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Summary Li2MnO3 has been considered to be a representative Li-rich compound with active debates on oxygen activities. Here, by evaluating the Mn and O states in the bulk and on the surface of Li2MnO3, we clarify that Mn(III/IV) redox dominates the reversible bulk redox in Li2MnO3, while the initial charge plateau is from surface reactions with oxygen release and carbonate decomposition. No lattice oxygen redox is involved at any electrochemical stage. The carbonate formation and decomposition indicate the catalytic property of the Li2MnO3 surface, which inspires Li-CO2/air batteries with Li2MnO3 acting as a superior electrocatalyst. The absence of lattice oxygen redox in Li2MnO3 questions the origin of the oxygen redox in Li-rich compounds, which is found to be of the same nature as that in conventional materials based on spectroscopic comparisons. These findings provide guidelines on understanding and controlling oxygen activities toward high-energy cathodes and suggest opportunities on using alkali-rich materials for catalytic reactions.

An increasing number of applications in scientific and other domains have moved or are in active transition to clouds, and the demand for big data transfers between geographically distributed cloud-based data centers is rapidly growing. Many modern backbone networks leverage logically centralized controllers based on software-defined networking (SDN) to provide advance bandwidth reservation for data transfer requests. How to fully utilize the bandwidth resources of the links connecting data centers with guaranteed quality of service for each user request is an important problem for cloud service providers. Most existing work focuses on bandwidth scheduling for a single request for data transfer or multiple requests using the same service model. In this work, we construct rigorous cost models to quantify user satisfaction degree, and formulate a generic problem of bandwidth scheduling for multiple deadline-constrained data transfer requests of different types to maximize the request scheduling success ratio while minimizing the data transfer completion time of each request. We prove this problem to be not only NP-complete but also non-approximable, and hence design a heuristic algorithm. For performance evaluation, we establish a proof-of-concept emulated SDN testbed and also generate large-scale simulation networks. Both experimental and simulation results show that the proposed scheduling scheme significantly outperforms existing methods in terms of user satisfaction degree and scheduling success ratio.

Summary Lithium-ion batteries with high energy density are being intensively pursued to meet the ever-growing demand for energy storage. However, the increase in energy density often comes with an elevated instability of electrode materials, causing major concerns about the reliability and safety of lithium-ion batteries. Here, we report a strategy for stabilizing cathode materials by modulating the vacant lattice sites on the particle surface. Using the high-voltage Li[Ni0.5Mn1.5]O4 as an example, we demonstrate that introduction of a 10-nm epitaxial surface layer with Al3+ in the empty 16c octahedral sites of the spinel Li[Ni0.5Mn1.5]O4 suppresses structural degradation during cycling by increasing the surface stability without interfering with the Li+ diffusion around the Al3+ sites. Control of the Al3+ concentration in the surface region was shown to be a facile process. The process was shown to stabilize long-term cycling of Li[Ni0.5Mn1.5]O4 to 5 V versus Li+/Li0.

The demand of sustainable power supply requires high-performance cost-effective energy storage technologies. Here we report a high-rate long-life low-cost sodium-ion battery full-cell system by innovating both the anode and the electrolyte. The redox couple of manganese(I/II) in Prussian blue analogs enables a high-rate and stable anode. Soft X-ray absorption spectroscopy and resonant inelastic X-ray scattering provide direct evidence suggesting the existence of monovalent manganese in the charged anode. There is a strong hybridization between cyano ligands and manganese-3d states, which benefits the electronic property for improving rate performance. Additionally, we employ an organic–aqueous cosolvent electrolyte to solve the long-standing solubility issue of Prussian blue analogs. A full-cell sodium-ion battery with low-cost Prussian blue analogs in both electrodes and co-solvent electrolyte retains 95% of its initial discharge capacity after 1000 cycles at 1C and 95% depth of discharge. The revealed manganese(I/II) redox couple inspires conceptual innovations of batteries based on atypical oxidation states., Sodium ion batteries offer more cost-effective storage than lithium and could be used for grid-scale energy storage. Here, the authors demonstrate a full cell based on a MnHCMn anode and an organic-aqueous cosolvent electrolyte. X-ray spectroscopy evidence further suggests the presence of Mn(I).

