Sensitive X-ray Absorption Spectroscopy Research Articles

Detailed study of reactively sputtered ScN thin films at room temperature

To contemplate an alternative approach for the minimization of diffusion at high temperature, present findings impart viability of room-temperature deposited reactively sputtered ScN thin films. The adopted route endows precise control over the RN2 flow for a methodical structural phase evolution from Sc→ScN and probe the correlated physical aspects of the highly textured ScN samples. In the nitrided regime i.e. at RN2 = 2.5–100% flow, incorporation of unintentional oxygen defects was evidenced from surface sensitive soft x-ray absorption spectroscopy study, though less compared to their metal (RN2 = 0%) and interstitial (RN2 = 1.6%) counterparts, due to higher Gibbs free energy for Sc-O-N formation with no trace of ligand field splitting around the O K-edge spectra. To eradicate the skepticism of appearance of N K-edge (401.6 eV) and Sc L-edge (402.2 eV) absorption spectra adjacent to each other, the first-ever Sc K-edge study has been adopted to validate complementary insight on the metrical parameters of the Sc-N system. Optical bandgaps of the polycrystalline ScN thin film samples were found to vary between 2.25 and 2.62 eV as obtained from the UV–vis spectroscopy, whereas, the nano-indentation hardness and modulus of the as-deposited samples lie between 15–34 GPa and 152–476 GPa, respectively following a linearly increasing trend of resistance to plastic deformations with an exception at 34 GPa in case of RN2 = 5% sample. Besides, contrary to other early 3d transition metal nitrides (TiN, VN, CrN), a comprehensive comparison of noticeably large homogeneity range in Sc-N has been outlined to apprehend the minuscule lattice expansion over the large RN2 realm.

Interfacial Degradation in NMC811-Graphite Batteries during Extended Cycling

To improve the energy density in Li-ion batteries, development of cathode materials with higher capacities are required, where much of the current focus is on Ni-rich lithium nickel manganese oxides (NMC). However, higher Nickel content is typically accompanied by accelerated degradation and thus poor cycle life, with undesired reactions at the electrode-electrolyte interfaces strongly implicated. There have been numerous attempts to mitigate this, using various methods including surface coatings, structural doping and advanced synthesis techniques such as core-shell or concentration gradient particles with much lower nickel concentrations at their surface. Designing improved solutions, requires a deeper mechanistic understanding of the underlying interfacial processes involved in degradation and their relative contributions. In this study these reactions are investigated in LiNi0.8Mn0.1Co0.1O2–graphite cells cycled using LP57 electrolyte (1 M LiPF6 in EC:EMC, 3:7) for different number of cycles, providing information about how interfacial degradation proceeds. Electrochemical measurements are combined with post mortem techniques including surface sensitive soft X-ray absorption spectroscopy, and photoelectron spectroscopy measured with different excitation energies. Based on fitted differential voltage analysis (DVA) data from the cells it is shown how the initial capacity fading during the first 300 cycles is almost fully attributable to excessive electrolyte reduction exhibited as SEI thickening, whereas further cycling leads to a larger contribution to capacity fading from loss of active NMC material. The initial capacity fading due to electrolyte reduction can be correlated with more plating of manganese, as compared to nickel, onto the graphite electrode, see figure. However, after 600 cycles both nickel and manganese are found in similar concentrations within the SEI. The changing ratio between the amount of plated nickel and manganese with cycle number indicates that manganese on the NMC surface initially dissolves more easily than the nickel, but as the unstable manganese ions are dissolved more and more nickel will contribute to the transition metal dissolution. The nickel ions in the SEI are also found to change their chemical surrounding as the cycling proceeded, showing that they do not form stable species and are thus likely to contribute to continued breakdown of the SEI. We compare the observed evolution of the SEI that results from cross talk between the NMC and graphite electrodes, to the case where EC-free electrolytes are used and graphite is replaced with Li4Ti5O12 (LTO) such that contributions from electrolyte reduction at the anode are expected to be minimised [1,2].In addition to the side reactions at the graphite other important side reactions take place on the NMC, some of which are potentially initiated by the graphite electrode [3]. For instance it was found that the Li2CO3 impurities present on the NMC surface decompose throughout the cells’ cycle life rather than just during the initial cycles. Previous studies have shown how Li2CO3 coatings severely decrease the cycle life, and therefore the observations here show that decomposition related to Li2CO3 is a problem that does not disappear during the formation cycling. Instead minimizing the initial Li2CO3 formation is critical. The insights obtained here on the evolution of surface layers coupled with the individual electrochemical degradation of the electrodes will advance the understanding of the Ni-rich NMC’s capacity retention behaviour and inform future efforts to achieve longer cycle lifetimes.[1] Björklund, E., Göttlinger, M., Edström, K., Younesi, R. & Brandell, D. Sulfolane‐Based Ethylene Carbonate‐Free Electrolytes for LiNi0.6Mn0.2Co0.2O2−Li4Ti5O12 Batteries. Batter. Supercaps 3, 201– 207 (2020).[2] Björklund, E., Göttlinger, M., Edström, K., Brandell, D. & Younesi, R. Investigation of Dimethyl Carbonate and Propylene Carbonate Mixtures for LiNi0.6 Mn0.2Co0.2O2-Li4 Ti5 O12 Cells. ChemElectroChem 6, 3429–3436 (2019).[3] Björklund, E., Brandell, D., Hahlin, M., Edström, K. & Younesi, R. How the Negative Electrode Influences Interfacial and Electrochemical Properties of LiNi1/3Co1/3Mn1/3O2 Cathodes in Li-Ion Batteries. J. Electrochem. Soc. 164, A3054–A3059 (2017). Figure 1

