Advanced metal oxide electrodes in Li-ion batteries usually show reversible capacities exceeding the theoretically expected ones. Despite many studies and tentative interpretations, the origin of this extra-capacity is not assessed yet. Lithium storage can be increased through different chemical processes developing in the electrodes during charging cycles. The solid electrolyte interface (SEI), formed already during the first lithium uptake, is usually considered to be a passivation layer preventing the oxidation of the electrodes while not participating in the lithium storage process. In this work, we combine high resolution soft X-ray absorption spectroscopy with tunable probing depth and photoemission spectroscopy to obtain profiles of the surface evolution of a well-known prototype conversion-alloying type mixed metal oxide (carbon coated ZnFeO) electrode. We show that a partially reversible layer of alkyl lithium carbonates is formed (∼5-7 nm) at the SEI surface when reaching higher Li storage levels. This layer acts as a Li reservoir and seems to give a significant contribution to the extra-capacity of the electrodes. This result further extends the role of the SEI layer in the functionality of Li-ion batteries.

found to be able to exchange Li + and e -both by conversion and alloying processes. As a consequence Fe, LiZn, Li 2 O are formed upon lithiation, which are fi nely dispersed into a carbonaceous matrix, [ 7 ] according to a reversible reaction involving nine lithium ions per formula unit of ZFO and resulting in a capacity of ≈1000 mAh g −1 . [ 7 ] While the lithiation kinetics have already been probed by electrochemical impedance spectroscopy (EIS) and X-ray diffraction (XRD) analysis, [ 5,7 ] very little is known about the evolution of passivation layer properties on ZFO-C.The aim of this work is to study the evolution of the SEI in this innovative anode material at selected charging steps by exploiting the surface sensitivity [10][11][12] of the soft X-ray absorption spectroscopy (XAS). This technique requires synchrotron radiation and was never used before for such a purpose, although it appears to be very suitable for a detailed depth profi ling of the SEI of advanced electrodes. In fact, XAS experiments in the 50-1000 eV photon energy range can be typically performed using both total electron (TEY) and total fl uorescence (TFY) yield techniques for which effective probing depths are around 2-10 nm and 70-200 nm, respectively. In this study, ex situ TEY and TFY X-ray absorption experiments have been conceived and realized to study the modifi cation of the signals related to the various atomic species in ZFO-C electrodes selected at different states of charge during the fi rst Li insertion process. XAS measurements have been preceded and corroborated by a complete electrochemical characterization including galvanostatic intermittent titration technique (GITT) and EIS, with the aim of correlating each XAS experiment with half-cell open-circuit potential (OCV) and charge, and to crosscheck the SEI evolution with the polarization of the electrodes.The samples for the experiments were prepared using carbon-coated ZFO nanoparticles (ZFO-C), obtained [ 7 ] by dispersing 1 g of ZFO powder (<100 nm, Aldrich Chemistry) in 1.5 mL of an aqueous carbon precursor solution of sucrose (Acros Organics), followed by an annealing step under inert gas atmosphere. The weight ratio was 1:0.75 for ZFO:Suc. The obtained dispersion was homogenized by means of a planetary ball mill (Vario-Planetary Mill Pulverisette 4, FRITSCH, 2× 45 min at 400/−800 rpm with 10 min rest in between). Subsequently, the dispersion was dried at 70 °C under ambient atmosphere. After grinding, the resulting composite powder was annealed in a tubular furnace (R50/250/12, Nabertherm) at 450 °C for 4 h under a constant argon gas stream. The heating rate was set to 3 °C min −1 . The material was investigated by SEM (Scanning Electron Microscopy) and TEM (Transmission Electron Microscopy) revealing that it is formed by nanoparticles of average linear dimensions of about 50 nm, with formation of some ZnFe 2 O 4 Li-ion batteries (LIBs) represent a reliable, affordable, and safe energy storage technology for use in portable application. However, current LIB active ...