The pursuit of high capacity rechargeable batteries which utilize strong oxidizing cathodes and reducing anodes create significant challenges on the electrochemical stability of the electrolyte. A viable rechargeable Li-ion battery system is realized through an in-situly formed passivation layer on the anode-electrolyte interface. Additionally, the surface and interface properties of the cathode determine the stability, rate, and voltage of the battery. Therefore, understanding and controlling reactions at the electrode–electrolyte interfaces is essential for developing high-capacity, long-lasting and safe battery system. In this review, we discuss some of the recent soft x-ray absorption spectroscopic results on electrode–electrolyte interfaces. By virtue of the sensitivity of soft x-ray to both low-Z elements and transition metals and its short probing depth, the spectroscopic results provide insight into the complex interactions between electrode and electrolyte, as well as the surface evolutions of the electrodes during electrochemical operations.

Oxide heterostructures and superlattices have attracted a great deal of attention in recent years owing to the rich exotic properties encountered at their interfaces. We focus on the potential of tunable correlated oxides by investigating the spectral function of the prototypical correlated metal SrVO3, using soft x-ray absorption spectroscopy (XAS) and resonant inelastic soft x-ray scattering (RIXS) to access both unoccupied and occupied electronic states, respectively. We demonstrate a remarkable level of tunability in the spectral function of SrVO3 by varying its thickness within the SrVO3/SrTiO3 superlattice, showing that the effects of electron correlation can be tuned from dominating the energy spectrum in a strongly correlated Mott-Hubbard insulator, towards a correlated metal. We show that the effects of dimensionality on the correlated properties of SrVO3 are augmented by interlayer coupling, yielding a highly flexible correlated oxide that may be readily married with other oxide systems.

With flexibility in tuning their electric and magnetic properties, multiferroics can be used in information exchange and storage in ways that are very different from the present electronic materials. Here we use resonant soft X-ray scattering spectroscopy to study the F -type ( 0 , τ , 0 ) and C -type (0, 1–2 τ , 0) diffraction peaks from sinusoidal antiferromagnetic spin order in multiferroic DyMnO 3 . By comparing the temperature dependence of ordering wave vectors τ , peak intensities I , and correlation lengths λ measured at Mn L 2 -, O K -, and Dy M 5 -edges, we show that the nearly perfect locking between the ordering wave vectors from Dy 4 f states and Mn 3 d orbitals manifesting the second harmonic diffraction peak implies the notable orbital involvement in the coupling between Mn and Dy spins. Our DFT calculations further suggest that the lattice response to different antiferromagnetic ground states (A-type versus E-type) is much weaker in TbMnO 3 , in agreement with previous claim that the symmetric exchange interaction can be an important factor for understanding the ferroelectricity in DyMnO 3 than in TbMnO 3 .

Undoped, Cr-doped, and Nb-doped LiMn(1.5)Ni(0.5)O4 (LNMO) is synthesized via a PVP (polyvinylpyrrolidone)-combustion method by calcinating at 1000 °C for 6 h. SEM images show that the morphology of LNMO particles is affected by Cr and Nb doping. Cr doping results in sharper edges and corners and smaller particle size, and Nb doping leads to smoother edges and corners and more rounded and larger particles. The crystal and electron structure is investigated by XRD- and synchrotron-based soft X-ray absorption spectroscopy (sXAS). Cr doping and light Nb doping (LiNb(0.02)Ni(0.49)Mn(1.49)O4) improve the rate performance of LNMO. To explore the reason for rate-performance improvement, we conducted potential intermittent titration technique (PITT) and electrochemical impedance spectroscopy (EIS) tests. The Li(+) chemical diffusion coefficient at different state of charge (SOC) is calculated and suggests that both Cr and light Nb doping speeds up Li(+) diffusion in LNMO particles. The impedance spectra show that both R(SEI) and R(ct) are reduced by Cr and light Nb doping. The cycling performance is improved by Cr or Nb doping, and Cr doping increases both Coulombic efficiency and energy efficiency of LNMO at 1 C cycling. The LiCr(0.1)Ni(0.45)Mn(1.45)O4 remains at 94.1% capacity after 500 cycles at 1 C, and during the cycling, the Coulombic efficiency and energy efficiency remain at over 99.7% and 97.5%, respectively.

