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.