X-ray absorption spectroscopy (XAS) as a local structural tool for the study of the electrochemical processes in battery materials is highlighted. Due to its elemental specificity and high penetration of the X-rays in the 4–35 keV range, XAS is particularly suited for this, allowing the study of battery materials using specifically developed in situ electrochemical cells. This energy is required to dislodge one core electron from transition metal or p-group atoms, which are commonly used as redox centers in positive and negative electrode materials. In such a simple picture, the ejected photoelectron is scattered by the surrounding atoms, producing characteristic traces in the X-ray absorption spectrum. Both positive and negative electrode materials (intercalation, alloy and conversion electrodes) can be studied. The chapter starts with an introduction of the context around battery studies, followed by a short explanation of the photoelectric effect at the basis of the X-ray absorption phenomenon and to specific features of XAS. A selection of XAS experiments conducted in the field of batteries will be then outlined, also emphasizing the effects due to nanoscale dimension of the material studied. Finally, a perspectives section will summarize the specific role that this spectroscopy has played in the battery community.
- X-ray absorption spectroscopy
- lithium batteries
- energy materials
- electrode materials
One of the most challenging difficulties that our planet has to face in the next decades is the sustainable use of energy. In particular, the demand for advanced energy storage devices has increased significantly, motivated by a variety of different needs of our technologically driven, highly mobile, energy challenged society. For instance, batteries are the devices that can solve the problems inherent to the intrinsic intermittency of renewable energy sources, since they can store the energy surplus produced in excess when the plant is operating and then feed it to the power grid when there is a peak of consumption. Moreover, they are also targeted to fulfill the ever growing demand of energy for portable applications (mobile phones and computers, and nowadays cars and trucks). The excellent performance and the well-established technology of lithium-ion batteries (LIBs) put them in a crucial position for supporting this new energy revolution. Several post-LIB systems, such as lithium-sulfur batteries (LSBs), lithium-oxygen batteries (LOBs) or sodium-ion batteries (NIBs), have also been proposed in the last years, as sustainable performing alternatives to LIBs.
Differently from other well-established battery technologies, such as alkaline or lead-acid batteries, LIBs (as well as the other post-LIB systems) are based on the famous “rocking chair” mechanism , where the Li+ cations are exchanged alternatively between the positive and the negative electrode during the discharge and the charge process, as shown in Figure 1 . In such a system, the two electrodes can be any sort of material that are able to undergo reversibly to a reduction/oxidation process at a specific high or low potential (for the positive or negative electrode, respectively) with the concomitant addition/elimination of Li+ cations. For this reason, many materials able to form lithiated phases have been proposed for playing the role of electrode materials.
The very large number of possible host materials for Li+ have generated a great deal of works on potential LIBs electrode materials, from the micro to the nanosized range, which may accommodate lithium via different reaction mechanisms, including intercalation [3–5], alloying [6–8] and conversion  reactions. In addition to the reaction mechanisms at the electrodes, other features concerning the electrolytes and their interaction with the electrodes, including the formation of the solid-electrolyte interphase (SEI) , which is of primordial importance for the stability and the cycle life of the battery, have been thoroughly studied.