While a rechargeable battery based on Na/CFx has been proposed, its reversible mechanism has remained unclear. Here, a new fluorinated carbon fiber material with the formula CF0.75 is used as a cathode material for rechargeable sodium batteries, delivering an initial discharge capacity of 705 mA·h g–1 with a high discharge plateau of 2.75 V and a reversible high discharge capacity of 350 mA·h g–1 at 20 mA g–1. The first discharge plateau of 2.75 V is the highest value reported in this family of materials so far, even slightly higher than that of commercial fluorinated graphite tested in a lithium battery (2.7 V). The origins of the observed high voltage of the material are explored by a combination of theoretical calculations and galvanostatic intermittent titration technique data and determined to be related to the disordered structure of the carbon fiber. Soft X-ray absorption spectroscopy and 19F magic angle spinning nuclear magnetic resonance characterization results disclose a full view of the conver...

Energy storage has become more and more a limiting factor of today's sustainable energy applications, including electric vehicles and green electric grid based on volatile solar and wind sources. The pressing demand of developing high-performance electrochemical energy storage solutions, i.e., batteries, relies on both fundamental understanding and practical developments from both the academy and industry. The formidable challenge of developing successful battery technology stems from the different requirements for different energy-storage applications. Energy density, power, stability, safety, and cost parameters all have to be balanced in batteries to meet the requirements of different applications. Therefore, multiple battery technologies based on different materials and mechanisms need to be developed and optimized. Incisive tools that could directly probe the chemical reactions in various battery materials are becoming critical to advance the field beyond its conventional trial-and-error approach. Here, we present detailed protocols for soft X-ray absorption spectroscopy (sXAS), soft X-ray emission spectroscopy (sXES), and resonant inelastic X-ray scattering (RIXS) experiments, which are inherently elemental-sensitive probes of the transition-metal 3d and anion 2p states in battery compounds. We provide the details on the experimental techniques and demonstrations revealing the key chemical states in battery materials through these soft X-ray spectroscopy techniques.

Recent research has explored combining conventional transition-metal redox with anionic lattice oxygen redox as a new and exciting direction to search for high-capacity lithium-ion cathodes. Here, we probe the poorly understood electrochemical activity of anionic oxygen from a material perspective by elucidating the effect of the transition metal on oxygen redox activity. We study two lithium-rich layered oxides, specifically lithium nickel metal oxides where metal is either manganese or ruthenium, which possess a similar structure and discharge characteristics, but exhibit distinctly different charge profiles. By combining X-ray spectroscopy with operando differential electrochemical mass spectrometry, we reveal completely different oxygen redox activity in each material, likely resulting from the different interaction between the lattice oxygen and transition metals. This work provides additional insights into the complex mechanism of oxygen redox and development of advanced high-capacity lithium-ion cathodes., A reversible oxygen redox process contributes extra capacity and understanding this behavior is of high importance. Here, aided by resonant inelastic X-ray scattering, the authors reveal the distinctive anionic oxygen activity of battery electrodes with different transition metals.

We describe a new experimental technique that allows for soft x-ray spectroscopy studies (∼100-1000 eV) of high pressure liquid (∼100 bars). We achieve this through a liquid cell with a 100 nm-thick Si3N4 membrane window, which is sandwiched by two identical O-rings for vacuum sealing. The thin Si3N4 membrane allows soft x-rays to penetrate, while separating the high-pressure liquid under investigation from the vacuum required for soft x-ray transmission and detection. The burst pressure of the Si3N4 membrane increases with decreasing size and more specifically is inversely proportional to the side length of the square window. It also increases proportionally with the membrane thickness. Pressures > 60 bars could be achieved for 100 nm-thick square Si3N4 windows that are smaller than 65 μm. However, above a certain pressure, the failure of the Si wafer becomes the limiting factor. The failure pressure of the Si wafer is sensitive to the wafer thickness. Moreover, the deformation of the Si3N4 membrane is quantified using vertical scanning interferometry. As an example of the performance of the high-pressure liquid cell optimized for total-fluorescence detected soft x-ray absorption spectroscopy (sXAS), the sXAS spectra at the Ca L edge (∼350 eV) of a CaCl2 aqueous solution are collected under different pressures up to 41 bars.

