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Surface impurity species, most notably Li2CO3, that develop on layered oxide positive electrode materials with atmospheric aging have been reported to be highly detrimental to the subsequent electrochemical performance. LiNi0.8Co0.15Al0.05O2 (NCA) was used as a model layered oxide compound to evaluate the growth and subsequent electrochemical impact of H2O, LiHCO3, LiOH and Li2CO3. Methodical high temperature annealing enabled the systematic removal of each impurity specie, thus permitting the determination of each specie's individual effect on the host material's electrochemical performance. Extensive cycling of exposed and annealed materials emphasized the cycle life degradation and capacity loss induced by each impurity, while rate capability measurements correlated the electrode impedance to the impurity species present. Based on these characterization results, this work attempts to clarify decades of ambiguity over the growth mechanisms and the electrochemical impact of the specific surface impurity species formed during powder storage in various environments. [ABSTRACT FROM AUTHOR]

Lithium-ion batteries (LIBs) provide high-energy-density electrochemical energy storage, which plays a central role in advancing technologies ranging from portable electronics to electric vehicles (EVs). However, a demand for lighter, more compact devices and for extended range EVs continues to fuel the need for higher energy density storage systems. Li2VO2F, which is synthesized in its lithiated state, allows two-electron transfer per formula during the electrochemical reaction providing a high theoretical capacity of 462 mAh/g. Herein, the synthesis and electrochemical performance of Li2VO2F are optimized. The thermal stability of Li2VO2F, which is related to the safety of a battery is studied by thermal gravimetric analysis. The structure and vanadium oxidation state evolution along Li cycling are studied by ex-situ X-ray diffraction and absorption techniques. It is shown that the rock-salt structure of pristine Li2VO2F is stable up to at least 250°C, and is preserved upon Li cycling, which proceeds by the solid-solution mechanism. However, not all Li can be removed from the structure upon charge to 4.5 V, limiting the experimentally obtained capacity. [ABSTRACT FROM AUTHOR]

An endstation with two high-efficiency soft x-ray spectrographs was developed at Beamline 8.0.1 of the Advanced Light Source, Lawrence Berkeley National Laboratory. The endstation is capable of performing soft x-ray absorption spectroscopy, emission spectroscopy, and, in particular, resonant inelastic soft x-ray scattering (RIXS). Two slit-less variable line-spacing grating spectrographs are installed at different detection geometries. The endstation covers the photon energy range from 80 to 1500 eV. For studying transition-metal oxides, the large detection energy window allows a simultaneous collection of x-ray emission spectra with energies ranging from the O K-edge to the Ni L-edge without moving any mechanical components. The record-high efficiency enables the recording of comprehensive two-dimensional RIXS maps with good statistics within a short acquisition time. By virtue of the large energy window and high throughput of the spectrographs, partial fluorescence yield and inverse partial fluorescence yield signals could be obtained for all transition metal L-edges including Mn. Moreover, the different geometries of these two spectrographs (parallel and perpendicular to the horizontal polarization of the beamline) provide contrasts in RIXS features with two different momentum transfers. [ABSTRACT FROM AUTHOR]