In such a picture, many characterization methods have been proposed and efficiently used, either simply
X-ray absorption spectroscopy (XAS) can also be counted among the characterization tools used in the field of batteries. Indeed, it is one of the techniques of choice for retrieving structural and electronic information, especially when the materials or some of the species formed through the electrochemical reactions are not crystalline and cannot be studied by diffraction techniques. The main important characteristics of XAS are: (i) its element specificity, which allows the study of a particular element by concentrating on its K (or in some cases L) absorption edge; (ii) the possibility of tuning it to different sites (for instance Fe and P in LiFePO4), thus providing sources of complementary information on the same compound; (iii) the physico-chemical information contained in the near-edge structure of the XAS signals, which can be used to reveal the formal oxidation state and the local symmetry of the probed atom; (iv) the possibility of doing
To the best of our knowledge, the first use of XAS in the field of batteries dates back to the paper of Mc Breen
2. X-ray absorption spectroscopy (XAS)
XAS, also known as X-ray absorption fine structure (abbreviated as XAFS) spectroscopy, is a powerful tool that provides information on a very local scale (4–5 Å) around a selected atomic species and is well suited for the characterization of not only crystals but also materials that possess little or no long-range translational order. It is based on the absorption: when a sample is exposed to X-rays, it will absorb part of the incoming photon beams, which is mainly generated by the photoelectric effect for energy in the hard X-rays regimes (3–50 KeV). XAS is even selective for the atomic species and also allows us to tune the X-rays beam selectively to a specific atomic core (the absorption energy of next elements are sufficiently spaced), and therefore it probes the local structure around only the selected element that are contained within a material. The element-specific characteristic of XAS, providing both chemical and structural information at the same time, differentiates it from other techniques, such as the X-ray scattering. In this respect, it serves as a unique tool for the investigation of battery materials during charge-discharge cycles.
XAS experiment measures the absorption coefficient μ as a function of energy
XAS (or XAFS) is generally used to refer to the entire spectrum, which is constituted by the edge region called X-ray absorption near edge spectroscopy (XANES), which is limited at the first 80–100 eV above the edge, and a post-edge region extended X-ray absorption fine structure (EXAFS), which is extended up to 1000 eV above the absorption edge. The distinction between XANES and EXAFS remains arbitrary, but some important approximations in the theory allow us to interpret the extended spectra in a more quantitative way than is currently possible for the near-edge spectra. The XANES region, comprising the pre-edge and the absorption edge itself, is strongly sensitive to oxidation state and coordination chemistry of the absorbing atom of interest. The EXAFS region has been largely exploited to gain quantitative structural information such as first shell distance of the metal site and the coordination number. EXAFS comprises periodic undulations in the absorption spectrum that decay in intensity as the incident energy increases well over (~1000 eV) the absorption edge. These undulations arise from the scattering of the emitted photoelectron with the surrounding atoms. A striking feature of XAFS is that this technique can be applied to all states of matter, and for both crystalline and amorphous materials, it has been used with great success in many research fields, such as liquids , catalysis [23–25], biology , inorganic metal complexes  and electrochemical interfaces . Several excellent books are also available [29–32]. The website of the International XAFS society is reachable at http://www.ixasportal.net/ixas/.
When discussing XAS, we are primarily concerned with the absorption coefficient
Normalization to the density of the material results quite convenient, as different states of matter may be analyzed: the mass absorption coefficient
X-rays ionize and the absorbing atom turns to an excited ion after the electron liberation. Relaxation may occur in two different ways: (i) the core-hole may be filled by a higher-energy electron and the energy difference is released as a second photon, whose energy is smaller compared to that of the primary absorption, for an inner transition occurs (the detection of which is at the basis of another x-ray analytical technique, X-ray Fluorescence Spectroscopy—XFS) or (ii) an Auger secondary electron may be freed, after having absorbed the second photon. The measurement of these electrons is made possible by Auger spectrometers. In the soft X-ray region (<2 keV), the Auger process is more likely to occur, unlike for higher energies where X-ray fluorescence dominates.