The excessive volume changes during cell cycling of Si-based anode in lithium ion batteries impeded its application. One major reason for the cell failure is particle isolation during volume shrinkage in delithiation process, which makes strong adhesion between polymer binder and anode active material particles a highly desirable property. Here, a biomimetic side-chain conductive polymer incorporating catechol, a key adhesive component of the mussel holdfast protein, was synthesized. Atomic force microscopy-based single-molecule force measurements of mussel-inspired conductive polymer binder contacting a silica surface revealed a similar adhesion toward substrate when compared with an effective Si anode binder, homo-poly(acrylic acid), with the added benefit of being electronically conductive. Electrochemical experiments showed a very stable cycling of Si-alloy anodes realized via this biomimetic conducting polymer binder, leading to a high loading Si anode with a good rate performance. We attribute the ability of the Si-based anode to tolerate the volume changes during cycling to the excellent mechanical integrity afforded by the strong interfacial adhesion of the biomimetic conducting polymer.

© 2018 The Royal Society of Chemistry. We report the first direct experimental evidence of the dynamic behavior of the solid-electrolyte-interphase (SEI) on copper electrodes upon electrochemical cycling. Synchrotron-based soft X-ray absorption spectroscopy (sXAS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) consistently show that both the chemical composition and the thickness of the SEI change with electrochemical potential throughout the slow formation process. In particular, sXAS results show that the nascent carbonate species in SEI show redox reversibility and decompose during the delithiation (oxidation) process, which leads to a significant shrinking of the SEI thickness as confirmed by TOF-SIMS. Meanwhile, the carbonates also matures and become more and more inactive at every lithiation (reduction) process. These experimental observations reveal unambiguously that SEI formation is much more complicated than a simple and monotonous deposition of electrolyte decomposition product; instead, it could be an oscillating process with a breathing growth.

2D vanadium carbide MXene containing surface functional groups (denoted as V2CTx, where Tx are surface functional groups) is synthesized and studied as anode material for Na-ion batteries. V2CTx anode exhibits reversible charge storage with good cycling stability and high rate capability through electrochemical test. The charge storage mechanism of V2CTx material during Na+ intercalation/deintercalation and the redox reaction of vanadium are studied using a combination of synchrotron based X-ray diffraction, hard X-ray absorption near edge spectroscopy (XANES), and soft X-ray absorption spectroscopy (sXAS). Experimental evidence of a major contribution of redox reaction of vanadium to the charge storage and the reversible capacity of V2CTx during sodiation/desodiation process are provided through V K-edge XANES and V L2,3-edge sXAS results. A correlation between the CO32− content and the Na+ intercalation/deintercalation states in the V2CTx electrode observed from C and O K-edge in sXAS results implies that some additional charge storage reactions may take place between the Na+-intercalated V2CTx and the carbonate-based nonaqueous electrolyte. The results of this study provide valuable information for the further studies on V2CTx as anode material for Na-ion batteries and capacitors.

© 2017 Author(s). 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.

