**1. Introduction**

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

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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 [2], where the Li<sup>+</sup> 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<sup>+</sup> cations. For this reason, many materials able to form lithiated phases have been proposed for playing the role of electrode materials.

**Figure 1.** Schematic representation of the discharge of a Li<sup>+</sup> -ion battery: a graphite-based negative electrode undergoes Li<sup>+</sup> deintercalation, according to the following reaction:.

$$\mathsf{Til}^\*\mathsf{C} \cong \mathsf{C} \star \times \mathsf{L} \mathrm{i}^\*\star \mathrm{x} \mathrm{e}^-$$

Li<sup>+</sup> ions migrate toward the Li1-xCoO2 positive electrode forming the reduced LiCoO<sup>2</sup> :

$$\text{Li}\_{1-x}\text{CoO}\_x \star \text{x} \text{Li}^\* \star \text{x} \text{ e}^- \text{\#}\text{LiCoO}\_x$$

Typical electrolytes are based on a lithium salt (*e.g.* LiPF<sup>6</sup> ) dissolved in a mixture of liquid carbonates (Ethylene carbonate, propylene carbonate, etc.) Reproduction from Ref. [1].

The very large number of possible host materials for Li<sup>+</sup> 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 [9] 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) [10], which is of primordial importance for the stability and the cycle life of the battery, have been thoroughly studied.

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" mecha-

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, respec-

able to form lithiated phases have been proposed for playing the role of electrode materials.

cations are exchanged alternatively between the positive and the negative

cations. For this reason, many materials


:

) dissolved in a mixture of liquid carbonates (Ethylene carbon-

nism [2], where the Li<sup>+</sup>

tively) with the concomitant addition/elimination of Li<sup>+</sup>

52 X-ray Characterization of Nanostructured Energy Materials by Synchrotron Radiation

**Figure 1.** Schematic representation of the discharge of a Li<sup>+</sup>

deintercalation, according to the following reaction:.

Typical electrolytes are based on a lithium salt (*e.g.* LiPF<sup>6</sup>

ate, propylene carbonate, etc.) Reproduction from Ref. [1].

ions migrate toward the Li1-xCoO2

**Lix**

Li1 <sup>−</sup> <sup>x</sup>

**C**⇆**C +** x Li<sup>+</sup> + x e−

positive electrode forming the reduced LiCoO<sup>2</sup>

CoO2 + x Li<sup>+</sup> + x e− ⇆ LiCoO<sup>2</sup>

Li<sup>+</sup>

Li<sup>+</sup>

In such a picture, many characterization methods have been proposed and efficiently used, either simply *ex situ*, *in situ* or under even *operando* conditions for the characterization of the starting materials and of their reaction mechanisms such as X-ray diffraction (XRD) [11], infrared [12], Raman [13], Mössbauer [14] and X-ray photoelectron spectroscopy [15, 16].

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 physicochemical 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 *operando* measurements by collecting XAS spectra during electrochemical cycling using specifically developed *in situ* cells. In this case, the physico-chemical properties and the local structure of the studied element can be monitored at all moments during the charge and discharge processes.

To the best of our knowledge, the first use of XAS in the field of batteries dates back to the paper of Mc Breen *et al.* [17]. Several reviews have appeared more recently, resuming the principal advances allowed by the application of XAS in this research field [18–21]. In this chapter, after a short presentation of the techniques and of the relative experimental methods, a selection of XAS experiments conducted in the field of batteries will highlight the potentiality of the technique in the *in situ* characterization of nanosized, nanostructured and badly organized materials. This knowledge is necessary to obtain a precise description of the electrochemical mechanisms governing battery's chemistry.
