X-ray Absorption Spectroscopy Results Research Articles

(Invited) Oxygen Defects in Oxide-Based Lithium Ion Battery Cathodes: Formation and Influences on Electrochemical Properties

IntroductionDemands on high energy density batteries increase to exploit renewable energies efficiently and develop zero-emission vehicles [1]. Among technological issues for next-generation batteries, we focus on the development of cathode active materials. So far, oxide-based cathode materials have been used in lithium ion batteries because of their excellent electrochemical properties. However, the formation of oxygen defects like oxygen vacancy is inevitable in oxides. There are two important motivations for the study of oxygen defect formation in oxide-based cathodes, one is the safety of batteries and the other is the battery performance. For the safety, it was reported that the reaction between organic liquid electrolytes and released oxygen from oxide-based cathodes is exothermic and cause abnormal heat generation [2]. To prevent catastrophic thermal runaway of battery systems, understanding oxygen release mechanism is important. For the influence on battery performance, it was reported that preliminary reduction treatments for Li-rich cathodes improved capacity, rate capability and capacity retention [3]. To establish the guideline for the oxygen defect engineering for advanced battery materials, it is necessary to understand influences of oxygen defects on electrochemical properties. In the presentation, oxygen vacancy formation behavior in oxide-based cathodes and influences of oxygen vacancy on battery performances are discussed.ExperimentalLi1.2Mn0.6Ni0.2O2 and Li1.2Ni0.13Co0.13Mn0.54O2 were synthesized by solid state reaction of carbonate precursors and LiOH·H2O. The oxygen vacancy formation was evaluated by thermogravimetry and the coulometric titration using yttria stabilized zirconia. The absolute oxygen content of the sample was determined by iodometric titration. Samples with different oxygen vacancy concentration were also prepared by the coulometric titration method. Influences of oxygen vacancy on crystal and electronic structures were evaluated by X-ray diffraction (XRD) and soft X-ray absorption spectroscopy (XAS). Charge/discharge measurements were performed on the pristine and the oxygen-deficient cathodes.Results and discussionFigure 1 shows the variation of the oxygen content in Li1.2Mn0.6Ni0.2O2 and Li1.2Ni0.13Co0.13Mn0.54O2. In the figure, the vertical axis, O/M, represents the molar ratio of oxygen in and metal components (Li, Ni, Co and Mn) in the target material, meaning that O/M = 1 represents the oxygen stoichiometric composition for Li1.2Mn0.6Ni0.2O2 and Li1.2Ni0.13Co0.13Mn0.54O2, and the horizontal axis represents the equilibrium P(O2) for each oxygen content. With decreasing O/M, P(O2) decreased and reached a constant value regardless of the relative oxygen content. The oxygen nonstoichiometric data in Figure 1 can be roughly classified into two regions depending on the slope of the curve. In the higher P(O2) region where the O/M vs. log P(O2) curve showed a certain slope, meaning both oxygen content and P(O2) changes simultaneously. In the lower P(O2) region, on the other hand, P(O2) is almost invariant regardless of O/M. The high P(O2) region is the oxygen deficient nonstoichiometry region where the oxygen vacancies were randomly distributed in the oxide-based cathodes. On the contrary, the low P(O2) region is the multiphase co-existing region where the sample is decomposed by the electrochemical oxygen extraction and co-exist with the decomposition phases. In this case, the equilibrium P(O2) is fixed with that of the multiphase co-existing state regardless of apparent O/M. The reduction stability of Li1.2Mn0.6Ni0.2O2 and Li1.2Ni0.13Co0.13Mn0.54O2 were also confirmed by XRD. The reduction stability limit of Li1.2Mn0.6Ni0.2O2 and Li1.2Ni0.13Co0.13Mn0.54O2 were log(P(O2)/bar) = -17.7 and -20.7, respectively. From these evaluations, it can be concluded that Li1.2Mn0.6Ni0.2O2 and Li1.2Ni0.13Co0.13Mn0.54O2 can accept oxygen vacancy up to about 2 mol% and about 3 mol%, respectively. The results of XRD, XAS and battery tests will be shown in the presentation, and influences of oxygen defects on crystal and electronic structures and electrochemical performance will be discussed.