We present detailed results of a multiple-scattering (MS) extended x-ray absorption fine structure (EXAFS) data analysis of crystalline and nanocrystalline platinum. Advanced MS EXAFS analysis has been applied to raw x-ray absorption data including the background, using the expansion of the absorption cross section in terms of local two-body and three-body configurations. Present EXAFS results on bulk Pt are found to be in agreement with previous structural and vibrational data, and has been used as a reference for reliable structural refinement of nanosized systems. EXAFS structural refinement of Pt nanoparticles has been performed in combination with electron microscopy and x-ray diffraction, showing the importance of considering the actual size distribution and morphology of the samples. Present samples were unsupported and supported Pt nanocrystalline systems with size distributions showing clusters of quasispherical shape in the 1-7 nm range. In particular, EXAFS spectra have been analyzed accounting for the reduction of the coordination number and degeneracy of three-body configurations, resulting from the measured size distribution and expected surface atom contributions. The importance of a correct account of the reduction of the number of neighbors for calculating MS contributions is emphasized in the paper. EXAFS results have been found compatible with x-ray diffraction and transmission electron microscopy investigations. We estimate that EXAFS could be used to study cluster shapes only for sizes below 2 nm using present methods and quality of the experimental data. We have also shown that the local distribution of distances and angles probed by EXAFS is broader than in bulk Pt, with first-neighbor bond length variance and asymmetry increasing upon reducing the particle size. Methods and results presented in this paper have been found to be successful for a robust structural refinement of monatomic nanocrystalline systems and represents a solid starting point for analyzing subtle structural and dynamical local changes occurring during in situ experiments involving nanomaterials for specific applications like supported nanocatalysts

Molecular dynamics (MD) simulations and extended x-ray absorption fine structure (EXAFS) investigations of the structure of lead-silicate glasses, xPbO(1 − x)SiO2, have been undertaken to elucidate the problem of partially contradicting experimental findings reported in the literature about basic structural units and their interconnection. The MD simulations were performed in a wide range of compositions, x = 0.1–0.9. The atoms were assumed to interact by a two-body Born–Mayer–Huggins interaction potential. The EXAFS measurements were performed for x = 0.3, 0.5 and 0.7, and also for pure crystalline (red) PbO at the L3-edge of Pb. The absorption spectra were analysed within the GNXAS approach.Our EXAFS and MD results are in good agreement, and support some previous suggestions that: (1) the PbO4 groups are the dominant structural units in lead-silicate glasses for any concentration and (2) at lower PbO concentrations the co-existence of the PbO4 and PbO3 groups is possible.The medium-range ordering in the simulated glasses has also been investigated in detail. The connectivity of the SiO4 tetrahedra network breaks at about x = 0.45, whereas the Pb structural units form a continuous (mainly edge-sharing) network even at relatively low PbO concentrations (x > 0.2). The cation–anion ring statistics is also discussed.

A commercial fuel cell has been successfully modified to carry out X-ray absorption spectroscopy (XAS) measurements under optimized in operando conditions. The design is conceived for the performance of XAS experiments in transmission mode over a wide range of X-ray energies above 6 keV, owing to the reduced absorption of the cell. The wide angular aperture allows the collection of XAS in fluorescence mode and of X-ray diffraction patterns when needed. Details of the design of the cell and its performances are given. The quality of the extended X-ray absorption fine-structure spectra under working conditions has been verified at the ESRF and ELETTRA synchrotron radiation facilities, showing that relatively fast and low-noise transmission measurements on electrodes over a wide range of catalyst concentrations and energies are feasible.

