**4.3. Study of Li-sulfur batteries by S K-edge XAS**

also form throughout the whole electrochemical cycle. An example of a complicated reaction

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 [62]. 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 trans-

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 *fcc* lattice of Ni metal. Such a fit, however, gives

effective number of Ni neighbors is much smaller than 12, in line with the formation of Ni nanoparticles with a significant fraction of surface atoms. Such reduced coordination numbers are often observed for supported metal nanoparticles in heterogeneous catalysts with sizes below about 2 nm [64]. The nanosized nature of the Ni particles is also confirmed by the absence of the following coordination shells in the FT signal. The presence of Ni nanoparticles at the end of lithiation and their following (partial) reaction during delithiation to reform

, allowed the author to confirm that NiSb<sup>2</sup>

**Figure 7.** *Operando* evolution of the Ni K-edge EXAFS spectra (left) and corresponding phase-uncorrected FT signals

darker to brighter spectra (only selected spectra are shown for the sake of clearness). Reproduced from Ref. [63].

is directly correlated to the coordination number, such a low value indicates that the

, which reacts reversibly with lithium to form nickel metal and

exhibits a main contribution with a dominant peak at about

<sup>2</sup> = 0.34, *i.e.*, less than half of the usually observed value.

vs. Li metal. Evolution with lithiation is shown on going from

is a veritable conversion

mechanism is that of NiSb2

form (FT) signal of pristine NiSb<sup>2</sup>

an amplitude reduction factor S0

a nanosized form of NiSb<sup>2</sup>

(right) during the first galvanostatic lithiation of NiSb<sup>2</sup>

Sb providing a theoretical capacity of 532 mAh/g [62].

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

*Operando* Ni K-edge XAS was thus used to address this issue [63].

Li3

Since S0 2

material.

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 *et al*. [65], many groups have developed first the use of XANES and more recently that of EXAFS for the study of such systems. In LSB, the positive electrode material is elemental sulfur, which can react with lithium to produce Li<sup>2</sup> S for a theoretical capacity of 1672 mAh/g at about 2.5 V [66]. The practical capacity of such systems is unfortunately much lower, never exceeding 1200 mAh/g. Moreover, LSB suffer from several other drawbacks: the main one is surely the diffusion of polysulfides (Li<sup>2</sup> Sn), produced during the first steps of the reduction of sulfur and highly soluble in the electrolyte, which cause the well-known "shuttle" phenomenon strongly limiting the capacity [67]. Second, solid Li<sup>2</sup> S and elemental sulfur are both insulating and cannot be used as such in normal composite electrodes, fabricated as mixtures of sulfur and carbon powder on aluminum foil current collectors, since their continuous dissolution/precipitation during cycling gradually disconnects part of the active mass, making sulfur progressively electrochemically inactive [68, 69]. All these disadvantages cause rapid capacity fading and low columbic efficiency of LSB.

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 [70]. 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 [71]. 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]. *Operando* XANES spectra fitted by linear combinations of reference XANES spectra of pure sulfur, synthetic polysulfides and Li<sup>2</sup> S allowed following both the evolution of the sulfur species and of their relative ratio along the discharge and the charge process, as well as the variation of the concentration of sulfur in both the cathode and the electrolyte, in line with the diffusion of the polysulfides in the whole battery. More recently, it was also used for the detection of the formation of sulfur radical species [82–84], which were confirmed also by *operando* Raman spectroscopy [53].

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 and the low-voltage discharge plateaus and to confirm the formation of Li<sup>2</sup> S only from the beginning of the low-voltage plateau and to follow its concentration in the electrode.

**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. From Ref. [85].

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).
