2.2. Small-angle X-ray scattering (SAXS)

The structural characterization of nanoscale systems is a very active area of research these days not only in energy storage material system but in variety of other scientific disciplines as well. Nanoscale systems can be studied with real-space imaging or reciprocal space scattering techniques. X-ray scattering techniques provide reciprocal space data, whereas electron microscopy-based imaging techniques usually provide real-space data. A distribution of electron density at nanometer length scales will scatter an X-ray beam to low angles, while that in the atomic scale will scatter to high angles. Therefore, small-angle X-ray scattering (SAXS) is a technique to study material structures at small angles or large distances. SAXS is a powerful technique to determine, not only the object's size, size distribution, shape, surface structure, relative positions of particles, but it can also be used for the structure factor analysis. The size distribution function is a key piece of information that can be obtained from SAXS. Collected data can be fitted, when the shape of a particle is known or can be assumed, to get the size distribution. SAXS form factor analysis provides useful information at the single-particle level; the structure factor allows to figure out the organization of particle systems in the structure.

In recent years, the development of synchrotron radiation X-ray sources has made possible to adopt novel approaches to utilize X-ray scattering technique for nanoparticle research. SAXS is nondestructive and provides structural data averaged over macroscopic sample volumes. Modern synchrotron radiation-based SAXS is capable of structural characterization of sample in its working state because of its tunable flux and energy that is particularly useful for nanoparticle research especially for electrochemical energy storage systems.

#### 2.2.1. Case study

SAXS is a useful characterizing technique for characterization of Li-ion batteries and other energy storage materials. Conventional Li-ion batteries suffer from capacity loss due to several failure mechanisms associated with the strain induced in anode and cathode materials upon electrochemical cycling. Ordered mesoporous materials have been considered as potential candidates for the next-generation electrode materials. There are several advantages associated with mesoporous electrode materials, for example, the ordered framework of mesopores which can act as a physical buffer for the volume changes, and it reduces the diffusion path length to promote easy Li and electron transport. These structures offer intrinsic high specific surface area that provides large active surface between electrolyte and electrode material. SAXS is an ideal technique to study ordered mesoporous structures. Recently, Park et al. [9] have developed an in situ synchrotron-based small-angle X-ray scattering (SAXS) technique to investigate the nanostructural changes of ordered mesoporous materials during cycling for further understanding the Li storage reactions.

Information on nanostructural changes of an electrode material from SAXS allows to understand fine details of nanostructured electrode dynamics during electrochemical cycling. They performed in situ SAXS studies on the meso-CoxSn<sup>y</sup> anode materials to probe the mesoscopic structural changes during its electrochemical cycling to understand the behavior of the entire electrode with different Co contents. In situ SAXS data including contour projection for each meso-CoxSn<sup>y</sup> composition during the first cycle are shown in Figure 3. All the in situ SAXS patterns indicate that the present meso-CoxSn<sup>y</sup> materials retain highly ordered meso-structures, even though the intermetallic electrodes are known to form Li alloys during lithium insertion. There are no significant changes in the relative scattering intensities of SAXS patterns, when a discharge current is applied, until the discharge potential reaches to 0.2 V. While discharging below 0.2 V, the scattering peaks move slightly toward the lower angle, and their intensity is decreases. These results indicate that the meso-structures of all the meso-CoxSn<sup>y</sup> electrodes are retained until 0.2 V and then small expansion of mesoscopic cell volume and somewhat loss of mesostructural periodicity take place during the Li-Sn alloying reaction. Both the intensities and positions seem to be recovered to the initial state after the complete cycle, indicating the structural stability of meso-CoxSn<sup>y</sup> electrodes.

2.2. Small-angle X-ray scattering (SAXS)

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

2.2.1. Case study

further understanding the Li storage reactions.