Coordination Number Regulation of Molybdenum Single-Atom Nanozyme Peroxidase-like Specificity

Summary Nanozymes are promising alternatives to natural enzymes, but their use remains limited owing to poor specificity. Overcoming this and controlling the targeted enzyme-like performance of traditional nanozymes is extremely challenging due to the intrinsic structural complexity of these systems. We report theoretical design and experimental realization of a series of heterogeneous molybdenum single-atom nanozymes (named MoSA–Nx–C), wherein we find that the peroxidase-like specificity is well regulated by the coordination numbers of single Mo sites. The resulting MoSA–N3–C catalyst shows exclusive peroxidase-like behavior. It achieves this behavior via a homolytic pathway, whereas MoSA–N2–C and MoSA–N4–C catalysts have a different heterolytic pathway. The mechanism of this coordination-number-dependent enzymatic specificity is attributed to geometrical structure differences and orientation relationships of the frontier molecular orbitals toward these MoSA–Nx–C peroxidase mimics. This study demonstrates the rational design of peroxidase-specific nanozymes and precise regulation of their enzymatic properties.

Insight into Prolonged Cycling Life of 4 V All‐Solid‐State Polymer Batteries by a High‐Voltage Stable Binder

AbstractPolyethylene oxide (PEO) based solid polymer electrolytes (SPEs) are incompatible with the 4 V class cathodes such as LiCoO2 due to the limited electrochemical oxidation window of PEO. Herein, a number of binders including commonly used binders PEO, polyvinylidene fluoride (PVDF), and carboxyl‐rich polymer (CRP) binders such as sodium alginate (Na‐alginate) and sodium carboxymethyl cellulose, are studied for application in the 4 V class all‐solid‐state polymer batteries (ASSPBs). The results show ASSPBs with CRP binders exhibit superior cycling performance up to 1000 cycles (60% capacity retention, almost 10 times higher than those with PEO and PVDF binders). Synchrotron‐based X‐ray absorption spectroscopy (XAS), morphology studies and density functional theory studies indicate that, with their carboxyl groups, CRPs can strongly bind the electrode materials together, and work as coating materials to protect the cathode/SPE interface. Cyclic voltammetry studies indicate that CRP binders are more stable at high voltage compared to PEO and PVDF. The stability under high voltage and the coating property of CRP binders contribute to stable cathode/SPE interfaces as disclosed by the X‐ray photoelectron spectroscopy and Co L‐edge XAS results, enabling long cycling life, high performance 4 V class ASSPBs.

XAS and XPS analysis of double magnetic transition, canonical spin glass behavior and magnetoresistance in LaMn1-xCoxO3 (0.1 ≤ x ≤ 0.5) system

Polycrystalline LaMn1-xCoxO3 (0.1≤ x ≤ 0.5) samples were synthesized using conventional ceramic method. Rietveld refined X-ray diffraction pattern revealed the single-phase orthorhombic crystal structure of all the samples with the space group Pbnm. Temperature-dependent magnetic measurements performed in field cooled (FC) and zero field cooled (ZFC) mode at 102 Oe exhibit the onset of double transition in x = 0.3–0.5 compositions. The ordering temperature rises with an increase in Co concentration. FC and ZFC studies show the presence of glassy state below the ordering temperature in all samples; confirmed using a. c. susceptibility measurements. The a. c. susceptibility data are analyzed using power law and the existence of canonical spin glass is revealed. Magnetic hysteresis studies demonstrate the enhanced ferromagnetism amid the presence of unsaturated magnetization with an increase in Co doping. The presence of double transition and spin glass state is attributed to the competing ferromagnetic and anti-ferromagnetic interactions between the Co and Mn ions present in the system. The system also depicts the presence of appreciable value of magnetoresistance ~42% at 8 T magnetic field in x = 0.5 sample. These properties are interpreted through valence and spin states of Mn and Co ions, being confirmed from electronic structure studies using X-ray absorption spectroscopy (XAS) at L3,2- edges of respective ions along with O K-edge for all samples (0.1≤ x ≤ 0.5). After meticulous analysis and conjoining the results obtained from magnetization and XAS studies, it is found that cobalt is present in high spin Co2+ and high/low spin Co3+-state. Charge transfer multiplet calculation done at L3,2 edges of Mn and Co ions confirm the presence of Mn3+/Mn4+ and Co2+/Co3+ states consistent with XAS results. X-ray photoelectron spectroscopy performed at Mn2p, Co2p, and O1s -edges further substantiate the reasons behind the properties exhibited by the present system.