Full, reversible intercalation of two Li+ has not yet been achieved in promising VOPO4 electrodes. A pronounced Li+ gradient has been reported in the low voltage window (i.e., second lithium reaction) that is thought to originate from disrupted kinetics in the high voltage regime (i.e., first lithium reaction). Here, we employ a combination of hard and soft x-ray photoelectron and absorption spectroscopy techniques to depth profile solid state synthesized LiVOPO4 cycled within the low voltage window only. Analysis of the vanadium environment revealed no evidence of a Li+ gradient, which combined with almost full theoretical capacity confirms that disrupted kinetics in the high voltage window are responsible for hindering full two lithium insertion. Furthermore, we argue that the uniform Li+ intercalation is a prerequisite for the formation of intermediate phases Li1.50VOPO4 and Li1.75VOPO4. The evolution from LiVOPO4 to Li2VOPO4 via the intermediate phases is confirmed by direct comparison between O K-edge absorption spectroscopy and density functional theory. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) monodispersed sea urchin-like Ru-doped rutile TiO2 hierarchical architectures composed of radially aligned, densely-packed TiO2 nanorods have been successfully synthesized via an acid-hydrothermal method at low temperature without the assistance of any structure-directing agent and post annealing treatment. The addition of a minuscule concentration of ruthenium dopants remarkably catalyzes the formation of the 3D urchin structure and drives the enhanced photocatalytic H2 production under visible light irradiation, not possible on undoped and bulk rutile TiO2. Increasing ruthenium doping dosage not only increases the surface area up to 166 m2 g−1 but also induces enhanced photoresponse in the regime of visible and near infrared light. The doping introduces defect impurity levels, i.e. oxygen vacancy and under-coordinated Ti3+, significantly below the conduction band of TiO2, and ruthenium species act as electron donors/acceptors that accelerate the photogenerated hole and electron transfer and efficiently suppress the rapid charge recombination, therefore improving the visible-light-driven activity. [ABSTRACT FROM AUTHOR]

Inside Front Cover: Mechanism of the Resistivity Switching Induced by the Joule Heating in Crystalline NbO2 (Adv. The cover image of article number 2200067 by Wei-Cheng Lee and co-workers schematically demonstrates that the weakening of the Nb-Nb dimers dynamically driven by the Joule heating effect triggers local metal-to-insulator transitions responsible for the resistivity switching observed in NbO SB 2 sb . Nanoscale memristors exhibiting voltage-induced resistivity switching are a crucial component in the neuromorphic computation architecture. [Extracted from the article]

The band gap of earth‐abundant ZnSnN2 can be tuned between 1 and 2 eV by varying the growth conditions and resulting cation disorder. The optical absorption edges and carrier densities fall between model curves for cation‐ordered orthorhombic and disordered wurtzite ZnSnN2. Hard X‐ray photo­emission spectra suggest different degrees of cation disorder from comparison with hybrid DFT‐calculated densities of states. [ABSTRACT FROM AUTHOR]

Epitaxial ultra-thin oxide films can support large percent level strains well beyond their bulk counterparts, thereby enabling strain-engineering in oxides that can tailor various phenomena. At these reduced dimensions (typically < 10 nm), contributions from the substrate can dwarf the signal from the epilayer, making it difficult to distinguish the properties of the epilayer from the bulk. This is especially true for oxide on oxide systems. Here, we have employed a combination of hard X-ray photoelectron spectroscopy (HAXPES) and angular soft X-ray absorption spectroscopy (XAS) to study epitaxial VO2/TiO2 (100) films ranging from 7.5 to 1 nm. We observe a low-temperature (300 K) insulating phase with evidence of vanadium-vanadium (V-V) dimers and a high-temperature (400 K) metallic phase absent of V-V dimers irrespective of film thickness. Our results confirm that the metal insulator transition can exist at atomic dimensions and that biaxial strain can still be used to control the temperature of its transition when the interfaces are atomically sharp. More generally, our case study highlights the benefits of using non-destructive XAS and HAXPES to extract out information regarding the interfacial quality of the epilayers and spectroscopic signatures associated with exotic phenomena at these dimensions. [ABSTRACT FROM AUTHOR]

We have revealed the critical role of carbon coating in the stability and thermal behaviour of olivine MnPO4 obtained by chemical delithiation of LiMnPO4. (Li)MnPO4 samples with various particle sizes and carbon contents were studied. Carbon-free LiMnPO4 obtained by solid state synthesis in O2 becomes amorphous upon delithiation. Small amounts of carbon (0.3 wt%) help to stabilize the olivine structure, so that completely delithiated crystalline olivine MnPO4 can be obtained. Larger amount of carbon (2 wt%) prevents full delithiation. Heating in air, O2, or N2 results in structural disorder (<300 °C), formation of an intermediate sarcopside Mn3(PO4)2 phase (350-450 °C), and complete decomposition to Mn2P2O7 on extended heating at 400 °C. Carbon coating protects MnPO4 from reacting with environmental water, which is detrimental to its structural stability. [ABSTRACT FROM AUTHOR]