2.1. Extended X-ray absorption fine structure (EXAFS)
When X-ray is absorbed by a core-level electron, a photoelectron with wavevector
The appendix 0 indicates the value for an isolated atom. It is remarkable here that due to its
If other atoms are located in the vicinity of the absorber (the central atom), the photoelectron is scattered by the neighbors (yellow atoms) and so does every atom in the material. The incoming and the scattered wave interferes either constructively or destructively as a function of the energy of the X-ray beam. Therefore, the observed absorption coefficient is expected to vary periodically as a function of the energy as depicted at the bottom right of Figure 3 . In the latter case, the total absorption coefficient
This allows one to extract the oscillations from a raw experimental spectrum:
For practical purpose, the denominator is often replaced by
Within this simple description, the EXAFS can be represented by an oscillation, which of course can be described by terms of amplitude and phase. In a first approximation, the amplitude term depends on the nature and the number of near neighbors around the central atoms and the phase on the mutual distance photoabsorber scatterer. This leads to a simple expression for EXAFS in terms of different parameters affecting the fine structure:
Several effects have to be taken into account to complete the description of real systems, and they all can be considered damping terms. They are (i) the structural and thermal disorder; (ii) the limited mean free path of the photoelectron; and (iii) the relaxation of all the other electrons in the absorbing atom in response to the hole in the core level. The first term is due to the fact that atoms in matter vibrate around their equilibrium position depending on temperature. This atomic motion reduces the EXAFS amplitude, and a term called the EXAFS Debye-Waller factor
This is valid for the plane wave approximation,
EXAFS data analysis is normally done by using code programs, which permit to calculate the theoretical EXAFS spectrum based on
Two more considerations should be made on EXAFS data analysis. The first is that XAS (and therefore the results obtained by an EXAFS analysis) is a bulk technique and thus all the atoms irradiated by the beam contribute to the overall XAS spectrum. The same is true in the case of a multicomponent system (for instance two phases in equilibrium of a polymorphic species). Each component or phase gives its contributions. An example to disclose the simple component of a species, such as in the case of gold nanoparticles and its precursors, appeared . Alternatively, an efficient use of chemometry has been proposed for the analysis of XAS data in such cases . This approach has interesting implication for the interpretation of spectra recorded during an
The second consideration concerns the EXAFS data analysis of nanoparticles and nanostructures [41, 42]. This issue has been addressed for metal nanoparticles first , evidencing that by decreasing the size of the material there is a significant effect on the observed coordination number, due to the increased surface/bulk ratio. A specific example of this effect on a battery material will be presented in the case study section.
2.2. X-ray absorption near edge spectroscopy (XANES)
The XANES region is sensitive to the geometrical structure of the metal center but also probes its effective charge. It turns out that the position of the edge (which can be evaluated by the edge inflection point) is shifted to higher energies when the formal valence of the photo-absorber increases. Below the absorption edge, the presence of pre-edge structures can be observed . The occurrence of this peak in a metal (first raw transition metal) K-edge is due to 1s-3d electronic transition  that is electric-dipole forbidden but quadrupole allowed. Its intensity can be used as a probe for geometry, as the geometrical distortion of the metal core from centrosymmetric coordination favors the transition, while the energy position is relative to the metal core formal oxidation state. This fact is frequently used for investigating the charge associated to positive- and negative-electrode materials during reduction and oxidation reactions in batteries.
If we now consider the form of the absorption edge, it can be seen that it reflects the empty density of states and it strongly depends on the coordination, while the forms of the absorption traces up to 60–80 eV are due to the multiple scattering resonances of the ejected photoelectron. Several computer codes can simulate the XANES spectrum, such as above-mentioned FEFF, MXAN , FDMNES  and CTM4XAS , which are useful for the analysis of metal L-edges.
