Incorporation of Ni into natural goethite: An investigation by X-ray absorption spectroscopy

1. Introduction Li-rich type (manganese) oxides are one of the most featured cathodes for lithium ion batteries in recent years, owing to their high capacity and cyclability. In these cathodes, partial substitution of manganese by other transition metals such as Ni and Co has been proposed and shown to be effective in improving the performance, [1] however, the role of such metals in the battery performance has not been clarified in detail.We have synthesized Ni ion substituted Li excess manganese oxides (Li2Mn1-xNixO3) as a simple model of Li2MeO3 phase in solid solution cathode to understand the effect of introduced Ni by using the combination of in situ X-ray absorption spectroscopy (XAS) and X-ray diffraction spectroscopy (XDS) analysis. Diffraction anomalous fine structure (DAFS) spectra are achieved by XDS analysis, that there were quantitatively showed potential-dependent transbilayer migration of nickel and manganese and their reversibility. 2. Experimental procedure The active material of nickel substituted Li2MnO3 (Li2Mn1-x NixO3, x = 0 ~ 0.25) was prepared from solid phase calcination at 1173 K for 12 h in air. The working electrode consisted of active material (80 wt.%), acetylene black (10 wt.%) and polyvinylidene difluoride (PVdF) binder (10 wt.%), coated on aluminum current foil.The electrochemical measurements of the electrode were employed using aluminum pouch type cells, metallic lithium as counter and reference electrodes, 1 mol dm-3 LiPF6 in a 3:7 mixture of EC/DMC, and a polyolefin film as a separator. The electrochemical charge and discharge tests were performed at room temperature and the potential range was between 2.0 and 4.8 V.XAS measurements were performed by the three-electrode cell with charging and discharging processes (in Operando). The XAS measurements of the Ni-K and Mn-K edge region were performed using a solid-state detector at beamlines BL28XU in SPring-8 (Hyogo, Japan).The XDS measurements were conducted at beamline BL28XU, SPring-8, Japan. A two-dimensional Pilatus detector (Dectris Co., Ltd.) was mounted onto an arm of a multi-axis goniometer and was used to measure the diffraction profile, and two ionization chamber was used for the absorption correction of the DAFS spectra. 3. Results and Discussion The obtained X-ray absorption near edge structure (XANES) region spectra at N-K energy region of Li2Mn1-x NixO3 (x = 0.25) in the 1st charging and discharging processes are shown in Fig. 1. The edge energy was shifted to high energy, which is suggested that Ni element function as a charge compensation species. However the end of charging region around 4.7 V to 4.8 V was shifted to low energy as seen in previous XAS report [2], which means the reduction of nickel on average in this region.As our group have published before,[3] XANES-like spectra only from selected sites (i.e. DAFS spectra) can be extracted from the XDS analysis. Fig. 2 shows the DAFS spectra of initial sample. The Ni-K edge XANES-like spectra shows that there was cation mixing in the lithium layer of the pristine material. In contrast, Mn-K edge XANES-like spectra shows that manganese mostly occupied in transition metal layer. XDS analysis of the fully charged sample indicated that Ni element further migrated to the lithium layer.These analytical approaches are important to understand the metal migration behavior as a function of operation potential together with the charge compensation mechanism during electrochemical charging and discharging.