Valence tautomeric transition of bis(o-dioxolene) cobalt complex in solid state and solution

Valence tautomer transition occurs mainly in 3d metalorganic complexes with redox-active ligands and makes them potential candidates for single-molecular switches. The transition occurs under temperature, pressure, or light-induced stimuli and is strongly affected by the intermolecular interactions. However single-crystal x-ray diffraction is not always applicable to such systems when crystal structure is destroyed upon transition or system is studied in the solution. Such an example is bis(o-semiquinonato) cobalt complex with TEMPO-functionalized iminopyridine ancillary ligand. In this work we apply two complementary techniques—ligand-sensitive Fourier transform infrared spectroscopy (FTIR) and metal sensitive Co K-edge x-ray absorption spectroscopy (XAS). In a solid state, a temperature hysteresis of magnetization larger than 40 K was observed upon cyclic cooling-heating. So, the temperature of phase transition upon cooling is about 40 K lower than that upon heating. In solution, the x-ray absorption spectra for high-temperature and low-temperature states were similar to that in the solid form, but the hysteresis was absent. Two methods are can probe valence tautomer transition, but XAS has an advantage for the liquid phase analysis and FTIR has larger sensitivity to the ligand related interactions in solid.

Search for enhanced magnetism at the interface between Bi2Se3 and EuSe

Enhanced magnetism has recently been reported for the topological-insulator/ferromagnet interface ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$/EuS with Curie temperatures claimed to be raised above room temperature from the bulk EuS value of 16 K. Here we investigate the analogous interface ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$/EuSe. EuSe is a low-temperature layered ferrimagnet that is particularly sensitive to external perturbations. We find that superconducting quantum interference device (SQUID) magnetometry of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$/EuSe heterostructures reveals precisely the magnetic phase diagram known from EuSe, including the ferrimagnetic phase below 5 K, without any apparent changes from the bulk behavior. Choosing a temperature of 10 K to search for magnetic enhancement, we determine an upper limit for a possible magnetic coercive field of 3 mT. Using interface sensitive x-ray absorption spectroscopy we verify the magnetic divalent configuration of the Eu at the interface without contamination by ${\mathrm{Eu}}^{3+}$, and by x-ray magnetic circular dichroism (XMCD) we confirm at the interface the magnetic hysteresis obtained by SQUID. XMCD data obtained at 10 K in a magnetic field of 6 T indicate a magnetic spin moment of ${m}_{z,\mathrm{spin}}=7\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_{B}/{\mathrm{Eu}}^{2+}$, in good agreement with the SQUID data and the expected theoretical moment of ${\mathrm{Eu}}^{2+}$. Subsequent XMCD measurements in zero field show, however, that sizable remanent magnetization is absent at the interface for temperatures down to about 10 K.

