Li-rich cathode materials are potential candidates for next generation Li-ion batteries. However, they exhibit large voltage hysteresis on the 1 st charge/discharge cycle involving a substantial (up to 1V) loss of voltage and therefore energy density. For Na cathodes, e.g. Na0.75[Li0.25Mn0.75]O2, voltage hysteresis can be explained by formation of molecular O2 trapped in voids within the particles. Here we show that this is also the case for Li1.2Ni0.13Co0.13Mn0.54O2. RIXS and 17 O MAS NMR show that molecular O2, rather than O2 2-, forms within the particles on oxidation of O 2at 4.6 V vs Li + /Li on charge. These O2 molecules are reduced back to O 2on discharge but at the lower voltage of 3.75 V explaining the voltage hysteresis in Li-rich cathodes. 17 O MAS NMR indicates a quantity of bulk O2 consistent with the O-redox charge capacity minus the small quantity of O2 loss from the surface. The implication is that O2, trapped in the bulk and lost from the surface, can explain O-redox.

The energy density of Li-ion batteries can be improved by storing charge at high voltages through the oxidation of oxide ions in the cathode material. However, oxidising O 2triggers irreversible structural rearrangements in the bulk and an associated loss of the high voltage plateau replacing it with a lower discharge voltage, as well as a loss of O2 accompanied by densification at the surface. Here we consider various models for O-redox proposed in the literature before describing a single unified model involving O 2oxidation to form O2, which is trapped in the bulk with the balance evolving from the surface. The model extends the O2 formation and evolution at the surface, which is well-known and well-characterised, into the electrode particle bulk as caged O2 that can be reversibly reduced and oxidised. This converged understanding allows us to propose practical strategies for avoiding O-redox-induced instability offering potential routes towards more reversible high energy density Li-ion cathodes.Since the discovery of 'anomalous' extra capacity to store charge in 3d transition metal oxide Li-rich cathode materials in the early 2000s, 1-4 there has been intense research interest seeking to understand the origin of the effect. [5][6][7][8][9] Over the years, these compounds have grown in number extending to include materials based on 4d and 5d transition metal oxides. [10][11][12][13] In the case of a conventional Li transition metal oxide intercalation cathode, Li + ions are extracted on charging, with charge-compensation by oxidation of the transition metal ion, the process is reversed on discharge, e.g. Li1-xMn2O4 (0 < x < 1). In contrast, the Li-rich cathodes, such as Li1.2Ni0.2Mn0.6O2, Li1.3Nb0.3Mn0.4O3, Li2RuO3 and Li2Ir0.5Sn0.5O3, extend the capacity to store charge by oxidation of the O 2ions. 10,[13][14][15][16] The ability of anions to undergo redox reactions is not without precedent, for example the S 2-/S2 2reaction in sulphides is well known but the phenomenon was not recognised in oxides until more recently. 17 The oxidation of O 2in cathode materials is typically accompanied by a high voltage plateau (usually ~4.5 V vs Li + /Li for 3d cathodes) on charge followed by an S-shaped discharge profile, Fig. 1. Early models posited that the charging plateau was associated completely with the irreversible loss of oxygen from the lattice (O-loss), which alongside extraction of Li + gives rise to the net loss of Li2O. 2,4 Online mass spectrometry showed that O2 gas was released from the surface of the material. 18 Later work showed that there was an insufficient degree of reduction observed of the transition metal ions on re-lithiation to explain the large discharge capacity 19 and quantitative studies also revealed an insufficient amount of evolved O2 to account for the charging capacity associated with the plateau. 14,20 Consequently, the idea was developed that reversible oxidation and reduction of O 2ions in the bulk compensate for the extraction and reinsertion of Li + beyond the limit of TM redox. 5,...

Layered Li-rich transition metal oxides undergo O-redox, involving the oxidation of the O2− ions charge compensated by extraction of Li+ ions. Recent results have shown that for 3d transition metal oxides the oxidized O2− forms molecular O2 trapped in the bulk particles. Other forms of oxidised O2− such as O22− or (O–O)n− with long bonds have been proposed, based especially on work on 4 and 5d transition metal oxides, where TM–O bonding is more covalent. Here, we show, using high resolution RIXS that molecular O2 is formed in the bulk particles on O2‒ oxidation in the archetypal Li-rich ruthenates and iridate compounds, Li2RuO3, Li2Ru0.5Sn0.5O3 and Li2Ir0.5Sn0.5O3. The results indicate that O-redox occurs across 3, 4, and 5d transition metal oxides, forming O2, i.e. the greater covalency of the 4d and 5d compounds still favours O2. RIXS and XAS data for Li2IrO3 are consistent with a charge compensation mechanism associated primarily with Ir redox up to and beyond the 5+ oxidation state, with no evidence of O–O dimerization.