Structural models obtained using classical molecular dynamics (MD) simulations and realistic interatomic potentials for solid metals are tested using experimental results obtained by x-ray absorption spectroscopy (XAS). Accurate L-edge extended x-ray absorption fine-structure (EXAFS) measurements of Pb grains dispersed in BN and graphite matrices have been collected for temperatures up to the melting point. The thermal expansion of the grains was measured by energy-dispersive x-ray diffraction techniques and found to be coincident with that of pure Pb up to the limit of the present measurements. L3-edge EXAFS measurements of solid Pb at various temperatures have been analysed using advanced data-analysis techniques (GNXAS) based on exact spherical-wave multiple-scattering simulation of the absorption cross-section. Realistic structural models for solid Pb were obtained from MD simulations using an empirical pair potential (Dzugutov, Larsson and Ebbsjo (DLE)), a tight-binding (TB) square-root functional, and an embedded-atom (EA) model potential parametrized by us. The short-range pair distribution function g(r) reconstructed by means of EXAFS is compared with those obtained by MD simulations. The empirical DLE potential, originally designed for the liquid state, is too soft, showing too-large values for the average distance R, variance σ2, and skewness β. The TB and EA potentials are both compatible with XAS data as regards the average distance and skewness of the first neighbours. The distance variance, associated with the thermal vibration amplitudes, is underestimated for the TB potential, while the EA model is found to be in agreement with XAS data. The present results are also compared with those from a previous EXAFS study on solid lead, where the cumulant expansion and a simple one-dimensional anharmonic oscillator model were used. The need for realistic interaction models and appropriate simulation schemes for reliable XAS data analysis is emphasized, while differences from and improvements with respect to previous approaches are only briefly discussed.

We present an X-ray absorption spectroscopy (XAS) study of a low Pt content catalyst layer (Pt loading 0.1 mg cm(-2)) operating at the cathode of a proton exchange membrane fuel cell (PEMFC). This catalyst is based on the use of a mesoporous inorganic matrix as a support for the catalyst Pt nanoparticles. Due to the high Pt dilution, in situ measurements of its structural properties by XAS are challenging and suitable experimental strategies must be devised for this purpose. In particular, we show that accurate XAS in situ fluorescence measurements can be obtained using an optimized fuel cell, suitable protocols for alignment of a focused X-ray beam and an appropriate filter for the background signal of the other atomic species contained in the electrodes. Details, advantages and limitations of the XAS technique for in situ measurements are discussed. Analysis of the near-edge XAS and EXAFS (extended X-ray absorption fine structure) data, corroborated by a HRTEM (high-resolution transmission electron microscopy) study, shows that the Pt particles have a local structure compatible with that of bulk Pt (fcc) and coordination numbers match those expected for particles with typical sizes in the 1.5-2.0 nm range. Substantial changes in the oxidation state and in local atomic arrangement of the Pt particles are found for different applied potentials. The catalyst support, containing W atoms, exhibits a partial reduction upon PEMFC activation, thus mimicking the catalyst behavior. This indicates a possible role of the mesoporous matrix in favouring the oxygen reduction reaction (ORR) and stimulates further research on active catalyst supports.

International audienceThis work reports a detailed investigation of the local structure and chemical disorder of a Pt3±δCo thin film and Pt3±δCo nanoparticles. We have used a combination of techniques including x-ray absorption spectroscopy (XAS), x-ray diffraction (XRD), and high-resolution transmission electron microscopy (TEM). High-quality XAS spectra at the Co K edge and Pt L3 edge have been analyzed using double-edge multiple-scattering data analysis. Structural extended x-ray absorption fine structure (EXAFS) refinements have been performed accounting for the reduction of the coordination numbers and degeneracy of three-atom configurations, resulting from the measured size distribution and stoichiometry. The important effect of chemical ordering on pair and three-atom configurations has been studied using computer simulations based on a simple model accounting for substitutional disorder, defined by an order parameter s. It has been found that individual EXAFS signals related to the minority species (Co) are extremely sensitive to substitutional disorder so their intensities, especially those of the collinear three-atom configurations, can be used as a measure of the ordering level. The thin film has been found to be chemically disordered (s 0.4), in agreement with previous estimates. The Pt3±δCo nanoalloy has been found to be partially ordered (s = 0.6 ± 0.1) while the local structure around Co atoms is characterized by a higher level of structural disorder as compared to the bulk-like thin film. The robust approach for nanomaterial characterization used in this work combining different techniques can, in principle, be applied for structural refinements of any binary nanocrystalline functional system