Detailing the Self-Discharge of a Cathode Based on a Prussian Blue Analogue

Prussian Blue analogues (PBAs) are a promising class of electrode active materials for batteries. Among them, copper nitroprusside, Cu[Fe(CN)5NO], has recently been investigated for its peculiar redox system, which also involves the nitrosyl ligand as a non-innocent ligand, in addition to the electroactivity of the metal sites, Cu and Fe. This paper studies the dynamics of the electrode, employing surface sensitive X-ray Photoelectron spectroscopy (XPS) and bulk sensitive X-ray absorption spectroscopy (XAS) techniques. XPS provided chemical information on the layers formed on electrode surfaces following the self-discharge process of the cathode material in the presence of the electrolyte. These layers consist mainly of electrolyte degradation products, such as LiF, LixPOyFz and LixPFy. Moreover, as evidenced by XAS and XPS, reduction at both metal sites takes place in the bulk and in the surface of the material, clearly evidencing that a self-discharge process is occurring. We observed faster processes and higher amounts of reduced species and decomposition products in the case of samples with a higher amount of coordination water.

Defects and oxygen vacancies tailored structural, optical and electronic structure properties of Co-doped ZnO nanoparticle samples probed using soft X-ray absorption spectroscopy

The structural, optical and electronic structure properties of all the Zn1-xCoxO nanoparticles [x = 0.01, 0.03, and 0.05], synthesized using co-precipitation method have been studied using X-ray diffraction (XRD), Surface Enhanced Raman Spectroscopy (SERS), UV–Vis–NIR spectroscopy and X-ray Absorption Spectroscopy measurements. The XRD and SERS analysis have confirmed the formation of hexagonal wurtzite structure (space group C46v). UV–Vis–NIR spectra indicate the blue shifting of the absorbance edge and the band gap is found to decrease with doping. The electronic structure of undoped and Co-doped ZnO nanoparticles have been investigated using site selective and element sensitive X-ray absorption spectroscopy. From the XAS spectra at Zn L3,2, Co L3,2 and O K-edges we have found that Zn ions are in +2 valence state in undoped and Co-doped ZnO nanoparticles. From the XAS analysis it can be seen that proper incorporation of Co-ion in the host lattice take place in t2geg state Co+2 (d7) under tetrahedral symmetry and maintain the crystal symmetry with lattice distortion. The Co-ion doping in the ZnO nano-matrix leads to creation of various defects & oxygen vacancies, which results in the blue shift in band-gap energy makes them useful for various applications e.g. photocatalytic water splitting, hydrogen generation, spintronics and optoelectronics.

Quantitative evaluation of transient valence orbital occupations in a 3d transition metal complex as seen from the metal and ligand perspective

It is demonstrated for the case of photo-excited ferrocyanide how time-resolved soft X-ray absorption spectroscopy in transmission geometry at the ligand K-edge and metal L3-edge provides quantitatively equivalent valence electronic structure information, where signatures of photo-oxidation are assessed locally at the metal as well as the ligand. This allows for a direct and independent quantification of the number of photo-oxidized molecules at two soft X-ray absorption edges highlighting the sensitivity of X-ray absorption spectroscopy to the valence orbital occupation of 3d transition metal complexes throughout the soft X-ray range.

Epitaxial stabilization of ultra thin films of high entropy perovskite

High entropy oxides (HEOs) are a class of materials, containing equimolar portions of five or more transition metal and/or rare-earth elements. We report here about the layer-by-layer growth of HEO [(La0.2Pr0.2Nd0.2Sm0.2Eu0.2)NiO3] thin films on NdGaO3 substrates by pulsed laser deposition. The combined characterizations with in situ reflection high energy electron diffraction, atomic force microscopy, and x-ray diffraction affirm the single crystalline nature of the film with smooth surface morphology. The desired +3 oxidation of Ni has been confirmed by an element sensitive x-ray absorption spectroscopy measurement. Temperature dependent electrical transport measurements revealed a first order metal-insulator transition with the transition temperature very similar to the undoped NdNiO3. Since both these systems have a comparable tolerance factor, this work demonstrates that the electronic behaviors of A-site disordered perovskite-HEOs are primarily controlled by the average tolerance factor.

Picosecond timescale tracking of pentacene triplet excitons with chemical sensitivity

Singlet fission is a photophysical process in which an optically excited singlet exciton is converted into two triplet excitons. Singlet fission sensitized solar cells are expected to display a greatly enhanced power conversion efficiency compared to conventional single-junction cells, but the efficient design of such devices relies on the selection of materials capable of harvesting triplets generated in the fission chromophore. To this aim, the possibility of measuring triplet exciton dynamics with chemical selectivity paves the way for the rational design of complex heterojunctions, with optimized triplet conversion. Here we exploit the chemical sensitivity of X-ray absorption spectroscopy to track triplet exciton dynamics at the picosecond timescale in multilayer films of pentacene, the archetypal singlet fission material. We experimentally identify the signature of the triplet exciton in the Carbon K-edge absorption spectrum and measure its lifetime of about 300 ps. Our results are supported by state-of-the-art ab initio calculations.