The article argues that lattice oxygen redox (OR) reactions and other oxygen activities should be studied and treated separately to achieve viable OR-based battery electrodes. Topics discussed include consideration of transition metal (TM) redox in oxide cathodes as practical in positive electrodes, differentiation between lattice and nonlattice OR activities; and the lack of clarification of the lattice OR activities.

The design of cathodes for intercalation batteries requires consideration of both atomistic and electronic structure to facilitate redox at specific transition metal sites along with the concomitant diffusion of cations and electrons. Cation intercalation often brings about energy dissipative phase transformations that give rise to substantial intercalation gradients as well as multiscale phase and strain inhomogeneities. The layered α-V2O5 phase is considered to be a classical intercalation host but is plagued by sluggish diffusion kinetics and a series of intercalation-induced phase transitions that require considerable lattice distortion. Here, we demonstrate that a 1D tunnel-structured ζphase polymorph of V2O5 provides a stark study in contrast and can reversibly accommodate Li-ions without a large distortion of the structural framework and with substantial mitigation of polaronic confinement. Entirely homogeneous lithiation is evidenced across multiple cathode particles (in contrast to α-V2O5 particles wherein lithiation-induced phase transformations induce phase segregation). Barriers to Li-ion as well as polaron diffusion are substantially diminished for metastable ζV2O5 in comparison to the thermodynamically stable α-V2O5 phase. The rigid tunnel framework, relatively small changes in coordination environment of intercalated Li-ions across the diffusion pathways defined by the 1D tunnels, and degeneracy of V 3d states at the bottom of the conduction band reduce electron localization that is a major impediment to charge transport in α-V2O5. The 1D ζphase thus facilitates a continuous lithiation pathway that is markedly different from the successive intercalation-induced phase transitions observed in α-V2O5. The results here illustrate the importance of electronic structure in mediating charge transport in oxide cathode materials and demonstrates that a metastable polymorph with higher energy bonding motifs that define frustrated coordination environments can serve as an attractive intercalation host. [ABSTRACT FROM AUTHOR]

Enabling practical utilization of layered R3̅m positive electrodes near full delithiation requires an enhanced understanding of the complex electrode-electrolyte interactions that often induce failure. Using Li[Ni0.8Co0.15Al0.05]O2 (NCA) as a model layered compound, the chemical and structural stability in a strenuous thermal and electrochemical environment was explored. Operando microcalorimetry and electrochemical impedance spectroscopy identified a fingerprint for a structural decomposition and transition-metal dissolution reaction that occurs on the positive electrode at full delithiation. Surface-sensitive characterization techniques, including X-ray absorption spectroscopy and high-resolution transmission electron microscopy, measured a structural and morphological transformation of the surface and subsurface regions of NCA. Despite the bulk structural integrity being maintained, NCA surface degradation at a high state of charge induces excessive transition-metal dissolution and significant positive electrode impedance development, resulting in a rapid decrease in electrochemical performance. Additionally, the impact of electrolyte salt, positive electrode surface area, and surface Li2CO3 content on the magnitude and character of the dissolution reaction was studied. [ABSTRACT FROM AUTHOR]