No abstract

attractive for future grid-level energy storage applications. Metallic Zn, as the ideal anode for AZBs, has the highest theoretical capacity (5851 mAh mL −1 ). It is also non-toxic, non-flammable, abundant, and has good electrical conductivity and water stability. [1][2][3][4][5] However, conventional metallic Zn anodes suffer from severe dendrite formation during cycling, causing serious problems like poor reversibility, voltage hysteresis, increased parasitic reactions, shorting-induced battery failures, and other issues. [1,3,6] These dendritic structures, either rarefied needle, or non-planar platelet deposits, preferentially form at irregular or defective areas of the electrode where the localized current density is highest and the initial nucleation event is most likely, [7] and is exacerbated by cycling at high current densities and capacities. [8,9] Strategies for controlling and suppressing dendritic growth have revolved around manipulating the electrolyte, typically by inclusion of additives, [10][11][12][13][14][15] or by engineering the electrode into a high-surface-area sponge, [16][17][18] or with a protective surface coating, [19] in order to suppress dendrite formation.Despite being one of the most promising candidates for grid-level energy storage, practical aqueous zinc batteries are limited by dendrite formation, which leads to significantly compromised safety and cycling performance. In this study, by using single-crystal Zn-metal anodes, reversible electrodeposition of planar Zn with a high capacity of 8 mAh cm −2 can be achieved at an unprecedentedly high current density of 200 mA cm −2 . This dendrite-free electrode is well maintained even after prolonged cycling (>1200 cycles at 50 mA cm −2 ). Such excellent electrochemical performance is due to single-crystal Zn suppressing the major sources of defect generation during electroplating and heavily favoring planar deposition morphologies. As so few defect sites form, including those that would normally be found along grain boundaries or to accommodate lattice mismatch, there is little opportunity for dendritic structures to nucleate, even under extreme plating rates. This scarcity of defects is in part due to perfect atomic-stitching between merging Zn islands, ensuring no defective shallow-angle grain boundaries are formed and thus removing a significant source of non-planar Zn nucleation. It is demonstrated that an ideal high-rate Zn anode should offer perfect lattice matching as this facilitates planar epitaxial Zn growth and minimizes the formation of any defective regions.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202202552.

Multivalent cation rechargeable batteries, including those based on Ca, Mg, Al, etc., have attracted considerable interest as candidates for beyond Li-ion. Recent developments have realized promising electrolyte compositions for rechargeable Ca batteries; however, an in-depth understanding of the Ca plating and stripping behavior, and the mechanisms by which adverse dendritic growth may occur, remains underdeveloped. In this work, via in-situ transmission electron microscopy, we have captured the real-time nucleation, growth, and dissolution of Ca, the formation of dead Ca, and demonstrated the critical role of current density and the solid-electrolyte interphase layer in controlling the plating morphology. In particular, the interface was found to influence Ca deposition morphology, and can lead 2 to Ca dendrite growth under unexpected conditions. These observations allow us to propose a model explaining the preferred conditions for reversible and efficient Ca plating.Multivalent cation batteries based on Mg, Ca, Al, etc. have attracted significant interest as potential candidates to replace Li-ion batteries in recent years. [1][2][3][4][5] These metallic anodes have much higher natural metal abundancy, and are reported to be much less prone to dendrite formation compared with metallic Li anode, [3][4][5][6][7][8][9][10][11] potentially due to their lower self-diffusion barriers. 1,12,13 The Ca-ion system has demonstrated significant potential. It has a comparable volumetric capacity to Li, and compared with other multivalent systems like Mg, it also has the advantages of higher earth abundance, lower reductive potential and lower charge density. 1 Despite this, the development of Ca-ion batteries has been slow in part due to issues with the anode, where most studied electrolytes react with metallic Ca, rapidly forming surface passivation layers comprised of CaCl2, Ca(OH)2, or CaCO3, that block Ca ion diffusion and make further plating impossible. [14][15][16][17] However, recent breakthroughs in electrolyte research have brought renewed interest in Ca-ion batteries. [18][19][20][21] These works have demonstrated promising Ca-based electrolytes that are capable of continuous plating and stripping with relatively high efficiency at moderately elevated 17 or room-temperatures. 4,11,22 While most previous studies demonstrated fairly smooth plating morphology, [3][4][5][6][7][8][9][10][11] a recent paper by Davidson et al. 23 showed that dendrites do grow in Mg-ion electrolyte. This challenges the widely accepted belief that multivalent systems do not form dendrites easily. Since research into Ca-ion electrolytes is at an early stage, little work has been done to systematically study their plating and stripping processes. This study explores the electroplating morphology and mechanism within the Ca-ion system via in situ transmission electron microscopy (TEM) to evaluate the feasibility of employing metallic Ca anodes, and to provide a deeper understanding of this system for future optimization.

Oxide ions in transition metal oxide materials can store charge at high voltage offering one of the very few routes to battery cathodes with higher energy density. However, oxidation of O 2on charging is accompanied by condensation of oxidised oxide ions to form molecular O2 trapped in the material; as a consequence, the discharge voltage is much lower than charge, leading to undesirable voltage hysteresis. Here, we capture the nature of the electron-holes on O 2before O2 formation, by exploiting the suppressed transition metal rearrangement in ribbon-ordered Na0.6[Li0.2Mn0.8]O2. We show using SQUID, 17 O NMR, DFT and RIXS that the electron-holes formed on oxidation of O 2are delocalised and distributed across the oxide ions coordinated to two Mn (O-Mn2) that are arranged in ribbons in the transition metal layers. Furthermore, we track these delocalised hole states as they gradually localise in the structure in the form of trapped molecular O2, over a period of days. Establishing the nature of hole states on oxide ions is important if truly reversible O-redox cathodes, with the same voltage on charge and discharge, are to be realised and used to increase the energy density of lithium batteries.

Operando synchrotron XRD and in situ ptycho-tomography of single NMC811 particle revealed the correlation between lattice strain and degradation.