Sodium ion batteries (SIBs) have emerged as suitable alternative energy storage systems to Li ion batteries (LIBs) due to the cost-effective and ample Na source. Sodium layered oxides have considerable attention as cathodes for Na-ion batteries due to easy synthesis and simple structure. O3-type NaTMO2 materials, where single transition metal is an oxidizable element such as Cr, Mn, Fe, Co, Ni [1], have the capability to reversible intercalation reaction of Na ions [2,3], which is different from their Li analogues system, where only nickel and cobalt do reversible intercalation of Li ions [4]. Furthermore, various mixed transition metals can be realized in the TM layer to create new oxide compounds [5,6]. Among them, O3-NaNi1/3Fe1/3Mn1/3O2 shows a capacity around 130 mAh/g with good capacity retention when cycled to 4.0 V [7]. In this study, layered sodium-ion battery cathode material, O3-type NaNi1/3Fe1/3Mn1/3O2, was systematically investigated by using synchrotron-based x-ray techniques to characterize the detailed redox mechanism during electrochemical process and to understand the role of each transition metal structural behavior in the ternary-element material. In Figure 1, reversible changes in the Ni, Fe, and Mn K-edge X-ray Absorption Near Edge Structure (XANES) spectra show that reversible electronic structural changes in the local level during Na+ deintercalation/intercalation in the voltage range of 2.0 – 4.0 V. Moreover, Ni and Fe elements are both active in Na1–xNi1/3Fe1/3Mn1/3O2 cathode and redox couples of Ni2+/Ni3+/Ni4+ and Fe3+/Fe4+ are responsible for the charge compensation mechanism. High Resolution Powder Diffraction (HRPD) results reveal that O3-type (R-3m) phase transforms into a P3 (R3m) structure coupled with Na+/vacancy ordering during charge and further elucidate the final P3-OP2 of phase transformation on over-sodiated state. Furthermore, in the structure with the small quantity of sodium, internal Fe ion migration occurs from octahedral site into tetrahedral site, which is possible due to formed vacant space by highly deintercalation of Na ions and this atomic level of movement causes the layered structure to have structural distortions. An in-depth analysis of the structural behavior and reaction mechanism for NaNi1/3Fe1/3Mn1/3O2 cathode material when the Na ions are used of the entire composition widens an electrochemical perspective and suggests a direction where we better understand the nature of structure which can be used as the cathode for the advanced rechargeable batteries with high energy density. From these experimental results, we will discuss structural evolution behavior of layered NaNi1/3Fe1/3Mn­1/3O2 cathode material. More detailed results and discussion including reaction process of Ni, Fe, and Mn in the material will be presented in the AiMES 2018 meeting. Figure 1. XANES spectra of (a, b) Ni K-edge, (c,d) Fe K-edge, and (e,f) Mn K-edge during the 1st cycle of Na+ deintercalation/intercalation process. Reference s : [1] C. Delmas, C. Fouassier, P. Hagenmuller, Phys. B + C 99B (1980) 81–85. [2] S. Komaba, C. Takei, T.Nakayama,A.Ogata,N. Yabuuchi, Electrochem. Commun. 12 (2010) 355–358. [3] N. Yabuuchi, H. Yoshida, S. Komaba, Electrochemistry 80 (2012) 716. [4] S.P. Ong, V.L. Chevrier, G. Hautier, A. Jain, C. Moore, S. Kim, et al., Energy Environ. Sci. 4 (2011) [5] I. Saadoune, A. Maazaz, M. Menetrier, C. Delmas, J. Solid State Chem. 117 (1996) 111–117. [6] M. Sathiya, K. Hemalatha, K. Ramesha, J.-M. Tarascon, A.S. Prakash, Chem. Mater. 24 (2012) 1846–1853. [7] D. Kim, E. Lee, M. Slater, W. Q. Lu, S. Rood, C. S. Johnson, Electrochem. Commun. (2012), 18, 66. Figure 1