Development of in situ A-SAXS and XAS Measurements of Pt Catalyst under Controlled Electrochemical Condition Using Channel Flow Electrode Cell

To improve the activity and durability, in situ characterizations of Pt catalysts used in polymer electrolyte fuel cells are necessary. The morphological and chemical states of the catalysts have been extensively studied by in situ small-angle X-ray scattering (SAXS) and X-ray absorption spectroscopy (XAS), for example, combined with electrochemical measurements. In the same analytical field, we conducted in situ anomalous SAXS (A-SAXS) and XAS measurements of a catalyst layer located in a channel flow electrode cell using a measurement system we developed. The obtained A-SAXS and XAS results were analysed to understand the morphological and chemical states of Pt nano particles. This system is expected to be used for electrochemical processes involving Pt nano particles.

Compositional study on the size distribution of nickel nanocrystals in borosilicate glasses

For three borosilicate glasses of different B-to-Si ratio, annealed under reducing atmosphere, the effect of glass composition on the size distribution of nickel nanocrystals formed therein was studied. Depth-dependent crystal size distributions were analyzed using transmission electron microscopy, while the glass structure was examined by optical and X-ray absorption spectroscopy (XAS) and related to magnetic measurements using a superconducting quantum interference device (SQUID). A B-to-Si ratio of 0.092 resulted in a uniform crystal size distribution of small width, both with depth and with time, therefore exhibiting a large share of small nanocrystals close to the superparamagnetic limit. XAS and SQUID results indicate a more effective reduction of Ni ions in the glass of higher boron oxide content due to a higher Tg and a mean lower oxidation state induced via lowering of the glass basicity.

Investigation of the Cs2(Mo,Te)O4 Solid Solution and Implications on the Joint Oxyde-Gaine System in Fast Neutron Reactors.

The formation of a thin layer, the so-called Joint Oxyde-Gaine (JOG), between the (U,Pu)O2 fuel pellets and the cladding has been observed in fast neutron reactors, due to the accumulation of volatile fission products. Cs2MoO4 is known to be one of the major components of the JOG, but other elements are also present, in particular tellurium and palladium. In this work, an investigation of the structural and thermodynamic properties of Cs2TeO4 and Cs2Mo1–xTexO4 solid solution is reported. The existence of a complete solubility between Cs2MoO4 and Cs2TeO4 is demonstrated, combining X-ray diffraction (XRD), neutron diffraction (ND), and X-ray absorption spectroscopy (XAS) results. High-temperature XRD measurements were moreover performed on Cs2TeO4, which revealed the existence of a α–β phase transition around 712 K. Thermal expansion coefficients were also obtained from these data. Finally, phase equilibra points in the Cs2MoO4–Cs2TeO4 pseudobinary phase diagram were collected using differential scanning calorimetry and used to develop a thermodynamic model for this system using a regular solution formalism.

Interdiffusion behaviors observation in TiN/ZrOxNy bilayer by XAS and ToF-SIMS

Interface behavior plays an important role in multilayer structure. The deep understanding of interface diffusion and reaction is necessary to improve the performance of device with multilayer structure. In the present work, we perform a detailed systematic study on interdiffusion and interaction between the TiN and ZrOxNy layers in the TiN/ZrOxNy bilayer system. A series of TiN/ZrOxNy bilayers with reverse deposition orders or with varied composition have been prepared by radio-frequency reactive magnetron sputtering. Synchrotron radiation x-ray absorption spectroscopy (XAS) and time of flight secondary ion mass spectrometry (ToF-SIMS) are both used to understand well interface diffusion behavior. The XAS results indicate that interaction and interdiffusion extent between TiN and ZrO2 (ZrOxNy) layers depend on deposition order of both layer. Herein the O atoms enter easily into the TiN layer by substituting the N atoms when the ZrO2 or ZrOxNy layer is deposited onto the TiN layer. No significant interaction between TiN and ZrN layers takes place in the TiN/ZrN sample. Both XAS results and ToF-SIMS depth profiles reveal that the interaction between TiN and ZrOxNy layers depends on the deposition order of the layers.