© 2017 Author(s). Over the past decade, the advances in grating-based soft X-ray spectrometers have revolutionized the soft X-ray spectroscopies in materials research. However, these novel spectrometers are mostly dedicated designs, which cannot be easily adopted for applications with diverging demands. Here we present a versatile spectrometer design concept based on the Hettrick-Underwood optical scheme that uses modular mechanical components. The spectrometer’s optics chamber can be used with gratings operated in either inside or outside orders, and the detector assembly can be reconfigured accordingly. The spectrometer can be designed to have high spectral resolution, exceeding 10 000 resolving power when using small source (∼1μm) and detector pixels (∼5μm) with high line density gratings (∼3000 lines/mm), or high throughput at moderate resolution. We report two such spectrometers with slightly different design goals and optical parameters in this paper. We show that the spectrometer with high throughput and large energy window is particularly useful for studying the sustainable energy materials. We demonstrate that the extensive resonant inelastic X-ray scattering (RIXS) map of battery cathode material LiNi1/3Co1/3Mn1/3O2 can be produced in few hours using such a spectrometer. Unlike analyzing only a handful of RIXS spectra taken at selected excitation photon energies across the elemental absorption edges to determine various spectral features like the localized dd excitations and non-resonant fluorescence emissions, these features can be easily identified in the RIXS maps. Studying such RIXS maps could reveal novel transition metal redox in battery compounds that are sometimes hard to be unambiguously identified in X-ray absorption and emission spectra. We propose that this modular spectrometer design can serve as the platform for further customization to meet specific scientific demands.

The mixed transition-metal layered compound, LiNi0.5Mn0.3Co0.2O2 (NMC532), is a promising high-energy cathode material. However, the required high-voltage (>4.3 V) cycling is accompanied by a rapid capacity fade associated with a complex redox mechanism that has not been clarified. Here we report soft x-ray absorption spectroscopy of NMC532 electrodes, both pristine and those charged to 4.2, 4.35, or 4.5 V in graphite/NMC532 cells. A quantitative sXAS analysis shows that about 20% of the nickel exists as Ni4+ in the as-synthesized NMC532. The Ni redox reaction contributes only to the experimental capacity obtained below 4.2 V, while Co redox reactions take place throughout the entire electrochemical cycling up to 4.5 V. In contrast to the changing ratio of the well-defined Ni2+, Ni3+ and Ni4+ ions, Co always displays ill-defined intermediate valence states in the charged NMC532 electrodes. This indicates an itinerant electron system in NMC electrodes related to the improved rate performance through Co doping. Additionally, about 20% of Ni2+ is found on the electrode surface at the high potential, which suggests that the electrode surface has either gone through surface reconstruction or reacted with the electrolyte at high voltage.

Aqueous electrochemical energy storage devices have attracted significant attention owing to their high safety, low cost and environmental friendliness. However, their applications have been limited by a narrow potential window (∼1.23 V), beyond which the hydrogen and oxygen evolution reactions occur. Here we report the formation of layered Mn5O8 pseudocapacitor electrode material with a well-ordered hydroxylated interphase. A symmetric full cell using such electrodes demonstrates a stable potential window of 3.0 V in an aqueous electrolyte, as well as high energy and power performance, nearly 100% coulombic efficiency and 85% energy efficiency after 25,000 charge–discharge cycles. The interplay between hydroxylated interphase on the surface and the unique bivalence structure of Mn5O8 suppresses the gas evolution reactions, offers a two-electron charge transfer via Mn2+/Mn4+ redox couple, and provides facile pathway for Na-ion transport via intra-/inter-layer defects of Mn5O8., Rechargeable aqueous electrochemical energy storage is a promising technology but suffers from a narrow potential window. Here the authors report a surface hydroxylated Mn5O8 pseudocapacitor electrode with bivalence structure that expands the potential window to deliver high energy and power performance.

There are some interesting discrepancies between what has been observed experimentally and the predictions of cluster theory. In particular, the F 1s x-ray absorption spectroscopy (XAS) of uranium tetrafluoride and the U 4d XAS branching ratio for oxidized uranium are two cases in point. These discrepancies may be driven by the overestimation of covalency in cluster calculations.