Introduction For development and popularization of electric vehicles, it is indispensable to improve the safety and the capacity of secondary batteries not only in a usual operating condition but also in harsh operating environments (e.g., at high temperature and in an overcharge condition). Basic knowledge regarding the electronic and bonding states of functional 3d transition metal (TM) elements is necessary to design and develop cathode materials of lithium-ion batteries in this direction. Core-level X-ray spectroscopies provide the element-selected information about electronic states of constituent elements. In this study, we applied X-ray spectroscopies to LiNiO2-based cathode materials to investigate the electronic and bonding states of Ni in different states of charge. Experimental All the samples for X-ray spectroscopies were prepared by using a two-electrode cell comprised of LiNi0.80Co0.15Al0.05O2 as an active material of a working electrode, Li metal as a counter electrode, and mixed solvent of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate containing 1.0 mol dm− 3 LiPF6. The cell was charged at a current density of 1 mA cm−2 (ca. 1 C rate) at room temperature. X-ray absorption spectroscopy (XAS) was performed at the Ni and Co K-edges at the BL33XU (the Toyota beamline) and at the Ni and Co L 2,3-edges and at the O K-edge at the BL27SU of SPring-8, Japan. X-ray emission spectroscopy was also carried out at the Ni K-edge at the BL39XU and the BL33XU of SPring-8. Local structural analysis based on extended X-ray absorption fine structure (EXAFS) was done by using ATHENA and ARTEMIS with the scattering phase shift and the effective scattering amplitudes calculated by FEFF9.05. Results The oxidation state of Ni is often estimated by using an empirical index of absorption energy, at which normalized absorbance is 0.5. This empirical method was applied to the present system, and revealed that the absorption energy at the Ni K-edge monotonically increased as the state of charge approached from 0 to 100 and slightly decreased as the cell was more overcharged. In general, the change in the electronic state of Ni involves that in local structure around Ni, which suggests that structural parameters such as coordination numbers and bond length for the first coordination Ni–O should be alternate indicators of the oxidation state of Ni. Actually, it was found that these structural parameters could be good indicators of average Ni valence. The structural parameters monotonically increased even in the overcharge region as the cell was charged, differently from the above-mentioned empirical absorption energy. The difference in the changing trend of the oxidation-state indicator and the electronic structure of Ni was further investigated by using 3d TM L 2,3-edge and O K-edge XAS and X-ray emission spectroscopy, which will be discussed in the conference. Acknowledgement X-ray spectroscopic studies were performed at the BL27SU, the BL33XU, and BL39XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) with the proposal numbers of 2014B1582, 2014B7008, and 2014B1574.

Operando X-ray absorption spectroscopy (XAS) can be utilized to probe the phase and structural changes of FeNiOx and similar transition metal oxide electrocatalysts during electrocatalytic reactions. However, capturing the temporal changes occurring in the operando chemistry of electrocatalysts has been little studied. In this work, we successfully capture the time-resolved changes at the Ni K-edge for both the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions of an aqueous phase synthesized FeNiOx bimetallic nanoparticle electrocatalyst. During a stepped voltage experiment, the temporal change in the Ni K-edge was observed as the applied voltage was stepped from 0.7 to 0.8 V versus the silver/silver chloride reference electrode, and the change observed is associated with the Ni redox transition from the 2+ to 3+/4+ oxidation state. Individual XAS spectra were obtained in 90 s, and a total of 9 scans post voltage step showed a unique transition from a hydroxide to an oxyhydroxide phase. The shift in absorption edge energy position and the changes in spectral shape for individual scans explain the typically observed broadening of the Ni K-edge XANES spectrum during time-averaged operando XAS, and a kinetic analysis revealed a first-order observed rate constant, |kobs|, of 0.00426 s–1 and a half-life (t1/2) of 163 s. Multivariate curve resolution-alternating least squares analysis followed by linear combination fitting analysis for time-averaged versus time-resolved Ni K-edge changes upon voltage step show distinct differences in estimated contributions from Ni2+ oxide/hydroxide phases. Detailed EXAFS modeling shows the phase transition from hydroxide to oxyhydroxide on top of the unchanged metallic core.

