3.1.1. Case study

3. Synchrotron radiation-based X-ray absorption techniques

X-ray absorption spectroscopy (XAS) is a powerful technique that can characterize aII forms of matter, irrespective of their degree of crystallinity. Traditionally, diffraction-based characterization methods are being used for structural investigations, and reliable structural information can be determined for materials that exhibit a long-range structural order. In contrast XAS can probe the local structure of disordered solids, liquids, as well as amorphous materials. XAS has vast application area ranging from coordination chemistry, catalysis, biology, and surface physics to material chemistry. One of the major advantages of XAS is its atomic selectivity which makes it possible to study the local structure of each different constituent of a sample.

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-

Sample preparation for XAS is very simple, and experiments can be performed in situ.

XAS spectrum can be divided in two parts, namely, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). In XANES phenomenon, an element-specific signal is generated, typically using a synchrotron radiation source. A core electron absorbs the energy of incident X-rays and gets excited beyond the Fermi level, leaving behind a core hole. The synchrotron radiation sources can provide energy that is right for desired electron transitions. When a sample is exposed to X-rays, it will absorb part of the

3.1. Hard X-ray absorption spectroscopy (HXAS)

Co0.5Sn0.5, meso-Co0.3Sn0.7, and meso-Co0.1Sn0.9, respectively [9].

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

Major challenges faced by Li-ion batteries are demand for high-energy density, capacity retention, safety, and low cost. In order to achieve the higher-energy density than that of currently commercialized ones, metal oxides are being considered as potential anode materials due to their high-energy density arising from conversion and alloying reactions [11]. SnO2 is a candidate anode material for future batteries, and previous studies show that SnO2 anode undergoes an irreversible conversion reaction in the initial cycle followed by a reversible alloying reaction of Sn [12, 13]:

$$\rm{SnO\_2 + 4Li^+ + 4e^- \to Sn + 2Li\_2O} \quad (711 \, mA/g) \tag{2}$$

$$\text{Sn} + \text{xLi}^+ + \text{ xe}^- \rightarrow \text{Li}\_x\text{Sn} \text{ (783mAh/g)} \text{ (0 << x << 4.4)}\tag{3}$$

Reaction based on Eq. (2) is the main reason for initial irreversible capacity of this anode material, whereas reaction based on Eq. (3) is responsible for reversible capacity of these electrode materials in the subsequent cycles. Surprisingly, reported capacity of nanostructured SnO2-based anode materials is higher than the abovementioned theoretical capacity (783 mAh/g). To understand this anomalous capacity, Kim et al. [14] conducted the synchrotron-based XAS experiments on mesoporous SnO2 anode material. The combination of XRD and XAS was used to probe the bulk and local structure. The XRD peaks almost disappeared (not shown here) after discharging below 0.7 V indicating that mesoporous SnO2 converts to an amorphous nano-LixSnO2 phase, so XRD alone was unable to further characterize this material.