3. Investigating a battery at work:
ex situand in situ( operando) studies
The simplest way to study the structural end electronic modification of a cathode or anode material is by
Figure 4 displays two different types of
4. Case studies in battery materials
Given the large amount of physico-chemical information that it usually carries, already mentioned in the previous sections, XAS has been largely applied to the study of battery materials [18, 19]. A few particular case studies, specifying specific features of this technique in particular cases involving nanostructured species, are presented in the following paragraphs. Itwill be stressed, in particular, the importance of performing
Ex situstudies of lithium-excess manganese layered oxides
The relative abundance of manganese coupled with their variety of oxides structures, which provides generally a three-dimensional array of edge-shared MnO6 octahedra for the lithium insertion and release, has aroused the interest of developing positive-electrode materials based on manganese oxide. Due to the well-known poor cycling capability of the spinel structure LiMn2O4, where a cooperative Jahn-Teller distortion of the Mn3+ ion causes a cubic-to-tetragonal phase transition leading to a rapid degradation of the electrode, an intensive research has been focused on alternative materials. Solid solutions of layered cathode materials such as the combination of Li2MnO3 and LiMO2 (M = Mn, Co, Ni, etc.) have been proposed as promising candidates for cheaper, higher capacity and safer positive electrode for lithium batteries. However, the occurrence of an initial activation process during the first delithiation step (first charge) is always accompanied by a large irreversibility in terms of specific capacity. To gain a deeper understanding of the initial activation step and to study the following delithiation-lithiation process, an electronic and local structural characterization of the host material is required and the XAS is the technique of choice. A series of electrodes with different lithium concentration (state of charge, SOC) were studied in a series of lithium-rich, cobalt-poor Li[Li0.2Ni0.16Mn0.56Co0.08]O2 electrode material (NMC), as an examples of
Figure 5 shows the voltage profile of the cell during charge-discharge operation. The numbered points in the curve indicates predetermined states of charge (SOC) at which cells were prepared for the XAS measurements. Figure 6 summarizes the XAS analysis conducted on the materials, where all the several portions of the X-ray absorption spectrum carry valuable information on the local and electronic structure: pre-edge, XANES and EXAFS. The pre-edge analysis (the Mn K-edge is displayed in the figure, showing two components) allowed the authors to check the variation of the Mn local site, in terms of symmetry and charge. XANES traces can provide the identification of the electroactive sites at different SOC and the EXAFS analyses the local structural information of the selected metal site. This information is complementary with respect to XRD which probes the long-range order in crystalline materials.
The study here highlighted demonstrates that the manganese is not taking part of the initial electrochemical oxidation process, but a complete Ni2+/Ni4+ and a partial Co3+/Co4+ redox processes occur during the first charge of the battery. The electrochemical performance of the material, considering the full and partial redox inactivity of Mn and Co, also reveals the participation of oxygen in the overall electrochemical redox process. Analysis of EXAFS at the three metal edges has revealed that the first charge of the lithium-rich cathode can be described by two separate reactions occurring at the two components, Li2MnO3 and LiMO2: an activation of the Li2MnO3 component with a phase transition to an
4.2. Study of the conversion reaction in electrode materials: the case of NiSb2
A particularly interesting case for the application of
Conversion reactions were first verified for transition metal oxides , but are rather common also for other chalcogenides, pnictogenides and carbon group semimetals. Conversion materials,
MaSbb compounds are expected to react with lithium by forming a matrix of Li3Sb in which nanoparticles of the transition metal M are embedded. Actual reaction mechanisms, however, can be more complex and often dependent on the specific compound. For instance, several conversion pnictogenides, such as FeSb2  and MnSb , form intermediate lithiated insertion phases before starting the veritable conversion reaction, while additional phases could also form throughout the whole electrochemical cycle. An example of a complicated reaction mechanism is that of NiSb2, which reacts reversibly with lithium to form nickel metal and Li3Sb providing a theoretical capacity of 532 mAh/g .
In this material, the possible formation of an intermediate ternary insertion solid solution was suggested by a slight shift of the XRD reflections during the first part of the discharge . The complete amorphisation of the system during the conversion, however, made it impossible to follow the reaction by XRD. In particular, the formation of Ni nanoparticles at the end of discharge, which are expected for typical conversion reactions, could not be verified.