© 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. Materials can be transformed from one crystalline phase to another by using an electric field to control ion transfer, in a process that can be harnessed in applications such as batteries, smart windows and fuel cells. Increasing the number of transferrable ion species and of accessible crystalline phases could in principle greatly enrich material functionality. However, studies have so far focused mainly on the evolution and control of single ionic species (for example, oxygen, hydrogen or lithium ions). Here we describe the reversible and non-volatile electric-field control of dual-ion (oxygen and hydrogen) phase transformations, with associated electrochromic and magnetoelectric effects. We show that controlling the insertion and extraction of oxygen and hydrogen ions independently of each other can direct reversible phase transformations among three different material phases: the perovskite SrCoO 3 'I' (ref. 12), the brownmillerite SrCoO 2.5 (ref. 13), and a hitherto-unexplored phase, HSrCoO 2.5. By analysing the distinct optical absorption properties of these phases, we demonstrate selective manipulation of spectral transparency in the visible-light and infrared regions, revealing a dual-band electrochromic effect that could see application in smart windows. Moreover, the starkly different magnetic and electric properties of the three phases - HSrCoO 2.5 is a weakly ferromagnetic insulator, SrCoO 3 'I' is a ferromagnetic metal, and SrCoO 2.5 is an antiferromagnetic insulator - enable an unusual form of magnetoelectric coupling, allowing electric-field control of three different magnetic ground states. These findings open up opportunities for the electric-field control of multistate phase transformations with rich functionalities.

We present a picosecond time-resolved X-ray absorption spectroscopy (tr-XAS) setup designed for synchrotron-based studies of interfacial photochemical dynamics. The apparatus combines a high power, variable repetition rate picosecond laser system with a time-resolved X-ray fluorescence yield detection technique. Time-tagging of the detected fluorescence signals enables the parallel acquisition of X-ray absorption spectra at a variety of pump–probe delays employing the well-defined time structure of the X-ray pulse trains. The viability of the setup is demonstrated by resolving dynamic changes in the fine structure near the O1s X-ray absorption edge of cuprous oxide (Cu2O) after photo-excitation with a 355 nm laser pulse. Two distinct responses are detected. A pronounced, quasi-static, reversible change of the Cu2O O1s X-ray absorption spectrum by up to ∼30% compared to its static line shape corresponds to a redshift of the absorption edge by ∼1 eV. This value is small compared to the 2.2 eV band gap of Cu2O but in agreement with previously published results. The lifetime of this effect exceeds the laser pulse-to-pulse period of 8 μs, resulting in a quasi-static spectral change that persists as long as the sample is exposed to the laser light, and completely vanishes once the laser is blocked. Additionally, a short-lived response corresponding to a laser-induced shift of the main absorption line by ∼2 eV to lower energies appears within

Oxygen is known to play an important role in the multiferroicity of rare earth manganites; however, how this role changes with rare earth elements is still not fully understood. To address this question, we have used resonant soft x-ray scattering spectroscopy to study the F-type (0,τ,0) diffraction peak from the antiferromagnetic order in DyMnO3 and TbMnO3. We focus on the measurements at O K edge of these two manganites, supplemented by the results at Mn L2 and Dy M5 edge of DyMnO3. We show that the electronic states of different elements are coupled more strongly in DyMnO3 than in TbMnO3, presumably due to the stronger lattice distortion and the tendency to develop E-type antiferromagnetism in the ferroelectric state that promote the orbital hybridization. We also show that the anomaly in the correlation length of (0,τ,0) peak in DyMnO3 signifies the exchange interaction between Mn and rare earth spins. Our findings reveal the prominent role of oxygen orbitals in the multiferroicity of rare earth manganites and the distinct energetics between them.

The atomic-scale origin of the unusually high performance and long-term stability of wurtzite p-GaN oriented nanowire arrays is revealed. Nitrogen termination of both the polar (0001¯) top face and the nonpolar (101¯0) side faces of the nanowires is essential for long-term stability and high efficiency. Such a distinct atomic configuration ensures not only stability against (photo) oxidation in air and in water/electrolyte but, as importantly, also provides the necessary overall reverse crystal polarization needed for efficient hole extraction in p-GaN.