Full, reversible intercalation of two Li+ has not yet been achieved in promising VOPO4 electrodes. A pronounced Li+ gradient has been reported in the low voltage window (i.e., second lithium reaction) that is thought to originate from disrupted kinetics in the high voltage regime (i.e., first lithium reaction). Here, we employ a combination of hard and soft x-ray photoelectron and absorption spectroscopy techniques to depth profile solid state synthesized LiVOPO4 cycled within the low voltage window only. Analysis of the vanadium environment revealed no evidence of a Li+ gradient, which combined with almost full theoretical capacity confirms that disrupted kinetics in the high voltage window are responsible for hindering full two lithium insertion. Furthermore, we argue that the uniform Li+ intercalation is a prerequisite for the formation of intermediate phases Li1.50VOPO4 and Li1.75VOPO4. The evolution from LiVOPO4 to Li2VOPO4 via the intermediate phases is confirmed by direct comparison between O K-edge absorption spectroscopy and density functional theory. [ABSTRACT FROM AUTHOR]

Epitaxial ultra-thin oxide films can support large percent level strains well beyond their bulk counterparts, thereby enabling strain-engineering in oxides that can tailor various phenomena. At these reduced dimensions (typically < 10 nm), contributions from the substrate can dwarf the signal from the epilayer, making it difficult to distinguish the properties of the epilayer from the bulk. This is especially true for oxide on oxide systems. Here, we have employed a combination of hard X-ray photoelectron spectroscopy (HAXPES) and angular soft X-ray absorption spectroscopy (XAS) to study epitaxial VO2/TiO2 (100) films ranging from 7.5 to 1 nm. We observe a low-temperature (300 K) insulating phase with evidence of vanadium-vanadium (V-V) dimers and a high-temperature (400 K) metallic phase absent of V-V dimers irrespective of film thickness. Our results confirm that the metal insulator transition can exist at atomic dimensions and that biaxial strain can still be used to control the temperature of its transition when the interfaces are atomically sharp. More generally, our case study highlights the benefits of using non-destructive XAS and HAXPES to extract out information regarding the interfacial quality of the epilayers and spectroscopic signatures associated with exotic phenomena at these dimensions. [ABSTRACT FROM AUTHOR]

Recent reports have identified new metaphases of VO2 with strain and/or doping, suggesting the structural phase transition and the metal-to-insulator transition might be decoupled. Using epitaxially strained VO2/TiO2 (001) thin films, which display a bulklike abrupt metal-to-insulator transition and rutile to monoclinic transition structural phase transition, we employ x-ray standing waves combined with hard x-ray photoelectron spectroscopy to simultaneously measure the structural and electronic transitions. This x-ray standing waves study elegantly demonstrates the structural and electronic transitions occur concurrently within experimental limits (±1 K). [ABSTRACT FROM AUTHOR]

Metal-Nb2O5−x-metal memdiodes exhibiting rectification, hysteresis, and capacitance are demonstrated for applications in neuromorphic circuitry. These devices do not require any post-fabrication treatments such as filament creation by electroforming that would impede circuit scalability. Instead these devices operate due to Poole-Frenkel defect controlled transport where the high defect density is inherent to the Nb2O5−x deposition rather than post-fabrication treatments. Temperature dependent measurements reveal that the dominant trap energy is 0.22 eV suggesting it results from the oxygen deficiencies in the amorphous Nb2O5−x. Rectification occurs due to a transition from thermionic emission to tunneling current and is present even in thick devices (>100 nm) due to charge trapping which controls the tunneling distance. The turn-on voltage is linearly proportional to the Schottky barrier height and, in contrast to traditional metal-insulator-metal diodes, is logarithmically proportional to the device thickness. Hysteresis in the I-V curve occurs due to the current limited filling of traps. [ABSTRACT FROM AUTHOR]

The characteristics of the cooperative Mott-Peierls metal-insulator transition (MIT) of VO2 can be altered by employing epitaxial strain. While the most commonly used substrate for this purpose is isostructural rutile TiO2, thin films often suffer from interdiffusion of Ti ions near the interface. Exploiting this phenomena, we investigate the nature of interfacial V4+/Ti4+ cation intermixing and its effects on the MIT using scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS), soft x-ray absorption spectroscopy (XAS), and hard x-ray photoelectron spectroscopy (HAXPES), along with supporting density functional theory (DFT) calculations. We find that the reduced orbital occupancy in highly Ti incorporated VO2 is responsible for suppressing the MIT. Interdiffused films are found to be metallic at all measured temperatures, despite a resolute dimerization inferred from x-ray absorption data at lower temperatures. Our results demonstrate that the Mott physics can be suppressed in doped VO2, while a lattice dimerization remains thermodynamically favorable. [ABSTRACT FROM AUTHOR]

