**6. Perspectives**

A particularly interesting approach was, however, the application of EXAFS to the study of the electrochemical mechanism [85]. 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

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 [86]. All these examples underlined the powerful properties of XAS for the *operando* study of electrochemical mechanisms in batteries even at low energies (sulfur K-edge is at only 2.47 keV).

**Figure 8.** Variation of the average *S* coordination number during the first discharge. The average coordination of the most important polysulfides is reported for comparison. The vertical line represents the end of the high-voltage plateau.

Due to the increasing performance of many synchrotron beamlines specialized in *in situ* XAS studies, extremely large dataset containing many tens or hundreds of spectra associates to a single experiment are currently collected. This huge amount of data is calling for a suitable strategy for their treatment in reasonable time. For instance, the study of the charge or the discharge process of a battery produces something like 100–300 spectra, depending on the experimental conditions (data acquisition protocol and battery discharge rate). In similar cases, the

**5. The chemometric approach to the interpretation of XAS data**

From Ref. [85].

S only from the

and the low-voltage discharge plateaus and to confirm the formation of Li<sup>2</sup>

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

beginning of the low-voltage plateau and to follow its concentration in the electrode.

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 *in situ* studies, coupled with a suitable and unbiased data analysis such as the chemometric approach to XAFS data presented above; (iii) the new generations of software for EXAFS data analysis, which are capable of analyzing multiple scattering contributions with great efficiency and to perform simultaneous multiple edge fits; (iv) the development of reliable spectrometers at synchrotron radiation light sources enabling high resolution recording, allowing the collection of complementary information with ancillary advanced techniques such as resonant inelastic X-ray scattering (RIXS) [91].

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 [92].

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.