Li-rich layered oxides Li1+xM1-xO2 with M = Mn, Co and Ni may be used as positive electrode in Li-ion batteries. Mechanisms at the origin of the large capacity exhibited by these Li-excess compounds compared to that obtained for stoichiometric ones (300 mAh/g vs 200 mAh/g, respectively) have already been extensively described in literature [1-2]. These materials can be viewed as an intergrowth between [LiO6]∞ and [MO6]∞ octahedral layers. Overlithiation induces a Li2MnO3-type order into [MO6]∞slabs which facilitates oxygen ions oxidation at high potential (>4.4 V), a phenomenon responsible for the extra-capacity observed.The average structure of these compounds is well-known but the complexity of the local structure needs further understanding. In fact, the local environments govern activation barriers and can affect notably Li ion transport. At the nanoscale, Li1+xM1-xO2 can be described as LiMO2-Li2MnO3nanocomposites with a complex microstructure which depends on the nominal composition. In addition, structural changes and redox processes occurring during cycling depend not only on the initial local structure but also on the cycling conditions (rate and temperature). Thus, a better understanding of the relationship between local structure and electrochemical properties is needed for improving performances of these materials.In this work two electrodes with different starting microstructures and activated up to 4.6 V at two different temperatures were studied. The electrochemical phenomena observed for each compound during galvanostatic cycling were found to be quite different, especially in discharge.Structural and redox mechanisms taking place during cycling were studied in detail and compared. In order to follow changes in electronic and crystal structures under battery operation, in-situ time-resolved experiments were performed at the synchrotron SOLEIL using x-ray absorption spectroscopy (XAS) and x-ray diffraction (XRD) in operando mode. The evolution of microstructure (bulk and surface) during cycling was also monitored by high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS) and x-ray photoelectron spectroscopy (XPS) in ex-situmode.Li2MnO3-type order, probably responsible for extra-capacity in these materials, is detected in both starting compounds; however its evolution is found to be highly dependent on the cycling conditions. During charge, there is a progressive attenuation of the Li2MnO3-type order at high potential, even disappearing at the end of the first charge under severe activation conditions. Many stacking faults were observed by HRTEM, especially after activation at high temperature. In addition, spinel-type defects are visible at the edges of crystals (see figure) and are probably the cause of the voltage decrease observed during cycling. Indeed, redox reactions associated with lithium ion insertion into “special” sites, like spinel defects, may require to overcome a larger energy barrier. The analysis of the surface composition by XPS shows a surface enrichment in manganese which is in good agreement with the formation of spinel-type defects (LiMn2O4 vs. LiMO2). Furthermore, we also monitored the effect of temperature and microstructure on the redox mechanisms involved during cycling, in ex-situ and operando mode. The evolutions of Mn-K, Co-K and Ni-K edges were studied by XAS. While the activation temperature has an influence on the Co-K and Ni-K edges, the influence of the microstructure is only noticeable on the Mn K-edge. Finally, in order to get information on redox processes at the nanoscale, O-K and Mn-L2,3 edges were studied by EELS at different states of charge. Modifications of the fine structures of these two edges were observed during the entire activation cycle and quantitative information was deduced from L3/L2ratios analyze.All these results will be discussed in detail.

An FeNi (oxy)hydroxidecocatalyst overlayer was photoelectrochemicallydeposited on a thin-film hematite (α-Fe2O3) photoanode, leading to a cathodic shift of ∼100 mV in thephotocurrent onset potential. Operando X-ray absorption spectroscopy(XAS) at the Fe and Ni K-edges was used to study the changes in theoverlayer with potential in the dark and under illumination conditions.Potential or illumination only had a minor effect on the Fe oxidationstate, suggesting that Fe atoms do not accumulate significant amountof charge over the whole potential range. In contrast, the Ni K-edgespectra showed pronounced dependence on potential in the dark andunder illumination. The effect of illumination is to shift the onsetfor the Ni oxidation because of the generated photovoltage and suggeststhat holes that are photogenerated in hematite are transferred mainlyto the Ni atoms in the overlayer. The increase in the oxidation stateof Ni proceeds at potentials corresponding to the redox wave of Ni,which occurs immediately prior to the onset of the oxygen evolutionreaction (OER). Linear combination fitting analysis of the obtainedspectra suggests that the overlayer does not have to be fully oxidizedto promote oxygen evolution. Cathodic discharge measurements showthat the photogenerated charge is stored almost exclusively in theNi atoms within the volume of the overlayer.