Aqueous zinc electrolytes offer the potential for cheaper rechargeable batteries due to their safe compatibility with the high capacity metal anode; yet, they are stymied by irregular zinc deposition and consequent dendrite growth. Suppressing dendrite formation by tailoring the electrolyte is a proven approach from lithium batteries; yet, the underlying mechanistic understanding that guides such tailoring does not necessarily directly translate from one system to the other. Here, it is shown that the electrostatic shielding mechanism, a fundamental concept in electrolyte engineering for stable metal anodes, has different consequences for the plating morphology in aqueous zinc batteries. Operando electrochemical transmission electron microscopy is used to directly observe the zinc nucleation and growth under different electrolyte compositions and reveal that electrostatic shielding additive suppresses dendrites by inhibiting secondary zinc nucleation along the (100) edges of existing primary deposits and encouraging preferential deposition on the (002) faces, leading to a dense and block-like zinc morphology. The strong influence of the crystallography of Zn on the electrostatic shielding mechanism is further confirmed with Zn||Ti cells and density functional theory modeling. This work demonstrates the importance of considering the unique aspects of the aqueous zinc battery system when using concepts from other battery chemistries., (© 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

A Mn 2+ -Li-Nb disordered rock-salt oxide cathode is prepared by a solid-state reaction under 5% H 2 /N 2 , and its electrochemical property shows a high voltage plateau at 4.8 V, with irreversible structural changes in the 1st cycle due to O redox processes; this is supported by powder X-ray diffraction and ex situ laboratory Mn K-edge XANES data.

The surface of the layered transition metal oxide cathode plays an important role in its function and degradation. Modification of the surface structure and chemistry is often necessary to overcome the debilitating effect of the native surface. Here, we employ a chemical reduction method using CaI 2 to modify the native surface of single-crystalline layered transition metal oxide cathode particles. High-resolution transmission electron microscopy shows the formation of a conformal cubic phase at the particle surface, where the outmost layer is enriched with Ca. The modified surface significantly improves the long-term capacity retention at low rates of cycling, yet the rate capability is compromised by the impeded interfacial kinetics at high voltages. The lack of oxygen vacancy generation in the chemically induced surface phase transformation likely results in a dense surface layer that accounts for the improved electrochemical stability and impeded Li-ion diffusion. This work highlights the strong dependence of the electrode's (electro)chemical stability and intercalation kinetics on the surface structure and chemistry, which can be further tailored by the chemical reduction method.

Epitaxial growth is a powerful tool for synthesizing heterostructures and integrating multiple functionalities. However, interfacial mixing can readily occur and significantly modify the properties of layered structures, particularly for those containing energy storage materials with smaller cations. Here, we show a two-step sequence involving the growth of an epitaxial LiCoO 2 cathode layer followed by the deposition of a binary transition metal oxide. Orientation-controlled epitaxial synthesis of the model solid-state-electrolyte Li 2 WO 4 and anode material Li 4 Ti 5 O 12 occurs as WO 3 and TiO 2 nucleate and react with Li ions from the underlying cathode. We demonstrate that this lithiation-assisted epitaxy approach can be used for energy materials discovery and exploring different combinations of epitaxial interfaces that can serve as well-defined model systems for mechanistic studies of energy storage and conversion processes.