The EXAFS data collected during the first discharge are shown in Figure 7 . The fourier transform (FT) signal of pristine NiSb2 exhibits a main contribution with a dominant peak at about 2.4 Å and a second smaller peak slightly below 2 Å, and a second contribution with a dominant peak at 4.2 Å. During lithiation, the first contribution is gradually replaced by a peak pointing at about 2.2 Å, while the peak at 4.2 Å gradually disappears. The spectrum of the fully lithiated material was fitted using 12 Ni neighbors at 2.47(1) Å. This result agrees well with the Ni − Ni distance of 2.491 Å in the
At the end of this paper, the authors compared the
4.3. Study of Li-sulfur batteries by S K-edge XAS
One of the most interesting recent applications of XAS to electrochemical energy storage concerns the study of lithium-sulfur batteries (LSBs). Since the work of Jie
Several improvements have been suggested in the last years to tackle these drawbacks: one of them consisted in infiltrating molten sulfur into porous conductive carbon materials . This approach, however, does not allow large sulfur loadings, nor does it prevent the diffusion of polysulfides outside the pores. Moreover, it requires large amounts of electrolyte to wet the large volume of porous carbon and to solubilize the polysulfides, which greatly reduces the volumetric energy density of LSB. Most recently, multifunctional positive electrodes, enhancing the sulfur loading and promoting the interaction of polysulfides with the electrode host to prevent their diffusion in the electrolyte have been successfully proposed and studied . In all these studies, XAS has been largely used at different levels to investigate in detail the electrochemical mechanism and the diffusion (or retention) of polysulfides as well as the possible different failure paths.
Sulfur K-edge XANES, for instance, can be used as a semiquantitative analytical tool for LSB [72–81].
A particularly interesting approach was, however, the application of EXAFS to the study of the electrochemical mechanism . Such study was possible only due to the use of a specific sulfur-free electrolyte salt, which usually hindered the EXAFS contribution of the sulfur species evolving during cycling (cf. Figure 8 ). In this way, it was possible to clearly identify the type of polysulfides (long- or short-chain) formed in the electrode during the high-voltage and the low-voltage discharge plateaus and to confirm the formation of Li2S only from the beginning of the low-voltage plateau and to follow its concentration in the electrode.
Finally, XAS was very recently used for detecting the interaction of sulfur precursors with appropriately modified graphene oxide nanocomposites, leading to the immobilization of the sulfur species in the electrode, improving the overall cycling performance of the cell .
All these examples underlined the powerful properties of XAS for the
5. The chemometric approach to the interpretation of XAS data
Due to the increasing performance of many synchrotron beamlines specialized in
With the increasing demand of energy resources for both portable and storage purposes, there has been an extensive and increasingly diversification of materials and technology for the electrochemical power sources in the last five years. Not only lithium-ion technology but also sodium or even trivalent ions, also in aqueous media, are currently studied to obtain a good balancing between cost, safety, abundance and electrochemical performances. This chapter has underlined the strength of the XAFS probe to understand the dynamic of the both anode and cathode materials during the battery functioning, at atomic level. We feel that this core-level spectroscopy can even meet the increasing demand of deep understanding of different technologies and of new materials for batteries. This extraordinary versatility is due to: (i) the extremely selective local structure probe of XAS for the atomic species in crystalline, amorphous solid and liquid electrolyte; (ii) the unprecedented quality and speed of for data recording in synchrotron beamlines dedicated to
Moreover, new advanced synchrotron-based techniques are expected to be at the forefront of battery research in the future; among them, there will surely be X-ray transmission microscopy, which allows the simultaneous imaging and spatially resolved XAS study of electrode materials in batteries .
Finally, a personal consideration: in XAS, data analysis is usually considered as the bottleneck of the whole spectroscopic study. This holds true regardless of the simplicity or the difficulty of the oscillatory portion of the spectrum to be analyzed. Indeed, as long as a suitable structural model has not been established, an oscillation can be interpreted in several different ways. It is then recommended to newcomers not only to learn how to conduct XAS experiments, but also to perform appropriate data analyses by seeking the advice and collaboration of experts who are willing to share their knowledge and their experience.
Work supported by RFO funding (University of Bologna). Thanks are due to staff at both Sincrotrone Trieste and Synchrotron SOLEIL for assistance during the experiments and for providing synchrotron radiation.