poses a considerable challenge. In our combined experimental and computational study, a concomitant intercalation and conversion reactions of H + /Zn 2+ occurring at different voltages in the Zn/MnO2 system is established. The rapid capacity fading is unambiguously ascribed to the rate-limiting and irreversible conversion reactions at a lower voltage. By mitigating or avoiding the irreversible conversion reactions at the lower ...

Most P2-type layered oxides suffer from multiple voltage plateaus, due to Na+/vacancy-order superstructures caused by strong interplay between Na–Na electrostatic interactions and charge ordering in the transition metal layers. Here, Mg ions are successfully introduced into Na sites in addition to the conventional transition metal sites in P2-type Na0.7[Mn0.6Ni0.4]­O2 as new cathode materials for sodium-ion batteries. Mg ions in the Na layer serve as “pillars” to stabilize the layered structure, especially for high-voltage charging, meanwhile Mg ions in the transition metal layer can destroy charge ordering. More importantly, Mg ion occupation in both sodium and transition metal layers will be able to create “Na–O–Mg” and “Mg–O–Mg” configurations in layered structures, resulting in ionic O 2p character, which allocates these O 2p states on top of those interacting with transition metals in the O-valence band, thus promoting reversible oxygen redox. This innovative design contributes smooth voltage profiles and high structural stability. Na0.7Mg0.05[Mn0.6Ni0.2Mg0.15]­O2 exhibits superior electrochemical performance, especially good capacity retention at high current rate under a high cutoff voltage (4.2 V). A new P2 phase is formed after charge, rather than an O2 phase for the unsubstituted material. Besides, multiple intermediate phases are observed during high-rate charging. Na-ion transport kinetics are mainly affected by elemental-related redox couples and structural reorganization. These findings will open new opportunities for designing and optimizing layer-structured cathodes for sodium-ion batteries.

Sodium ion batteries, because of their sustainability attributes, could be an attractive alternative to Li-ion technology for specific applications. However, it remains challenging to design high energy density and moisture stable Na-based positive electrodes by implementing the anionic redox process that has recently boosted the capacity of Li-rich layered oxides.Here, we report the first anionic-redox active O3-NaLi1/3Mn2/3O2 phase obtained through a ceramic process by carefully controlling the delicate balance between synthesis conditions and stoichiometry. It shows a sustained reversible capacity of 190 mAh g −1 by redox processes on oxygen and manganese ions as deduced by combined HAXPES and mRIXS spectroscopy techniques. Remarkably, unlike any other anionic-redox layered oxides so far reported, O3-NaLi1/3Mn2/3O2 electrodes do not show voltage fade upon cycling. This finding is due to switching from the interlayer to intralayer migration of the Mn cations promoted by Li + displacement towards the alkali layer upon first Na + de-insertion. Another practical asset of this material stems from its moisture stability, hence facilitating its handling and electrode processing. Besides providing insightful fundamental findings pertaining to anion redox, this work offers future directions towards designing high energy density electrodes for advanced Na-ion batteries.