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Structure and oxidation state of the Ni-Fe cofactor of the NAD-reducing soluble hydrogenase (SH) from Ralstonia eutropha were studied employing X-ray absorption spectroscopy (XAS) at the Ni K-edge, EPR, and FTIR spectroscopy. The SH comprises a nonstandard (CN)Ni-Fe(CN)(3)(CO) site; its hydrogen-cleavage reaction is resistant against inhibition by dioxygen and carbon monoxide. Simulations of the XANES and EXAFS regions of XAS spectra revealed that, in the oxidized SH, the Ni(II) is six-coordinated ((CN)O(3)S(2)); only two of the four conserved cysteines, which bind the Ni in standard Ni-Fe hydrogenases, provide thiol ligands to the Ni. Upon the exceptionally rapid reductive activation of the SH by NADH, an oxygen species is detached from the Ni; hydrogen may subsequently bind to the vacant coordination site. Prolonged reducing conditions cause the two thiols that are remote from the Ni in the native SH to become direct Ni ligands, creating a standardlike Ni(II)(CysS)(4) site, which could be further reduced to form the Ni-C (Ni(III)-H(-)) state. The Ni-C state does not seem to be involved in hydrogen cleavage. Two site-directed mutants (HoxH-I64A, HoxH-L118F) revealed structural changes at their Ni sites and were employed to further dissect the role of the extra CN ligand at the Ni. It is proposed that the predominant coordination by (CN),O ligands stabilizes the Ni(II) oxidation state throughout the catalytic cycle and is a prerequisite for the rapid activation of the SH in the presence of oxygen.

In situ X-ray absorption spectroscopy (XAS) of the Mn and Ni K-edges and magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy have been carried out during the first charging and discharging process for the layered cathode material. The Ni K-edge structure in the X-ray absorption near-edge structure (XANES) spectrum exhibits a rigid positive energy shift with increased Li deintercalation level, while the Mn XANES spectra do not show any substantial energy changes. The Ni edge shifts back reversibly during discharge. Further Li-ion intercalation at (vs. Li) could be accomplished by reduction of the ions. The MAS NMR results showed the presence of Li in the layers, in addition to the expected sites for Li in the lithium layers. All the Li ions in the transition metal layers are removed on the first charge, leaving residual lithium ions in the lithium layers. © 2002 The Electrochemical Society. All rights reserved.

The structural parameters in selected cobalt and mixed cobalt/nickel hexacyanoferrates have been determined by X-ray absorption spectroscopy. The presence of two or three metals in the sample requires the use of a highly efficient multiple edge analysis. The typical structure of mixed hexacyanoferrates coupled with a suitable data analysis program, GNXAS, allow us to determine structural parameters considering a very high number of experimental points. The first data analysis of three contiguous edges (Fe, Co, and Ni K-edges), the structural parameters of which are entirely correlated, is presented. The advantages and limitations of the multiple edge approach are underlined and placed in the context of the previous studies. The CN bond length has been determined with a statistical error of few thousandths of an angstrom.

Improvement of electrochemical technologies is one of the most popular topics in the field of renewable energy. However, this process requires a deep understanding of the electrode-electrolyte interface behavior under operando conditions. X-ray absorption spectroscopy (XAS) is widely employed to characterize electrode materials, providing element-selective oxidation state and local structure. Several existing cells allow studies as close as possible to realistic operating conditions, but most of them rely on the deposition of the electrodes on conductive and X-ray transparent materials, from where the radiation impinges the sample. In this work, we present a new electrochemical flow-cell for operando XAS that can be used with X-ray opaque substrates, since the signal is effectively detected from the electrode surface, as the radiation passes through a thin layer of electrolyte (∼17 μm). The electrolyte can flow over the electrode, reducing bubble formation and avoiding strong reactant concentration gradients. We show that high-quality data can be obtained under operando conditions, thanks to the high efficiency of the cell from the hard X-ray regime down to ∼4 keV. We report as a case study the operando XAS investigation at the Fe and Ni K-edges on Ni-doped γ-Fe2O3 films, epitaxially grown on Pt substrates. The effect of the Ni content on the catalytic performances for the oxygen evolution reaction is discussed.