Layered lithium transition metal oxides derived from LiMO 2 (M = Co, Ni, Mn, etc.) have been widely adopted as the cathodes of Li-ion batteries for portable electronics, electric vehicles, and energy storage. Oxygen loss in the layered oxides is one of the major factors leading to cycling-induced structural degradation and its associated fade in electrochemical performance. Herein, we review recent progress in understanding the phenomena of oxygen loss and the resulting structural degradation in layered oxide cathodes. We first present the major driving forces leading to the oxygen loss and then describe the associated structural degradation resulting from the oxygen loss. We follow this analysis with a discussion of the kinetic pathways that enable oxygen loss, and then we address the resulting electrochemical fade. Finally, we review the possible approaches toward mitigating oxygen loss and the associated electrochemical fade as well as detail novel analytical methods for probing the oxygen loss.

The characterization of oxidized oxygen states through high-efficiency mapping of resonant inelastic X-ray scattering (mRIXS) has become a crucial approach for studying the oxygen redox activities in high-energy battery cathodes. However, this approach has been recently challenged due to the concern of irradiation damage. Here we revisited a typical Li-rich electrode, Li 1.144 Ni 0.136 Mn 0.544 Co 0.136 O 2 , in both lithiated and delithiated states and evaluated the X-ray irradiation effect in the lengthy mRIXS experiments. Our results show that irradiation cannot introduce any oxidized oxygen feature, and the features of oxidized oxygen are weakened with a high X-ray dose. The results confirm again that mRIXS detects the intrinsic oxidized oxygen state in battery electrodes. However, the distinct irradiation effects in different systems imply that irradiation could selectively target the least stable elemental or chemical states, which should be analyzed with caution in the study of active chemical states.

Vanadium dioxide (VO 2 ) features a pronounced, thermally-driven metal-to-insulator transition at 340 K. Employing epitaxial stress on rutile [Formula: see text] substrates, the transition can be tuned to occur close to room temperature. Striving for applications in oxide-electronic devices, the lateral homogeneity of such samples must be considered as an important prerequisite for efforts towards miniaturization. Moreover, the preparation of smooth surfaces is crucial for vertically stacked devices and, hence, the design of functional interfaces. Here, the surface morphology of [Formula: see text] films was analyzed by low-energy electron microscopy and diffraction as well as scanning probe microscopy. The formation of large terraces could be achieved under temperature-induced annealing, but also the occurrence of facets was observed and characterized. Further, we report on quasi-periodic arrangements of crack defects which evolve due to thermal stress under cooling. While these might impair some applicational endeavours, they may also present crystallographically well-oriented nano-templates of bulk-like properties for advanced approaches.

Solar fuel generation mediated by semiconductor heterostructures represents a promising strategy for sustainable energy conversion and storage. The design of semiconductor heterostructures for photocatalytic energy conversion requires the separation of photogenerated charge carriers in real space and their delivery to active catalytic sites at the appropriate overpotentials to initiate redox reactions. Operation of the desired sequence of light harvesting, charge separation, and charge transport events within heterostructures is governed by the thermodynamic energy offsets of the two components and their photoexcited charge-transfer reactivity, which determine the extent to which desirable processes can outcompete unproductive recombination channels. Here, we map energetic offsets and track the dynamics of electron transfer in MoS 2 /CdS architectures, prepared by interfacing two-dimensional MoS 2 nanosheets with CdS quantum dots (QDs), and correlate the observed charge separation to photocatalytic activity in the hydrogen evolution reaction. The energetic offsets between MoS 2 and CdS have been determined using hard and soft X-ray photoemission spectroscopy (XPS) in conjunction with density functional theory. A staggered type-II interface is observed, which facilitates electron and hole separation across the interface. Transient absorption spectroscopy measurements demonstrate ultrafast electron injection occurring within sub-5 ps from CdS QDs to MoS 2 , allowing for creation of a long-lived charge-separated state. The increase of electron concentration in MoS 2 is evidenced with the aid of spectroelectrochemical measurements and by identifying the distinctive signatures of electron-phonon scattering in picosecond-resolution transient absorption spectra. Ultrafast charge separation across the type-II interface of MoS 2 /CdS heterostructures enables a high Faradaic efficiency of ∼99.4 ± 1.2% to be achieved in the hydrogen evolution reaction (HER) and provides a 40-fold increase in the photocatalytic activity of dispersed photocatalysts for H 2 generation. The accurate mapping of thermodynamic driving forces and dynamics of charge transfer in these heterostructures suggests a means of engineering ultrafast electron transfer and effective charge separation to design viable photocatalytic architectures.

