**4.2. Study of the conversion reaction in electrode materials: the case of NiSb<sup>2</sup>**

A particularly interesting case for the application of *operando* XAS is that of electrode materials undergoing a so-called conversion reaction, which was reviewed a few years ago by Cabana *et al.* [9]. In a conversion reaction, lithium reacts with a binary compound containing a transition metal (M = Ti, Mn, Fe, Co, Ni, etc.) and a group *p* element (X = O, P, Sb, Sn, etc.), according to the following equation:

$$M\_{\tiny\textsc{a}}X\_{\texttt{b}} + (\mathsf{b} \cdot \mathsf{n})\mathit{Li} \not\cong M + \mathsf{b} \cdot \mathit{Li}\_{\textsc{n}}X$$

Conversion reactions were first verified for transition metal oxides [56], but are rather common also for other chalcogenides, pnictogenides and carbon group semimetals. Conversion materials, *i.e.*, materials reacting through the conversion reaction allow reversible capacities as high as 1500 mAh/g, exceeding that of graphite (372 mAh/g), the negative electrode material commonly used in commercial Li-ion batteries. They have thus been considered as possible alternatives for the development of new high-energy storage devices. Recent studies have shown that, for conversion reactions, due to the formation of nanosized species, the composites obtained at the end of discharge are particularly unstable [49] and therefore the use of *operando* techniques for the study of reaction mechanisms is essential. Transition metal antimonides of general formula M<sup>a</sup> Sbb form a family of conversion materials providing capacities between 450 and 600 mAh/g and can easily stand up to about 20 cycles at stable capacity before fading. The very large volume expansion (of about 300%) experienced during the reaction with lithium is probably at the origin of the rapid fading, causing the pulverization of the active material particles, with further degradation of the electronic wiring at high-rate and agglomeration of the active mass at low rate [57]. Several methods were used to improve the cycling life of antimonides such as nanostructuration of the electrodes [58], carbon coating and optimization of the formulation [59].

Ma Sbb compounds are expected to react with lithium by forming a matrix of Li3 Sb 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 FeSb<sup>2</sup> [60] and MnSb [61], form intermediate lithiated insertion phases before starting the veritable conversion reaction, while additional phases could

**Figure 6.** XAS data analysis for the cathode material. The picture displays analysis of the pre-edge data obtained at the Mn K-edge (left panel) including the fitting of the observed peaks (at the bottom). These data provide both charge and symmetry information around the investigated metal. Data at the right panel refer to XANES behavior at the Ni K-edge (up) and the best-fit of the EXAFS data in terms of single contribution to the total EXAFS oscillation (right). At the bottom the fourier transform (FT) behavior of the corresponding EXAFS is displayed. Reproduced from ELETTRA Highlights 2014–15, page 12.

**Figure 5.** Voltage profile of two successive charge and discharge curves of Li-rich NCM at 20 mA/g. Representative points of 1–10 in the process of XAS measurements are indicated. Reference and counter electrode: Li. electrolyte: 1

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

in EC/DMC. Temperature: 20°C ± 2°C. On the upper X axis the capacity detected in each step is reported.

M LiPF<sup>6</sup>

Reproduced from Ref. [54].

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 Li3 Sb providing a theoretical capacity of 532 mAh/g [62].

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

The EXAFS data collected during the first discharge are shown in **Figure 7**. The fourier transform (FT) signal of pristine NiSb<sup>2</sup> 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 *fcc* lattice of Ni metal. Such a fit, however, gives an amplitude reduction factor S0 <sup>2</sup> = 0.34, *i.e.*, less than half of the usually observed value. Since S0 2 is directly correlated to the coordination number, such a low value indicates that the 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 a nanosized form of NiSb<sup>2</sup> , allowed the author to confirm that NiSb<sup>2</sup> is a veritable conversion material.

**Figure 7.** *Operando* evolution of the Ni K-edge EXAFS spectra (left) and corresponding phase-uncorrected FT signals (right) during the first galvanostatic lithiation of NiSb<sup>2</sup> vs. Li metal. Evolution with lithiation is shown on going from darker to brighter spectra (only selected spectra are shown for the sake of clearness). Reproduced from Ref. [63].

At the end of this paper, the authors compared the *operando* spectra with those of *ex situ* samples cycled vs. Li about 5 days prior to the XAS measurement campaign, which turned out to be rather different in spite of the precautions taken in order to avoid the decomposition of the latter materials. This comparison underlines the importance of performing *in situ* measurements to get a realistic view of the reaction mechanism of battery materials. In fact, especially in the case of conversion materials, such investigations can be very complex because the species formed in cycling electrodes are usually very reactive and/or unstable.