Water-Processable P2-Na0.67Ni0.22Cu0.11Mn0.56Ti0.11O2 Cathode Material for Sodium Ion Batteries

Sodium ion batteries offer a low-cost, sustainable, and environment-friendly solution to large-scale electrochemical energy storage systems. Layered oxides represent a family of promising cathode materials with a potential to improve the energy and power densities while reducing the material cost of sodium ion batteries. However, due to the chemical and structural instability of layered oxides in an aqueous solution, the current battery electrode manufacturing requires expensive and hazardous organic solvents, which impedes the full benefit of the low-cost, sustainable, and eco-friendly advantages. We need an effective technology that empowers a cathode with water processable properties. In this study, we set a representative example, P2-Na0.67Ni0.22Cu0.11Mn0.56Ti0.11O2, to explore its performance under water-processing conditions. This material achieves a discharge capacity of 180 mAh/g and a discharge energy of 544 Wh/kg at 22°C. The aging experiments indicate its superior stability against water, having negligible bulk structural or chemical changes. The surface sensitive soft X-ray absorption spectroscopy shows that the P2-Na0.67Ni0.22Cu0.11Mn0.56Ti0.11O2 has stable surface chemistry in the aqueous solution. Moreover, the cells with water-processed cathodes delivered stable cycling performance with minor voltage decay, originating from the decreased cell impedance. Therefore, the present study sets a refined example to establish a low-cost, sustainable, and eco-friendly solution by developing water-processable electrode materials for sodium ion batteries.

Electrostatic potential and valence modulation in La0.7Sr0.3MnO3 thin films

The Mn valence in thin film La0.7Sr0.3MnO3 was studied as a function of film thickness in the range of 1–16 unit cells with a combination of non-destructive bulk and surface sensitive X-ray absorption spectroscopy techniques. Using a layer-by-layer valence model, it was found that while the bulk averaged valence hovers around its expected value of 3.3, a significant deviation occurs within several unit cells of the surface and interface. These results were supported by first principles calculations. The surface valence increases to up to Mn3.7+, whereas the interface valence reduces down to Mn2.5+. The change in valence from the expected bulk value is consistent with charge redistribution due to the polar discontinuity at the film-substrate interface. The comparison with theory employed here illustrates how this layer-by-layer valence evolves with film thickness and allows for a deeper understanding of the microscopic mechanisms at play in this effect. These results offer insight on how the two-dimensional electron gas is created in thin film oxide alloys and how the magnetic ordering is reduced with dimensionality.

The Mechanism of Impedance Increase in Nickel-Rich NMC Cathodes

The electrochemical performance of nickel-rich NMC cathode materials in Li-ion batteries depends on the kinetic control of side reactions at NMC-electrolyte/interface such as electrolyte oxidation, surface film formation, surface structure transformation and transition metal dissolution. However, the exact mechanism i.e. interdependence, sequence and impact of these interfacial phenomena is still poorly understood. The effect of surface reconstruction from layered- to spinel- or rock salt-phase(1, 2) in NMC materials during cycling on the kinetics of charge/discharge process remains unclear. As much as the exact chemical and physical properties of such surface reconstruction layer are still largely unknown, it is likely that it occurs simultaneously with other surface reactions and phenomena, which all together are difficult to deconvolute and evaluate. To understand the mechanism of the surface reconstruction layer formation in Ni-rich NMC material during battery storage and its possible effect on the electrode and cell impedance a surface modification was deliberately induced in NMC particles artificially a priori via exposure to the electrolyte at elevated temperatures. The reaction of the electrolyte with NMC powders and the phase reconstruction of NMC surface were probed by surface sensitive techniques, including Raman, IR, SEM and X-ray absorption spectroscopy in total electron yield mode (XAS-TEY). Accompanying electrochemical testing including electrochemical impedance spectroscopy (EIS) was performed with half-cell configuration coin cells. Through comparison of the surface chemical properties and electrochemical performance between pristine NMC and treated samples, we were able to demonstrate that this artificial reconstruction layer on NMC does not cause a critical resistance in the battery. The treatment was shown to demonstrate some positive effects on the performance: i) higher capacity retention and ii)less resistance growth during electrochemical cycling. Surface chemical analysis shows that treated electrodes had reduced amount of organic film formation and less subsequent growth of reconstruction layers during electrochemical cycling compared with pristine NMC, indicating that an artificial reconstruction layer might suppress other degradation process of the battery, such as film formation and metal dissolution. Acknowledgement This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Reference 1. F. Lin, I. M. Markus, D. Nordlund, T. C. Weng, M. D. Asta, H. L. L. Xin and M. M. Doeff, Nat Commun, 5(2014). 2. S. K. Jung, H. Gwon, J. Hong, K. Y. Park, D. H. Seo, H. Kim, J. Hyun, W. Yang and K. Kang, Adv Energy Mater, 4 (2014).