High-temperature oxidation of one- and two-component metallic systems studied by in-situ X-ray absorption spectroscopy

A FeNi (oxy)hydroxide co-catalyst overlayer was photoelectrochemically deposited on a thin film hematite (α-Fe2O3) photoanode, leading to a cathodic shift of ~100 mV in the photocurrent onset potential. Operando X-ray absorption spectroscopy (XAS) at the Fe and Ni K-edges was used to study the changes in the overlayer with potential, in dark and under illumination conditions. Potential or illumination only had a minor effect on the Fe oxidation state, suggesting that Fe atoms do not accumulate significant amount of charge over the whole potential range. In contrast, the Ni K-edge spectra showed pronounced dependence on potential in dark and under illumination. The effect of illumination is to shift the onset for the Ni oxidation because of the generated photovoltage, and suggests that holes which are photogenerated in hematite are transferred mainly to the Ni atoms in the overlayer. The increase in the oxidation state of Ni proceeds at potentials corresponding to the redox wave of Ni, which occurs immediately prior to the onset of the oxygen evolution reaction (OER). Linear fitting analysis of the obtained spectra suggests that the overlayer does not have to be fully oxidized to promote oxygen evolution. Cathodic discharge measurements show that the photogenerated charge is stored almost exclusively in the Ni atoms within the volume of the overlayer.

The X-ray absorption spectra (XAS) of LiCoO2, LiCo1/2Ni1/2O2 and LiNiO2 were examined together with X-ray diffraction (XRD). Co and Ni K-edge XANES spectra of LiCo1/2Ni1/2O2 are quite similar to that of LiCoO2 or LiNiO2, suggesting that electronic states of Co and Ni in LiCo1/2Ni1/2O2 are Co3+ and Ni3+. Analytical results of Co and Ni K-edge EXAFS oscillations on the first coordination shell of nickel and cobalt ions in LiCo1/2Ni1/2O2 indicate that the local environment around the targeted species is the same as that in LiCoO2 or LiNiO2. Since there is no doubt about the crystal and electronic structures of LiCoO2 and LiNiO2, the results indicate that LiCo1/2Ni1/2O2 consists of low-spin states of Co3+ and Ni3+ distributed at equivalent positions in triangular lattice of sites forming homogeneous transition metal oxide layers. Thus, XAS complements XRD in describing solid solution LiCo1/2Ni1/2O2 of LiCoO2 and LiNiO2. The electrochemical behaviors of LiCoO2, LiCo1/2Ni1/2O2 and LiNiO2 are also restated and the effects of the formation of solid solution on the change in lattice dimension during topotactic electrochemical reactions are discussed.

FexNi100-xOy electrocatalysts have become a focus for alkaline water electrolysis and the oxidative half reaction of oxygen evolution. Under alkaline conditions, the oxygen evolution reaction (OER) can be promoted by non-precious metal oxide and hydroxide electrocatalysts. In particular, electrocatalyst compositions from first row late transition metals such as iron, nickel, manganese, and cobalt have emerged as some of the most active catalysts for the OER, and the role of iron has been identified as key within multi-metallic compositions. In our research, we have focused specifically on the iron-nickel bimetallic composition and have developed synthesis methods to be able to control the bimetallic composition, surface chemistry, and three-dimensional morphology of a suite of FexNi100-xOy nanoparticle electrocatalysts. With this subset of highly-active FexNi100-xOy nanoparticle electrocatalysts, we have recently been focused on operando x-ray absorption spectroscopy (XAS) studies to understand how the chemistry of the iron and nickel species within these nanocatalysts changes as a result of exposure to the electrochemical environment. This talk will focus on a set of operando data obtained with a quick scan detector at beamline 9-3 at SSRL, where we were able to capture temporal, dynamic changes to the nickel chemistry as a function of applied voltage in the Faradaic region of the OER. Specifically, we have captured the change in the Ni K-edge that occurs as a result of the voltage stepping through the Ni redox reaction, where the Ni species in our electrocatalyst oxidizes from a 2+ to a 3+/4+ oxidation state. This redox reaction is associated with a phase transition from Ni(OH)2 to NiOOH. Our measurements were able to obtain 90 s scans during a 15 min time period of measurement, where we find that the Ni redox reaction and phase transition occurs during the first 10 minutes of measurement. Modeling of the extended x-ray absorption fine structure (EXAFS) region suggests that both Ni – O and Ni – M (M = metal) scattering path atomic pairs undergo a compression in distance. Further, the time scale of the phase transition suggests that the Ni redox reaction is mass transport limited. In this talk, I will discuss these results and present our analysis of the time-resolved x-ray absorption spectroscopy data.