Reaction mechanism and additional lithium storage of mesoporous MnO2 anode in Li batteries

Nanostructured transition metal oxides, employed as anode materials for lithium-ion batteries, exhibit a higher capacity than the theoretical capacity based on the conversion reaction. To date, the reasons behind this phenomenon are unclear. For the one-step evolution of anode material for lithium-ion batteries, it is essential to understand the lithium storage reaction mechanism of the anode material. Herein, we provide a detailed report on the lithium storage and release mechanism of MnO2, using synchrotron-based X-ray techniques. X-ray diffraction and X-ray absorption spectroscopy results indicate that during the first discharge, MnO2 is reduced in the order of MnO2 → LixMnO2 (1<X<2) → MnO → Mn metal, followed by a reversible reaction between Mn metal and Mn3O4. Furthermore, soft X-ray absorption spectroscopy results indicate that additional reversible formation-decomposition of the electrolyte-derived surface layer occurs and contributes to the reversible capacity of MnO2 after the first discharge. These findings contribute to further understanding of the reaction mechanism and additional lithium storage of MnO2 and suggest practical strategies for developing high energy density anode materials for next-generation Li batteries.

(Invited) Electrochemical Nitrogen Reduction to Ammonia on Transition Metal Nitrides

Renewable production of ammonia, a building block for most fertilizers, via the electrochemical nitrogen reduction reaction (ENRR) is desirable; however, selective electrocatalysts are lacking. Here we show that transition metal nitrides are a family of active, selective, and stable ENRR catalysts. In particular, vanadium nitride nanoparticles have an ENRR rate and a Faradaic efficiency of 3.3 × 10−10 mol s−1 cm−2 and 6.0% at −0.1 V within 1 h, respectively. ENRR with 15N2 as the feed produces both 14NH3 and 15NH3, which indicates that the reaction follows a Mars−van Krevelen mechanism. Ex situ X-ray photoelectron spectroscopy characterization of fresh and spent VN catalysts reveals that multiple vanadium oxide, oxynitride, and nitride species are present on the surface and identified vanadium oxynitride as the active phase in the ENRR, which is supported by operando X-ray absorption spectroscopic results. Quantitative isotopic label studies are conducted to determine the density of the initial and steady-state active surface N sites.

Unraveling the solvation geometries of the lanthanum(III) bistriflimide salt in ionic liquid/acetonitrile mixtures.

A synergic approach combining molecular dynamics (MD) and X-ray absorption spectroscopy (XAS) has been used to investigate the structural properties of the La(Tf2N)3 salt (where Tf2N = bistriflimide or bis(trifluoromethansulfonyl)imide) dissolved into several mixtures of acetonitrile and the 1,8-bis(3-methylimidazolium-1-yl)octane bistriflimide (C8(mim)2(Tf2N)2) ionic liquid (IL), with the IL molar fraction (χIL) ranging from 0 to 1. The XAS and MD results show that major changes take place in the La3+ first solvation shell when moving from pure acetonitrile to pure C8(mim)2(Tf2N)2. With increasing the IL concentration of the mixture, the La3+ first shell complex progressively loses acetonitrile molecules to accommodate more and more oxygen atoms of the Tf2N- anions. Except in pure C8(mim)2(Tf2N)2, La3+ is always able to coordinate both acetonitrile and Tf2N- anions, with a ratio between the two different ligands strongly dependent on the IL content. Moreover, the La3+ ion prefers to form a 10-coordinated first shell complex in all the investigated systems, with a slightly different geometry of the cluster depending on the composition of the La3+ first solvation shell. In particular, when moving from pure acetonitrile to pure C8(mim)2(Tf2N)2, the La3+ first solvation shell passes from a bicapped square antiprism geometry where all the Tf2N- anions act only as monodentate ligands, to a "1 + 5 + 4" structure in which the Tf2N- anion binds La3+ both in a monodentate and bidentate fashion. The great adaptability shown by the La3+ solvation structure allows it to reach the optimal balance among many different forces at play involving all of the different species present in the mixtures.

Sorption of americium / europium onto magnetite under saline conditions: Batch experiments, surface complexation modelling and X-ray absorption spectroscopy study