Evolution of the Cathode-Electrolyte Interface of Li1 - xNi0.8Co0.15Al0.05O2 Electrodes Due to Electrochemical and Thermal Stress

The electrode-electrolyte interfaces (EEIs) play important roles in lithium ion batteries, often determining the capacity retention, safety, and lifetime. The EEI of the positive electrode is far less understood than its negative counterpart. In addition to possible electrolyte oxidation products, layered cathodes often exhibit pronounced surface phase transformations that can also limit the performance of the battery. For example, recent experiments have correlated charge-transfer impedance growth with surface phase transformations at Li1-xNi0.80Co0.15Al0.05O2(NCA) surfaces, which result from surface oxygen loss. [1] Electrolyte reactions with the active material play a role in surface oxygen loss as coating layers [2] and electrolyte additives [3] can mitigate impedance growth and capacity fade. Determining reaction pathways that promote surface oxygen loss is essential to optimizing the selection of coating layers or additives that further improve capacity retention. Here we report on our surface studies of NCA, to examine the evolution of the cathode-electrolyte interface (CEI) for stressed NCA electrodes held at high voltages (≥4.5V). Using a combination of surface sensitive x-ray absorption spectroscopy and x-ray photoelectron spectroscopy, we correlate transition metal reduction with electrolyte decomposition species at the cathode surface. From comparison of NCA electrodes held under various constant voltage conditions, we examine the influence of voltage and temperature on the formation of electrolyte decomposition species. Our work provides insight into possible reaction mechanisms that influence changes of the CEI layer for electrodes under thermal and electrochemical stress. This work was supported as part of NECCES, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0012583 [1] S. Sallis et al., App. Phys. Lett. 108 (2016) 263902 [2] I. D. Scott et al., Nano Letts. 11 (2011) 414 [3] R. Wagner et al., Appl. Mater. Interfaces. (2016) 30871 Figure 1

Single-Crystal Based Diagnostics for Li-Ion Battery Cathode Development

Lithium transition-metal oxides (Li-TM oxides) are presently the most promising high energy density cathode materials for rechargeable lithium batteries. When operated at high voltages (>4.3 V), however, these oxides suffer from structural instability, extensive side reactions with the electrolyte, poor cyclability and severe thermal runaway reactions. [1, 2] In order to develop successful strategies addressing these issues, there is a clear need in fundamental knowledge of the relationships between the material’s specific physical properties/reaction mechanisms and its reactivity. This is difficult to obtain on conventional aggregated secondary particles because isolating specific physical properties of interest from less controlled ones, such as particle porosity, grain boundaries, primary particle size and size distribution, is often challenging if not impossible. To this end, we developed a unique diagnostic approach combining carefully prepared cathode model samples and the start-of-the-art analysis techniques with high spatial resolution and chemical specificity. In this presentation, we will discuss the synthesis of high-voltage Ni/Mn spinel (LMNO) and high-capacity Li-and Mn-rich layered oxide (LMR-NMC) crystals with various physical characteristics. [3] We demonstrate the effective use of complementary microscopy and spectroscopy techniques at multi-length scale, such as aberration corrected (scanning) transmission electron microscopy, electron energy loss spectroscopy, X-ray energy dispersive spectroscopy, surface sensitive soft X-ray absorption spectroscopy and X-ray photoelectron spectroscopy, full-field transmission X-ray microscopy and X-ray absorption near edge structure imaging, to determine elemental, chemical and atomic-level structural make-up of the entire bulk as well as the surface properties of the crystals. [4, 5] Combined with electrochemical analysis, insights on the phase transformation mechanism during Li extraction/reinsertion and the key factors influencing the reactions occurring at the cathode/electrolyte interface were obtained. [6] Future directions and research in addressing some of the pressing challenges facing Li-TM oxide cathode development will also be discussed.

