The phenomenon of X-ray absorption is indeed at the origin of discovery of X-ray radiation itself by William Conrad Röntgen in 1895. Nevertheless, it is only in 1971 that a seminal paper allows the development of the corresponding spectroscopy, in conjunction with the use of synchrotron radiation. Since then, X-ray absorption spectroscopy (XAS) has become a widely used technique in many scientific fields. The purpose of this chapter is to give a good overview of XAS from theory, experiments, to data treatment and applications in photochemistry. After a brief introduction devoted to the history of XAS, X-ray matter interactions will be described. Then, we will focus on XAS itself, from basic knowledge to the more recent experimental developments.

It is a common practice to alter the undesirable behavior of clayey soils (i.e. swelling-shrinkage and high plasticity) with lime addition. However, the reaction mechanism involved in the early stage (first minute up to 24 h after mixing) of lime treatment is still not well explored. In the present work, the surface chemistry of lime ([Ca] = -22 mmol/l and pH = 12.63) treated kaolinites (Kaol) was studied using argon and nitrogen gas adsorption. The effect of pH ([Ca] = 22 mmol/l and. pH = 12.63 vs. [Ca] = 22 mmol/l and pH = 7) was also examined. The derivative low-pressure adsorption isotherms were analyzed using derivative isotherm summation (DIS), focusing on the analysis of surface heterogeneity. After the treatment with lime, the specific surface area and adsorption energy distribution of Kaol were modified. The adsorption of the calcium cation at both investigated pH (i.e. pH 7 and pH 12.63) takes place on the basal and lateral face of Kaol, however, the species of adsorbed calcium cation appeared to be different. The treatment also leads to a decrease in basal surface area due to basal face-basal face particle aggregation. The basal surface of Kaol treated with lime exhibited polar sites when probed with nitrogen molecule. These sites may have promoted linkage and flocculation of Kaol particles, which in turn play a role in the short-term modification of the macroscopic behavior of lime treated kaolins.

Soils stabilization with lime addition is a widespread technique in geotechnical field. Better understanding of interaction mechanism is central for optimization of the technique. In the present work, the short-term kaolinitelime solution interfacial chemistry and the effect on the dissolution and long-term reaction has been investigated. Calcium adsorption from saturated lime solution (i.e. simulated alkaline pore solution ([Ca] = 22 mmol/l and pH = 12.63)) characterized by initial fast uptake followed by slower adsorption rate curves, whereas only initial fast uptake is identified from chloride solution ([Ca] = 22 mmol/l and pH = 7). Species of calcium available for adsorption are strongly dependent on the pH conditions (i.e. Ca2+ at pH 7 and CaOH+ at pH 12.63). The adsorption increased with increasing initial concentration of calcium. On the other hand, the more the quantity of calcium adsorbed, the less the measured concentration of silicon and aluminum in the supernatant. The adsorbed calcium distribution has been mapped using mu-XRF. Regardless of the quantity of calcium adsorbed, homogeneous distribution has been identified suggesting coating of kaolinite particle by adsorbed calcium. The coating in turn prevented the kaolinite particles from alkaline attack and retards long-term pozzolanic reaction.

The original goal of our study is to synthesize by co‐evaporation the phase that could be formed at the interface between polycrystalline p‐Cu(In,Ga)Se2 treated with KF and n‐CdS. Hence, a new buffer layer, CdIn2S4 (C24), deposited by co‐evaporation is presented for the use in thin film solar cells, exhibiting device efficiencies as high as 16.2%, comparable to that obtained on a reference standard CdS‐buffered device. The physico‐chemical and optical properties of close to stoichiometry 400 nm‐thick films of C24 show similar properties to what has been reported in the literature for single crystals. The layer stack used for solar cells is investigated by transmission electron microscopy, showing the formation of an ultrathin Cd‐deficient C24 layer at the CIGSe/C24 interface, while a clear lattice match is observed at the C24/ZnO interface. Advanced electrical characterizations of the devices suggest that the output voltage and fill factor of the solar cells based on Cu(In,Ga)Se2/(PVD)C24 are limited by tunneling‐enhanced recombination through extended band tail states. These results open new routes to explain the superiority of wet processes used for the junction formation compared to vacuum‐based approaches.