HypothesisThis study investigates the adsorption of americium and its chemical analogue europium on magnetite, which is expected to form as a major long-term steel canister corrosion product under anoxic and highly saline conditions. ExperimentsThe sorption of europium on magnetite (solid/liquid ratio = 0.5 g/L) was investigated batch wise in NaCl brines with ionic strength I = 1 m, 3.5 m, and 6.67 m, as a function of pHm for two europium concentrations (6 × 10−10m, 1.2 × 10−5m). Information on the chemical nature of the surface species was obtained by X-ray absorption spectroscopy (XAS) at the americium L3-edge. FindingsRetention of europium by magnetite of >99.5% was found above pHm 6.4 for all ionic strengths for europium concentration of 6 × 10−10m. No ionic strength effect was observed in this pHm range. At 1.2 × 10−5m europium concentration, 95 ± 4% sorption was found above pHm 7.5 for I = 1 m and above pHm 8.0 for I = 3.5 m and 6.67 m. A small ionic strength effect was observed in this case. X-ray absorption spectroscopy (XAS) results are consistent with the batch sorption experiment outcomes, showing an insignificant effect of ionic strength on the pHm dependent sorption. Results from potentiometric titrations of the solid phase, batch sorption experiments and spectroscopy were interpreted consistently with a charge distribution multi-site (CD-MUSIC) triple layer surface complexation model assuming surface coordination of the metal ion via a tridentate binding mode.

Annealing effect on phase transition and thermochromic properties of VO2 thin films

Abstract Vanadium dioxide (VO2) is one kind of desired thermochromic material for many smart devices because of its notable temperature-responsive infrared modulation and metal−insulator transition with underlying structural phase transition (SPT). Understanding on the tuning of these properties in an anticipated manner is essential to accomplish device realization. Here, we report the annealing time induced modifications in SPT and the thermochromic properties of VO2 thin films. Using RF sputtering deposition, VO2 thin films were grown at room temperature and their ex-situ annealing was carried out at 600 °C for different time from 2 min to 60 min. Structural, electronic and thermochromic properties of these films were investigated. VO2 thin films samples annealed for longer time exhibit larger crystallite size, higher surface roughness and SPT temperature, and the reduced hysteresis width of SPT during heating and cooling cycle. X-ray absorption spectroscopy results indicate that the variation in the annealing time do not alter the electronic structure significantly. Nevertheless, VO2 thin film samples with prolonged annealing display higher IR transmittance modulation across the SPT.

Effect of nonstoichiometry on crystal structure, charge and spin states of cobalt ions in Tb1−yBa1+yCo2−xO5.5−δ: Neutron diffraction and soft X-ray absorption spectroscopy studies

Neutron diffraction and soft X-ray absorption spectroscopy have been applied for studying crystal structure, charges, and spin states of cobalt ions in nonstoichiometric Tb1−yBa1+yCo2−xO5.5−δ cobaltites. The following single-phase samples with defects in anion and cation sublattices were synthesized and investigated: TbBaCo2O5.38, TbBaCo1.95O5.31, TbBaCo1.90O5.24, and Tb0.95Ba1.05Co1.90O5.21. Co and Tb charge states have been estimated from soft X-ray absorption spectra. The anisotropic nature of changes of lattice parameters depending on the content of cobalt and oxygen vacancies has been established. A correlation between cobalt and oxygen vacancies in double layered cobaltites and a role of apical and planar oxygen in the formation of the Tb1−yBa1+yCo2−xO5.5−δ structure have been shown. Oxygen vacancies are found to be unevenly distributed across the unit cell. Most vacancies are formed in the O (4u) and O (2s) planar positions. The O (1g) apical positions are also partly vacant. The high spin state of Co3+ ions in octahedra has been found to be realized. This conclusion obtained from neutron diffraction data is consistent with the results of X-ray absorption spectroscopy.

Distribution and speciation of iron in Fe-modified biochars and its application in removal of As(V), As(III), Cr(VI), and Hg(II): An X-ray absorption study

Characterization of the spatial distribution and speciation of iron (Fe) in Fe-modified biochars is critical for understanding the mechanisms of contaminant removal. Here, synchrotron-based techniques were applied to characterize the spatial distribution and speciation of Fe in biochars modified by FeCl3 or FeSO4 and pyrolyzed at 300, 600, and 900 °C, respectively. Confocal micro-X-ray fluorescence imaging (CMXRFI) results indicated Fe, sulfur (S), and chlorine (Cl) diffused into the basic porous structure of the biochars and aggregated to the surface as pyrolysis temperature increased. Fe K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra revealed maghemite (γ-Fe2O3) as the primary Fe species in the modified biochars and Fe(0) was observed when pyrolyzed at 600 or 900 °C. Unmodified and FeCl3-modified biochars pyrolyzed at 900 °C were evaluated in the removal of arsenate (As(V)), arsenite (As(III)), hexavalent chromium (Cr(VI)) and Hg(II) from aqueous solution and Fe-modification enhanced the removal efficiency from 42.0%, 62.5%, 19.6%, and 97.0%, respectively, to all 99.9%. X-ray absorption spectroscopy results indicate both adsorption and redox reaction contributed to the removal mechanisms. The present study provides a prospective and sustainable material and offers information relevant to tailoring Fe-modified biochars to specific environmental applications.