The structural characterization of nanoscale systems is a very active area of research these days not only in energy storage material system but in variety of other scientific disciplines as well. Nanoscale systems can be studied with real-space imaging or reciprocal space scattering techniques. X-ray scattering techniques provide reciprocal space data, whereas electron microscopy-based imaging techniques usually provide real-space data. A distribution of electron density at nanometer length scales will scatter an X-ray beam to low angles, while that in the atomic scale will scatter to high angles. Therefore, small-angle X-ray scattering (SAXS) is a technique to study material structures at small angles or large distances. SAXS is a powerful technique to determine, not only the object's size, size distribution, shape, surface structure, relative positions of particles, but it can also be used for the structure factor analysis. The size distribution function is a key piece of information that can be obtained from SAXS. Collected data can be fitted, when the shape of a particle is known or can be assumed, to get the size distribution. SAXS form factor analysis provides useful information at the single-particle level; the structure factor allows to figure out the organization of particle systems in the structure. In recent years, the development of synchrotron radiation X-ray sources has made possible to adopt novel approaches to utilize X-ray scattering technique for nanoparticle research. SAXS is nondestructive and provides structural data averaged over macroscopic sample volumes. Modern synchrotron radiation-based SAXS is capable of structural characterization of sample in its working state because of its tunable flux and energy that is particularly useful for

nanoparticle research especially for electrochemical energy storage systems.

SAXS is a useful characterizing technique for characterization of Li-ion batteries and other energy storage materials. Conventional Li-ion batteries suffer from capacity loss due to several failure mechanisms associated with the strain induced in anode and cathode materials upon electrochemical cycling. Ordered mesoporous materials have been considered as potential candidates for the next-generation electrode materials. There are several advantages associated with mesoporous electrode materials, for example, the ordered framework of mesopores which can act as a physical buffer for the volume changes, and it reduces the diffusion path length to promote easy Li and electron transport. These structures offer intrinsic high specific surface area that provides large active surface between electrolyte and electrode material. SAXS is an ideal technique to study ordered mesoporous structures. Recently, Park et al. [9] have developed an in situ synchrotron-based small-angle X-ray scattering (SAXS) technique to investigate the nanostructural changes of ordered mesoporous materials during cycling for

Information on nanostructural changes of an electrode material from SAXS allows to understand fine details of nanostructured electrode dynamics during electrochemical cycling. They performed in situ SAXS studies on the meso-CoxSn<sup>y</sup> anode materials to probe the mesoscopic structural changes during its electrochemical cycling to understand the behavior of the entire electrode with different Co contents. In situ SAXS data including contour projection for each meso-CoxSn<sup>y</sup> composition during the first cycle are shown in

Figure 3. Color-coded 3D contours and projection maps showing SAXS data collected from ordered meso-CoxSn<sup>y</sup> electrodes during in situ experiment: (a) meso-Co0.5Sn0.5, (b) meso-Co0.3Sn0.7, and (c) meso-Co0.1 Sn0.9 [9].

In order to get more insight of the in situ SAXS results, dQ/dV data, relative peak intensities, and mesoscopic lattice parameters were plotted against the cell voltage for the meso-CoxSn<sup>y</sup> electrodes as shown in Figure 4. This data indicates about 19% decrease of relative SAXS peak intensity and 14% expansion of meso-structural cell volume in the meso-Co0.5Sn0.5 electrode after the full discharge. In meso-Co0.3Sn0.7 electrode, there is only 13% change in the peak intensity, whereas the meso-structural cell volume expansion is 41% that is much larger than that of the meso-Co0.5Sn0.5. The initial discharge capacities of the meso-Co0.3Sn0.7 and meso-Co0.5Sn0.5 electrodes are 1321 and 822 mAh/g, respectively, due to the different amount of electroactive Sn. Figure 4(f) shows a significant 52% decrease in the SAXS peak intensity with relatively large volume change of 30% in the meso-Co0.1Sn0.9 electrode, while its initial discharge capacity is 1493 mAh/g. These in situ SAXS results for meso-CoxSn<sup>y</sup> electrodes during cycling directly provide roles of the inactive Co element as a chemical buffer; meanwhile, the well-defined nanoporous system acts as a physical buffer to accommodate the volume changes in the electrode. In situ SAXS reveals that the mesoscopic volume and meso-structural order change reversibly during cycling. It indicates that reliable nanostructure is developed and that relieves severe internal strain induced by huge volume change upon the repeated electrochemical reactions.

Figure 4. (a–c) Color-coded contour projection maps during in situ experiment with corresponding voltage profile and (d–f) the changes in lattice parameters and resolved peak-relative intensities with the corresponding dQ/dV plot for meso-Co0.5Sn0.5, meso-Co0.3Sn0.7, and meso-Co0.1Sn0.9, respectively [9].