Enhanced Rate Capability in Two Phase Reaction in LiFePO4 By Interfacial Modification Between Cathode and Electrolyte

Olivine-type LiFePO4 is considered as a promising cathode material for high power density lithium-ion battery. Lithium-ion intercalation proceeds thorough a two-phase reaction divided into LiFePO4 and FePO4. During the phase transition between LiFePO4 and FePO4, the nucleation of a new phase occurs at the electrode / electrolyte interface, which is reported to be the rate determining reaction.1 To improve the rate capability of LiFePO4, Park et al performed the surface modification by nitrogen on LiFePO4 particles.2 However, their improved mechanism is not clearly understood due to the difficulty in the observation of electrode / electrolyte interface. In order to analyze the electrode / electrolyte interface, thin-film electrodes and surface sensitive X-ray absorption spectroscopy are powerful tool.3 In this study, we prepared LiFePO4thin-film model electrodes to clarify the reaction kinetics and applied the interfacial modification of nitrogen. Their reaction mechanism is discussed with the information of X-ray absorption spectroscopy. LiFePO4 thin-film was prepared on a polycrystalline Au substrate by pulsed laser deposition method. The deposition was conducted under Ar atmosphere (0.0001 Pa) at 923K for 30 min. The prepared thin films were annealed under ammonia gas at 873 K. The obtained thin-film was characterized by X-ray diffraction and TEM. X-ray absorption spectroscopy at Co K-edge was performed at BL37XU, Spring-8. N K-edge X-ray absorption measurements were performed in Ritsumeikan SR-center. For the electrochemical analysis, three-electrode cells employing Li metal as a counter and a reference electrode, and 1 M LiClO4in propylene carbonate liquid electrolyte were prepared. Annealing of ammonia gas introduces nitrogen into LiFePO4 surface, which is confirmed by N K-edge X-ray absorption spectra. The rate capability of the nitrogen modified LiFePO4 thin-film electrodes is improved. The surface sensitive Fe K-edge X-ray absorption spectra show the increasing of oxidation state in iron, which implies the lattice mismatch is decreased in surface of LiFePO4. Therefore, nucleation reaction is enhanced during two phase reaction between LiFePO4 and FePO4, which is the origin of surface modification effect.

Thickness dependence of La0.7Sr0.3MnO3/PbZr0.2Ti0.8O3 magnetoelectric interfaces

Magnetoelectric materials have great potential to revolutionize electronic devices due to the coupling of their electric and magnetic properties. Thickness varying La0.7Sr0.3MnO3 (LSMO)/PbZr0.2Ti0.8O3 (PZT) heterostructures were built and measured in this article by valence sensitive x-ray absorption spectroscopy. The sizing effects of the heterostructures on the LSMO/PZT magnetoelectric interfaces were investigated through the behavior of Mn valence, a property associated with the LSMO magnetization. We found that Mn valence increases with both LSMO and PZT thickness. Piezoresponse force microscopy revealed a transition from monodomain to polydomain structure along the PZT thickness gradient. The ferroelectric surface charge may change with domain structure and its effects on Mn valence were simulated using a two-orbital double-exchange model. The screening of ferroelectric surface charge increases the electron charges in the interface region, and greatly changes the interfacial Mn valence, which likely plays a leading role in the interfacial magnetoelectric coupling. The LSMO thickness dependence was examined through the combination of two detection modes with drastically different attenuation depths. The different length scales of these techniques' sensitivity to the atomic valence were used to estimate the depth dependence Mn valence. A smaller interfacial Mn valence than the bulk was found by globally fitting the experimental results.