High quality single-crystal fcc-Cox(MgyZn1-y)1-xO1-v epitaxial thin films with high Co concentration up to x = 0.5 have been fabricated by molecular beam epitaxy. Systematic magnetic property characterization and soft X-ray absorption spectroscopy analysis indicate that the coexistence of ferromagnetic regions, superparamagnetic clusters, and non-magnetic boundaries in the as-prepared Cox(MgyZn1-y)1-xO1-v films is a consequence of the intrinsic inhomogeneous distribution of oxygen vacancies. Furthermore, the relative strength of multiple phases could be modulated by controlling the oxygen partial pressure during sample preparation. Armed with both controllable magnetic properties and tunable band-gap, Cox(MgyZn1-y)1-xO1-v films may have promising applications in future spintronics.

Prussian Blue analogues (PBAs) have shown promise as electrode materials for grid-scale batteries because of their high cycle life and rapid kinetics in aqueous-based electrolytes. However, these materials suffer from relatively low specific capacity, which may limit their practical applications. Here, we investigate strategies to improve the specific capacity of these materials while maintaining their cycling stability and elucidate mechanisms that enhance their electrochemical properties. In particular, we have studied the electrochemical and structural properties of manganese hexacyanoferrate (MnHCFe) and cobalt hexacyanoferrate (CoHCFe) in an aqueous, sodium-ion electrolyte. We also studied manganese–cobalt hexacyanoferrate (Mn–CoHCFe) solid solutions with different Mn/Co ratios that combine properties of both MnHCFe and CoHCFe. The materials have the characteristic open-framework crystal structure of PBAs, and their specific capacities can be significantly improved by electrochemically cycling (oxidizing and reducing) both the carbon-coordinated Fe and the nitrogen-coordinated Co or Mn ions. In situ synchrotron X-ray diffraction studies and ex situ soft X-ray absorption spectroscopy combined with an in-depth electrochemical characterization provide insight into the different electrochemical properties associated with the Fe, Co, and Mn redox couples. We show that cycling the C-coordinated Fe preserves the crystal structure and enables the outstanding kinetics and cycle life previously displayed by PBAs in aqueous electrolytes. On the other hand, the N-coordinated Co and Mn ions exhibit a slower kinetic regime due to structural distortions resulting from the weak N-coordinated crystal field, but they still contribute significantly towards increasing the specific capacity of the materials. These results provide the understanding needed to drive future development of PBAs for grid-scale applications that require extremely high cycle life and kinetics.

X-ray absorption spectroscopy (XAS) is combined with density functional theory (DFT) to determine the orbitals of one- and two-dimensional carbon π-systems (lycopene, beta-carotene, retinal, retinol, retinoic acid, coronene, triphenylene). Considerable fine structure is observed for the transition from the C 1s level to the lowest unoccupied molecular orbital (LUMO) and explained by DFT. The wave functions of the one-dimensional chain molecules display the node structure of a vibrating string. The XAS transition energy is decomposed into contributions from the C 1s core level, the π* final state, and the electron–hole interaction. For the latter, we develop a simple model that accurately represents a full Δ-self-consistent field (ΔSCF) calculation. The distortion of the LUMO because of its interaction with the C 1s hole is investigated. These results illustrate the electronic states of prototypical π-bonded carbon structures with low-dimensional character, such as those used in molecular complexes for solar cells, confined graphene structures, and molecular wires., This research was supported by the NSF through the University of Wisconsin Materials Research Science and Engineering Center (DMR-1121288). We also acknowledge support by the DOE (BES) for the synchrotron radiation experiments at the Advanced Light Source (ALS) under the Contracts DE-AC02-05CH11231 (ALS) and DE-SC0006931 (endstation). J.M.G.L. acknowledges support from the Spanish Ministry of Economy and Competitiveness under Projects FIS2012-30996 and FIS2013-46159-C3-1-P, Grupos Consolidados (IT578-13), and COST Action MP1306 (EUSpec).

This work follows upon the recent demonstration of the photocatalytic conversion of N2 to NH3 in aqueous solutions via H-terminated, polycrystalline diamond electrodes. X-ray absorption spectroscopy serves as an atom- and bond-specific probe of the local bonding of oxygen and nitrogen species at the surface of the electrodes. Ozone treatment of the electrodes creates a sharp C1s transition to an unoccupied surface state at 2.5 eV below the onset of the transitions to the bulk conduction band. A similar transition occurs at 2.6 eV below the onset after ammonia treatment. It is surprising to obtain such a well-defined surface state on complex, real-life electrode materials, which suggests a characteristic local bonding configuration. An extensive comparison with spectra from reference molecules reveals keto groups (C=O) at the ozone-treated surface, with the possibility of additional tertiary alcohols (C-OH). The electron–hole interaction is taken into account for estimating the absolute energy of the surfa...