In Situ Observation of the Insulator-To-Metal Transition and Nonequilibrium Phase Transition for Li1-xCoO2 Films with Preferred (003) Orientation Nanorods.

It is notoriously difficult to distinguish the stoichiometric LiCoO2 (LCO) with a O3-I structure from its lithium defective O3-II phase because of their similar crystal symmetry. Interestingly, moreover, the O3-II phase shows metallic conductivity, whereas the O3-I phase is an electronic insulator. How to effectively reveal the intrinsic mechanism of the conductivity difference and nonequilibrium phase transition induced by the lithium deintercalation is of vital importance for its practical application and development. Based on the developed technology of in situ peak force tunneling atomic force microscopy (PF-TUNA) in liquids, the phase transition from O3-I to O3-II and consequent insulator-to-metal transition of LCO thin-film electrodes with preferred (003) orientation nanorods designed and prepared via magnetron sputtering were observed under an organic electrolyte for the first time in this work. Then, studying the post-mortem LCO thin-film electrode by using ex situ time-dependent XRD and conductive atomic force microscopy, we find the phase relaxation of LCO electrodes after the nonequilibrium deintercalation, further proving the differences of the electronic conductivity and work function between the O3-I and O3-II phases. Moreover, X-ray absorption spectroscopy results indicate that the oxidation of Co ions and the increasing of O 2p-Co 3d hybridization in the O3-II phase lead to electrical conductivity improvement in Li1-xCoO2. Simultaneously, it is found that the nonequilibrium deintercalation at a high charging rate can result in phase-transition hysteresis and the O3-I/O3-II coexistence at the charging end, which is explained well by an ionic blockade model with an antiphase boundary. At last, this work strongly suggests that PF-TUNA can be used to reveal the unconventional phenomena on the solid/liquid interfaces.

Structural peculiarities, point defects and luminescence in Bi-doped CsCdX3 (X = Cl, Br) single crystals

Promising luminescent CsCdX3 (X = Cl, Br) single crystals, both nominally-pure and doped with bismuth, have been studied comprehensively by X-ray diffraction, X-ray synchrotron radiation, and X-ray absorption spectroscopy. Crystal structure refinement shows vacancies in the Cs crystallographic site and partial substitution of Cd2+ ions by Bi3+ ones in both CsCdCl3 and CsCdBr3 (point defects BiCd•), which is consistent with the results of X-ray absorption spectroscopy. The assumed presence of Bi1+ ions in the Cs1+ sites of doped CsCdX3 (X = Cl, Br) crystals is not confirmed. In the photoluminescence spectra of Bi-doped CsCdCl3 and CsCdBr3 crystals, a single band in the near-IR spectral range with a maximum around 1000 nm is caused by point defects BiCd•. Photoluminescence spectra and decay kinetics of Bi-doped CsCdX3 (X = Cl, Br) indicate their promising use as luminescent materials.

Oxygen vacancy mediated conductivity and charge transport properties of epitaxial Ba0.6La0.4TiO3− δ thin films

We report on the effects of the oxygen vacancy (VO) regarding the microstructure, conductivity, and charge transport mechanisms of epitaxial Ba0.6La0.4TiO3−δ (BLTO) films. The VO concentration can be largely regulated from 21.5% to 37.8% by varying the oxygen pressure (PO2) during film deposition. Resistivity-temperature and Hall effect measurements demonstrate that the BLTO films can be tuned remarkably from an insulator to a semiconductor, and even to a metallic conductor by regulating the VO concentration. The role of VO concentration in the charge transport mechanism is clarified. For films with low VO concentration, the charge transport is dominated by variable range hopping (VRH) at low temperatures, and it shows small polaron (SP) hopping at high temperatures. For films with high VO concentration, the carrier transport remains VRH at low temperatures, while it changes to SP hopping at moderate temperatures, and is dominated by thermal phonon scattering at high temperatures. Furthermore, the lower starting temperature of SP hopping for films with higher VO concentrations indicates that VO favors electron-phonon coupling. Different charge transport mechanisms are assumed to be due to different VO-induced defect energy levels in the BLTO films, which has been verified by their soft x-ray absorption spectroscopy results.