Bulk Structure and Surface Properties of Lithium- and Manganese-Rich Layered Oxides and Their Impact on Electrochemical Performance

Lithium- and manganese-rich layered oxides, Li1+x M1-x O2 (M=Mn, Ni, Co), are promising cathode materials for lithium-ion batteries due to their high capacities, typically more than 250 mAh/g. Despite the intense research and development in the past decade, their commercial adoption remains hindered by severe shortcomings including a large first-cycle irreversible capacity loss, [1] voltage and capacity fade, [2] DC resistance rise at low state of charge (SOC) and transition metal dissolution. [3] Studies have attributed these issues to the structural changes occurring during first-cycle activation and prolonged cycling, yet the crystal structure of the pristine oxides is still a matter of debate. Fundamental studies on the conventionally synthesized, aggregated secondary particles often lead to ambiguous results due to the structural complexity in this class of materials. In this presentation, we report the synthesis of well-formed, discrete layered Li1+x M1-x O2 crystals with various morphologies and surface properties using a molten salt method. Complementary microscopy and spectroscopy techniques at multi-length scale, including aberration corrected (scanning) transmission electron microscopy, electron energy loss spectroscopy and X-ray energy dispersive spectroscopy, were used to reveal the structural make-up of the bulk as well as the surface of the primary particles. Surface sensitive soft X-ray absorption spectroscopy was further used to reveal the roles of key surface properties, particularly surface composition, surface area and surface crystalline orientation, in the material challenges facing Li1+x M1-x O2 cathodes. Future directions and research to enable this class of materials will also be discussed.

Monitoring the Activation of a Flexible Metal–Organic Framework Using Structurally Sensitive Spectroscopy Techniques

A flexible metal–organic framework (MOF) comprised of Co(II) ions and 4,4′-oxybis(benzoic acid) linkers undergoes notable structure rearrangements upon removal of the metal-coordinated water molecules through activation. Two complementary structurally sensitive methods, X-ray absorption and Raman spectroscopy, are used to build a composite picture of the MOF activation process by revealing both the substantial local structural changes about the Co sites and the more subtle rearrangements of the organic linkers that are not apparent through standard powder X-ray diffraction studies.

High potential durability of LiNi0.5Mn1.5O4 electrodes studied by surface sensitive X-ray absorption spectroscopy

Phenomena at electrode/electrolyte interface of LiNi0.5Mn1.5O4 are studied by in situ total-reflection fluorescence X-ray absorption spectroscopy (TRF-XAS), ex situ X-ray photoelectron spectroscopy (XPS), and electrochemical tests. Flat and well-defined thin films of LiNi0.5Mn1.5O4 prepared by pulsed laser deposition (PLD) are used as model electrodes to facilitate the observation of the interface. The thin-film LiNi0.5Mn1.5O4 electrode showed good cycling characteristics at around 4.7 V vs. Li/Li+. The TRF-XAS measurements reveal that nickel and manganese species at the surface have almost the same chemical states and local environments as those in the bulk when in contact with organic electrolyte solutions (1 mol dm−3 LiClO4 in a 1:1 volumetric mixture of ethylene carbonate and diethyl carbonate). This is in sharp contrast to the behavior of a LiCoO2 electrode, in which the surface cobalt species is irreversibly reduced by soaking to the organic electrolyte solutions, leading to gradual material deterioration during the delithiation/lithiation cycling (D. Takamatsu et al., Angew. Chem. Int. Edit., 51 (2012) 11597). It is suggested that the electrolyte decomposition products detected by XPS form a protective layer to restrict the reduction of the surface species of LiNi0.5Mn1.5O4, leading to good cycling characteristics of LiNi0.5Mn1.5O4 in spite of its high operating potential.

The formation of Ti–O tetrahedra and band gap reduction in SiO2 via pulsed ion implantation

Titanium ions are implanted into amorphous SiO2 at two different fluences using pulsed ion implantation, and the resulting samples are annealed. Bulk sensitive soft X-ray absorption spectroscopy of the Ti L2,3 edge reveal strikingly different spectra for the two fluences. Spectral simulations using multiplet crystal field theory show clearly that for low fluence the Ti ions have a local octahedral coordination, while at higher fluence the formation of Ti4+–O tetrahedra dominates. Using O K-edge X-ray absorption and emission, the effect of the Ti states on the valence and conduction bands of the host SiO2 is revealed. With the introduction of Ti tetrahedra, the band gap reduces from about 8 eV to just over 4 eV, due entirely to the Ti 3d conduction band states. These results demonstrate the possibility to obtain Ti–O tetrahedra in silica by Ti ion implantation and a suitable thermal treatment, clarify the mechanism of band gap reduction with Ti doping in SiO2, and demonstrate the sensitivity of L-edge X-ray absorption with a multiplet crystal field theory analysis to the Ti coordination of novel materials.