Most battery positive electrodes operate with a 3d transition-metal (TM) reaction centre. A direct and quantitative probe of the TM states upon electrochemical cycling is valuable for understanding the detailed cycling mechanism and charge diffusion in the electrodes, which is related with many practical parameters of a battery. This review includes a comprehensive summary of our recent demonstrations of five different types of quantitative analysis of the TM states in battery electrodes based on soft x-ray absorption spectroscopy and multiplet calculations. In LiFePO4, a system of a well-known two-phase transformation type, the TM redox could be strictly determined through a simple linear combination of the two end-members. In Mn-based compounds, the Mn states could also be quantitatively evaluated, but a set of reference spectra with all the three possible Mn valences needs to be deliberately selected and considered in the fitting. Although the fluorescence signals suffer the self-absorption distortion, the multiplet calculations could consider the distortion effect, which allows a quantitative determination of the overall Ni oxidation state in the bulk. With the aid of multiplet calculations, one could also achieve a quasi-quantitative analysis of the Co redox evolution in LiCoO2 based on the energy position of the spectroscopic peak. The benefit of multiplet calculations is more important for studying electrode materials with TMs of mixed spin states, as exemplified by the quantitative analysis of the mixed spin Na2−x Fe2(CN)6 system. At the end, we showcase that such quantitative analysis could provide valuable information for optimizing the electrochemical performance of Na0.44MnO2 electrodes for Na-ion batteries. The methodology summarized in this review could be extended to other energy application systems with TM redox centre for detailed analysis, for example, fuel cell and catalytic materials.

The formidable challenge of developing high-performance battery system stems from the complication of battery operations, both mechanically and electronically. In the electrodes and at the electrode–electrolyte interfaces, chemical reactions take place with evolving electron states. In addition to the extensive studies of material synthesis, electrochemical, structural, and mechanical properties, soft x-ray spectroscopy provides unique opportunities for revealing the critical electron states in batteries. This review discusses some of the recent soft x-ray spectroscopic results on battery binder, transition-metal based positive electrodes, and the solid-electrolyte-interphase. By virtue of soft x-ray's sensitivity to electron states, the electronic property, the redox during electrochemical operations, and the chemical species of the interphases could be fingerprinted by soft x-ray spectroscopy. Understanding and innovating battery technologies need a multimodal approach, and soft x-ray spectroscopy is one of the incisive tools to probe the chemical and physical evolutions in batteries.

International audience; Soft X-ray absorption spectroscopy with different probe depth wasemployed to characterize the solid electrolyte interphases (SEIs) formedon β-Sn single crystals with two different surface orientations. Based oncomparative studies of C- K , O- K , and F- K absorption spectra between theSEIs and reference samples, SEI on Sn (100) mainly consists of porousLi 2 CO 3 species with electrolyte uptake, while SEI on Sn (001) essentiallyconsists of LiF and organic molecules, with a small amount of –CO 3 andelectrolyte buried inside. Theoretical calculation suggests that Sn (001)surface is more reactive than (100), especially after air exposure. The reactive(001) surface facilitates the decomposition of LiPF 6 to form a LiF layer.In contrast, SEI on (100) surface is predominately from the typical decompositionof carbonate-based electrolyte. While the LiF passivates Sn (001)electrode after one cycle, the porous carbonate layer on (100) surface doesnot prevent further decomposition of electrolyte after many cycles. Thisleads to drastically different electrochemical behavior and morphology ofthe two SEIs. The result is a direct proof that surface properties of activematerials could strongly impact the SEI formation on electrodes even withthe same electrolyte. Such effect could lead to distinct SEI formation andelectrochemical performance.