Arousing the Reactive Fe Sites in Pyrite (FeS2) via Integration of Electronic Structure Reconfiguration and in Situ Electrochemical Topotactic Transformation for Highly Efficient Oxygen Evolution Reaction

Despite significant advances in the development of highly efficient and robust oxygen evolution reaction (OER) electrocatalysts to replace noble-metal catalysts, commercializing OER catalysts with high catalytic activity for sustainable development still remains a great challenge. Especially, transition-metal Fe-based OER catalysts, despite their earth-abundant, cost-efficient, and environmentally benign superiorities over Co- and Ni-based materials, have received relatively insufficient attention because of their poor apparent OER activities. Herein, by rational design, we report Ni-modified pyrite (FeS2) spheres with yolk-shell structure that could serve as pre-electrocatalyst precursors to induce a highly active nickel-iron oxyhydroxide via in situ electrochemical topological transformation under the OER process. Notably, as confirmed by the results of X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations, Ni doping could effectively regulate the intrinsic electronic structure of FeS2 to realize a semiconductor-to-semimetal transition, which endows FeS2 with dramatically improved conductivity and water adsorption ability, providing prequisites for subsequent topological transformation. Moreover, systematic post-characterizations further reveal that the optimal Ni-FeS2-0.5 sample completely converts to amorphous Ni-doped FeOOH via an in situ electrochemical transformation with yolk-shell structure well-preserved under the OER conditions. The electronic structure modulation combined with electrochemical topotactic transformation strategies well stimulate the reactive Fe sites in Ni-FeS2-0.5, which show impressively low overpotentials of 250 and 326 mV to drive the current densities ( j) of 10 and 100 mA cm-2, respectively, and a Tafel slope as small as 34 mV dec-1 for the OER process. When assembled as a water electrolyzer for the overall water splitting, Ni-FeS2-0.5 can display a low voltage of 1.55 V to drive a current density of 10 mA cm-2, outperforming most of the transition-metal-based bifunctional electrocatalysts to date. This work may provide new insight into the rational design of other high-performance Fe-based OER electrocatalysts and inspire the exploration of cost-effective, ecofriendly electrocatalysts to meet the demand for future sustainable development.

Degradation Mechanism of Ni-Rich Layered Cathode Materials during High Voltage Cycling Revealed By Multi-Model Synchrotron X-Ray Imaging and Spectroscopy Techniques

Ni-rich layered cathode materials are important candidates for lithium-ion batteries used for high energy density applications, such as electric vehicles. However, these materials are suffering severe capacity fade caused by surface reconstruction, unstable cathode-electrolyte interface (CEI) formation and micro crack formation during cycling, especially when a high cut off charge voltage is applied (such as 4.6 and 4.8V). To understand the origin of this capacity fade, state-of-the-art diagnostic tools are applied to understand their structure, chemical and morphological properties of LiNi0.94Co0.06O2 (NC) and LiNi0.92Co0.06Al0.02O2 (NCA) cathode materials during cycling. Electrochemical measurement results show that the NCA cathode delivered higher capacity and stable cyclic performance even cycled at a high cut of voltage of 4.8V. In situ X-ray absorption spectroscopy (XAS) results of NC and NCA electrodes measured during the first and 51st charge/discharge cycling between the voltage range of 2.8-4.4V showed that both Ni and Co contribute to the capacity by going through the Ni3+/Ni4+ and Co3+/Co4+ redox couples. The Ni-K edge XAS spectra of NC cathode charged to 4.8V showed slightly reduction of Ni4+ during the high voltage charging. While, XAS spectra of NCA cathode did not showed reduction of Ni4+ when the cell charged to the higher voltage. More interestingly, C, F, O, Ni and Co soft XAS spectroscopy of NC and NCA electrodes at the different state of charge provided more comprehensive insight into surface and bulk chemical properties. The results showed that much thicker CEI layer formed on the NC surface compared with the NCA cathode, indicated that Al-doping effectively reduced the side reaction of cathode materials with the electrolyte. We have also compared the morphological and chemical evolution of NC and NCA secondary particles using full-filed in-situ transmission X-ray microscopy (TXM) during the initial cycle. Results will be presented at the meeting. Acknowledgement This project was supported by the U.S. Department of Energy, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies through Advanced Battery Material Research (BMR) program (Battery500 consortium) under Contract No. DESC0012704.