Recent breakthroughs in Cu(In,Ga)Se2 (CIGS) thin film solar cell energy conversion efficiency are related to the application of a potassium fluoride post‐deposition treatment (KF‐PDT) to the completed absorber. Using X‐ray photoelectron spectroscopy and Raman scattering, we compare CIGS layers prior and after the KF‐PDT in the case of a deterioration and an improvement of the solar cells photovoltaic performance. The purpose is to study and model the modification of the surface in both cases and address some of the required characteristics of the absorber, grown on soda lime glass by 3‐stage process, in order to take advantage of the treatment. We show that, in both cases, KF‐PDT induces the formation of GaF3, which is removed during the subsequent chemical bath deposition of CdS, explaining the Ga depleted absorber surface, already reported in literature. However, the presence or not of an ordered defect compound (ODC), correlated with the third stage duration during the CIGS growth, is shown to be crucial in the modifications of the surface induced by the treatment. When an ODC is present prior the treatment, KF‐PDT leads to the formation of a surface layer of In2Se3 containing K, and the photovoltaic performance of completed solar cells are improved. When no ODC is present prior KF‐PDT, no trace of K is found at the absorber surface after the treatment, copper (Cu) segregates into detrimental CuxSe phases, high amount of elemental Se is formed, and the photovoltaic performance are lowered. The role of the ODC during the KF‐PDT is finally discussed.

Cu(In,Ga)Se2-based thin film solar cells have reached 22.3% energy conversion efficiency. This outstanding level of performance has been made possible by the use of a so-called potassium fluoride postdeposition treatment (KF-PDT) after the absorber synthesis. Such a treatment, consisting of evaporating KF under Se atmosphere, has been suggested to enhance the formation of a KInSe2 surface layer. In this paper, we propose an alternative to the standard KF-PDT, in which we simultaneously evaporate KF and In under Se atmosphere. We compare by X-ray photoemission spectroscopy the modified absorber surfaces in both cases and discuss the advantages of this alternative in terms of robustness and rapidity of the process.

This paper deals with the effects of lime addition on clay materials properties, in terms of mechanical and physico-chemical evolution. A coupling between these two aspects was undertaken in order to explain the mechanical behaviors of lime-treated clay materials. The study was performed on two different clays, a kaolin and a Ca2+-bentonite. Lime addition leads to an increase of mechanical properties of these clays. These improvements are faster with Ca2+-bentonite, even if at long term the performance reached for kaolinite is higher. The coupling between macroscopic and physico-chemical aspects shows that these improvements are linked to the development of secondary compounds such as calcium silicate hydrates, hydrogarnet and calcium carboaluminate hydrate phases.

Cu(In,Ga)Se-2-based thin film solar cells have reached 22.3% energy conversion efficiency. This outstanding level of performance has been made possible by the use of a so-called potassium fluoride postdeposition treatment (KF-PDT) after the absorber synthesis. Such a treatment, consisting of evaporating KF under Se atmosphere, has been suggested to enhance the formation of a KInSe2 surface layer. In this paper, we propose an alternative to the standard KF-PDT, in which we simultaneously evaporate KF and In under Se atmosphere. We compare by Xray photoemission spectroscopy the modified absorber surfaces in both cases and discuss the advantages of this alternative in terms of robustness and rapidity of the process.

In the paper an experimental multi-scale investigation on the influence of pore water chemistry on the short-term behaviour of lime treated soil is presented. Sedimentation tests on kaolin suspensions under different chemical environment have been performed and interpreted, taking into account the effects of clay particles interactions on the soil skeleton formation. The experimental evidences at volume scale of the samples have been directly related to the results of micro-scale investigations. Modification of surface charge and its influence on soil fabric has been investigated by means of zeta-potential measurements, Dynamic Light Scattering (DLS) and Scanning Electron Microscopy (SEM). The effects of lime content, cation valence and source of calcium ions on the surface charge of kaolin particles have been considered. The addition of lime increased the average size of particles aggregates in the short term. The fabric evolution affected the formation of the soil skeleton during sedimentation, influencing its void ratio and compressibility, highlighting the link between microstructural features and mechanical behaviour of lime treated samples.