Epitaxial films of vanadium dioxide (VO 2 ) on rutile TiO 2 substrates provide a means of strain-engineering the transition pathways and stabilizing of the intermediate phases between monoclinic (insulating) M1 and rutile (metal) R end phases. In this work, we investigate structural behavior of epitaxial VO 2 thin films deposited on isostructural MgF 2 (001) and (110) substrates via temperature-dependent Raman microscopy analysis. The choice of MgF 2 substrate clearly reveals how elongation of V-V dimers accompanied by the shortening of V-O bonds triggers the intermediate M2 phase in the temperature range between 70-80 °C upon the heating-cooling cycles. Consistent with earlier claims of strain-induced electron correlation enhancement destabilizing the M2 phase our temperature-dependent Raman study supports a small temperature window for this phase. The similarity of the hysteretic behavior of structural and electronic transitions suggests that the structural transitions play key roles in the switching properties of epitaxial VO 2 thin films.

Recent reports have identified new metaphases of VO&#95;{2} with strain and/or doping, suggesting the structural phase transition and the metal-to-insulator transition might be decoupled. Using epitaxially strained VO&#95;{2}/TiO&#95;{2} (001) thin films, which display a bulklike abrupt metal-to-insulator transition and rutile to monoclinic transition structural phase transition, we employ x-ray standing waves combined with hard x-ray photoelectron spectroscopy to simultaneously measure the structural and electronic transitions. This x-ray standing waves study elegantly demonstrates the structural and electronic transitions occur concurrently within experimental limits (±1  K).

Sensitivity to the "bulk" oxygen core orbital makes hard X-ray photoelectron spectroscopy (HAXPES) an appealing technique for studying oxygen redox candidates. Various studies have reported an additional O 1s peak (530-531 eV) at high voltages, which has been considered a direct signature of the bulk oxygen redox process. Here, we find the emergence of a 530.4 eV O 1s HAXPES peak for three model cathodes-Li 2 MnO 3 , Li-rich NMC, and NMC 442-that shows no clear link to oxygen redox. Instead, the 530.4 eV peak for these three systems is attributed to transition metal reduction and electrolyte decomposition in the near-surface region. Claims of oxygen redox relying on photoelectron spectroscopy must explicitly account for the surface sensitivity of this technique and the extent of the cathode degradation layer.

The oxygen redox (OR) activity is conventionally considered detrimental to the stability and kinetics of batteries. However, OR reactions are often confused by irreversible oxygen oxidation. Here, based on high-efficiency mapping of resonant inelastic x-ray scattering of both the transition metal and oxygen, we distinguish the lattice OR in Na 0.6 [Li 0.2 Mn 0.8 ]O 2 and compare it with Na 2/3 [Mg 1/3 Mn 2/3 ]O 2 . Both systems display strong lattice OR activities but with distinct electrochemical stability. The comparison shows that the substantial capacity drop in Na 0.6 [Li 0.2 Mn 0.8 ]O 2 stems from non-lattice oxygen oxidations, and its voltage decay from an increasing Mn redox contribution upon cycling, contrasting those in Na 2/3 [Mg 1/3 Mn 2/3 ]O 2 . We conclude that lattice OR is not the ringleader of the stability issue. Instead, irreversible oxygen oxidation and the changing cationic reactions lead to the capacity and voltage fade. We argue that lattice OR and other oxygen activities should/could be studied and treated separately to achieve viable OR-based electrodes., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).)