In general, electrochemistry is based on electron exchange between different species (ions, atoms, molecules,grains…), in a liquid or solid environment, and this electron exchange is usually written in using the concept of oxidation state of the different elements constituting the species. Redox processes are thus introduced, where oxidation and reduction for a specie correspond to the loss or a gain of electrons, respectively. This concept is generally well adapted to electrochemistry because the oxidation state of an element corresponds to the number of electrons which is lost or gained in a chemical bond, supposed to be purely ionic, to form a cation or an anion, respectively. The electronic structure of an ion or a molecule is described by discrete energy levels which are occupied by a small and integer number of electrons. So, during an electrochemical process, electrons are added or removed and this electronic transfer can be explained by using quite simple concept. The problem is different in a solid, The problem is different in a solid for which the only way to describe the electronic structure is to use the band structure model: atomic orbitals of all elements are more or less mixed together to form the bands. Consequently, the number of valence electrons is very large and the ionic model is very often unsuitable. In this presentation we will show through different examples that use of the oxidation state can be misleading in order to explain the true behavior of a solid electrode material in lithium batteries. We will propose a more general description. First, we will focus on X-ray Absorption Spectroscopy (XAS), one of the most powerful tools to analyze precisely the state of charge of an element. It has been showed many times, on chalcogenides (1) and oxides (2), that this technique combines very nicely with the calculated electronic structure of a solid. It will be evidenced that in a redox process all elements of a material are modified by the electron transfer. Secondly, we will show how the oxidation state concept could lead the researchers to exclude a priori some reactions because they would put some elements in unusual low or high oxidation states (3). Finally, we will show recent results obtained on Li 2 MnO 3 and HE-NMC materials (4-6). From them, it appears clearly that the oxidation and reduction processes in solid are complex phenomenon. They generally impact all elements which have to be considered as possible redox centers. The general model we propose is based on solid electronic band structure for solid, considered as an electron reservoir which can be emptied (oxidation) or filled (reduction). The reservoir is built a priori by the orbitals of all elements. The fact that an electrochemical reaction occurs, becomes reversible, or leads to degradation of the material, has to be discussed in terms of energetic stability of a given atomic structure. This can be related to the shape and the filling of the electronic reservoir. References 1 – Z.Y. Wu, G. Ouvrard, S. Lemaux, P. Moreau, P. Gressier, F. Lemoigno and J. ROUXEL, Phys. Rev. Lett. 77 (1996) 2101-2104. 2- F. Boucher, N. Bourgeon, K. Delbé, P. Moreau, D. Guyomard and G. Ouvrard, Journal of Physics and Chemistry of Solids, 67 (2006) 1238. 3 – N. Tran, L. Croguennec, M. Ménétrier, F. Weill, Ph. Biensan, C. Jordy, and C. Delmas, Chem. Mater. 20 (2008) 4815–4825 4 – H. Koga, L. Croguennec, M. Ménétrier, P. Mannessiez, F. Weill, C. Delmas, and S. Belin J. Phys. Chem. C 118 (2014) 5700−5709 5 – Yukinori Koyamaa, Isao Tanakaa, Miki Nagaob, Ryoji Kanno Journal of Power Sources 189 (2009) 798–801 6 – A. Pradon, C. La Fontaine, S. Belin, P. E. Petit, P. Moreau , , L. Lajaunie, E. Dumont, E. Elkaim, C. Tessier, G. Ouvrard, M.T. Caldes, Submitted to Chemistry of Materials

Li-rich layered oxides Li1+xM1-xO2 (M = Mn, Co, Ni) can be considered as good candidates for use as positive electrode in LIBs regarding on their high specific capacity (250mAh/g), only achieved by loading at high potential (> 4.3 V). Under these charging conditions, a voltage plateau appears at the end of the charge and an extra capacity is obtained. However, the first cycle is performed with a significant irreversible capacity and coulombic efficiency ranges between 85-90 %. Moreover, Li-rich compounds undergo notable capacity loss and voltage decay during cycling. These drawbacks prevent these materials to be commercialized so far and need further understanding. Redox phenomena at the origin of voltage plateau, have been extensively studied in literature [1]. However, even if irreversible capacity is often related to irreversible structural transformations undertaken by these compounds during the plateau, some questions still remain. What is the exact nature of this structural transformation? Are all chemical elements involved in the same way into the irreversible structural changes? How does this transformation impact redox mechanisms?

Lime is a widely used chemical additive in the stabilization of problematic soils. However, the physico-chemical mechanism involved, particularly in the short-term, is still not fully understood. In the present work, the influence of hydrated lime (Ca(OH)2) on the rheological properties of kaolinite dispersion has been investigated. The influence of the type of cation and pH on the interfacial chemistry and particle interaction were also examined. The result showed that kaolinite predominantly adsorbs Ca2+ and CaOH+ at pH=7 and pH=12.6 respectively. With increasing concentration of Ca(OH)2, the value of storage modulus (G′), Bingham yield stress (τB) and cohesive energy density (Ec) initially decreased at lower concentration (≤5.5mmol/l) followed by increase at higher concentration (≥11mmol/l); in contrast these parameters decreased monotonously with increasing the concentration of NaOH. The strain hardening characteristic of loss modulus (G″) curve was found to be very sensitive to change in surface chemistry and the associated particle organization. The modification in the mechanical properties of the kaolinite with varying amount of Ca(OH)2 was attributed to the aggregation microstructure of kaolinite particles. The Ca-ion at higher pH promotes linkage between particles and provides the most efficient way to form dense, tightly packed flocs, which behave as individual coarse grained materials (silt or sand).

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. References: [1] H. Koga et al., Journal of the Electrochemical Society, 160 (2013) A1 [2] H. Yu et al., the Journal of Physical Chemistry Letters, 4 (2013) 1268

To improve the performance of the positive electrode in batteries, it is important to thoroughly characterize these materials during battery cycling, beyond just electrochemical characterization. Among the very few techniques available for operando studies, X-ray Absorption Spectroscopy (XAS) appears very powerful, with high spatial and time resolutions provided by synchrotron radiation. Moreover XAS is well known for its capabilities in charge transfer and structure determinations, both being the major changes induced by electrochemical cycling of electrode materials. By an appropriate combination of three XAS beamlines using a specially designed electrochemical cell, we have studied composite positive electrodes made with LiFePO4 as the active material. We directly observed the heterogeneity of the electrode during operation, some parts being delayed and others advanced, compared to the mean charge state of the electrode. A mapping of this heterogeneity was made at different length scales.

Lime is widely used as additive to improve the mechanical properties of natural soil used in earthworks. However, the physico-chemical mechanisms involved are yet not well understood. In order to develop and optimize this treatment method, a better understanding of the interaction between lime and the minerals of the soils, in particular clay minerals, is required. In this study, Ca-bentonite was treated with 2, 5 and 10wt.% of lime during 1 to 98days. Modifications in the Si local environment were then monitored by solid state nuclear magnetic resonance to investigate the pozzolanic reaction. All the soil mineral phases contribute to the release of Si and to the pozzolanic reaction, with a rapid and total consumption of Si-polymorph and an exacerbated dissolution of montmorillonite. Mechanism of C–S–H formation, function of the Ca content in the system, was found to match the sorosilicate-tobermorite model described in cement systems.

Operando study of lithium batteries Lithium battery materials evidence redox processes and structural changes directly related to the electrochemical performances: energy, capacity, reversibility and life time. Many characterization techniques are used in order to explain the general behaviour. Most of them are only operating after equilibrium of the battery, and even after withdrawal of the material under study. This paper shows how the high performances of synchrotron sources allow the materials characterization during the functioning of the battery, in using X-ray diffraction and absorption. In doing so, it is possible to directly correlate the structural and electronic characteristics of an electrode and its electrochemical performances.