Figure 6(a) shows selected Sn K-edge XANES and EXAFS patterns in the initial discharge region of the first cycle. The oxidation state of Sn in the mesoporous SnO2 is 4+. The reduction of Sn takes places in the beginning of discharge, and the Sn K-edge XANES spectra show prominent shift toward lower-energy values. This reduction of Sn during conversion reaction effects the local environment around the Sn atom. The first prominent peak in Sn K-edge EXAFS spectra corresponds to the Sn-O interaction in the first coordination shell, and the broad peaks in 2.2–3.9 Å region are due to the Sn-Sn, Sn-O, and Sn-Sn interactions in the subsequent coordination shells. The intensity of these peaks decrease significantly during discharge due to displacement of reacting species during the conversion reaction. Figure 6(b) shows XAS data obtained in the middle discharge region of the first cycle. In this region, Sn K-edge XANES spectra show only negligible shift toward lower-energy values. However, Sn K-edge intensities decrease in this region, showing the formation of metallic Sn. After discharging beyond 600 mAh/g, the intensity of the Sn-O peak decreases, a new peak at around 2.6 Å emerges, which corresponds to the Sn-Sn(Li) pair in the LixSn alloy, and the intensity of this new peak increases with the increase of the Li/Sn ratio. The intensity of the peaks representing the Sn-O peaks gradually drops, and that of the Sn-Sn(Li) peak increases during this discharge region. The representative peaks for Sn-O bond disappear when the discharge capacity reaches 1500 mAh/g, which shows the completion of the conversion reaction. So, the remaining discharge capacity can be assigned to the alloying reaction only. Figure 6(c) shows the XAS data obtained from the mesoporous SnO2 electrode in the last discharge region of the first cycle. XANES data obtained in this discharge region show a shift toward high energy of the Sn K-edge. During the alloying reaction, charge redistribution takes place to minimize the electrostatic energy which results in shifts of Sn K-edge. In the EXAFS spectra, the amplitude of the Sn-Sn peak continuously decreases in this discharge region. Due to increase in the Li/Sn ratio, the amount of Li around Sn increases. Li has a much smaller electron-scattering cross section compared to Sn. So, the intensity of the Sn-Sn(Li) peak decreases when the Li/Sn molar ratio exceeds 3 [15]. This trend of XANES and EXAFS data suggests that the capacity in this deep discharge region is obtained only by Li alloying in the LixSn phase until it achieves its nominal composition of Li4.4Sn.

Synchrotron Radiation-Based X-Ray Study on Energy Storage Materials http://dx.doi.org/10.5772/67029 87

Sn <sup>þ</sup> <sup>x</sup>Li<sup>þ</sup> <sup>þ</sup> <sup>x</sup>e<sup>−</sup> ! LixSn 783mAh ð Þ <sup>=</sup><sup>g</sup> ð Þ <sup>0</sup> << <sup>x</sup> << <sup>4</sup>:<sup>4</sup> (3)

Reaction based on Eq. (2) is the main reason for initial irreversible capacity of this anode material, whereas reaction based on Eq. (3) is responsible for reversible capacity of these electrode materials in the subsequent cycles. Surprisingly, reported capacity of nanostructured SnO2-based anode materials is higher than the abovementioned theoretical capacity (783 mAh/g). To understand this anomalous capacity, Kim et al. [14] conducted the synchrotron-based XAS experiments on mesoporous SnO2 anode material. The combination of XRD and XAS was used to probe the bulk and local structure. The XRD peaks almost disappeared (not shown here) after discharging below 0.7 V indicating that mesoporous SnO2 converts to an amorphous nano-LixSnO2 phase, so XRD alone was unable to further characterize this

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

Figure 6(a) shows selected Sn K-edge XANES and EXAFS patterns in the initial discharge region of the first cycle. The oxidation state of Sn in the mesoporous SnO2 is 4+. The reduction of Sn takes places in the beginning of discharge, and the Sn K-edge XANES spectra show prominent shift toward lower-energy values. This reduction of Sn during conversion reaction effects the local environment around the Sn atom. The first prominent peak in Sn K-edge EXAFS spectra corresponds to the Sn-O interaction in the first coordination shell, and the broad peaks in 2.2–3.9 Å region are due to the Sn-Sn, Sn-O, and Sn-Sn interactions in the subsequent coordination shells. The intensity of these peaks decrease significantly during discharge due to displacement of reacting species during the conversion reaction. Figure 6(b) shows XAS data obtained in the middle discharge region of the first cycle. In this region, Sn K-edge XANES spectra show only negligible shift toward lower-energy values. However, Sn K-edge intensities decrease in this region, showing the formation of metallic Sn. After discharging beyond 600 mAh/g, the intensity of the Sn-O peak decreases, a new peak at around 2.6 Å emerges, which corresponds to the Sn-Sn(Li) pair in the LixSn alloy, and the intensity of this new peak increases with the increase of the Li/Sn ratio. The intensity of the peaks representing the Sn-O peaks gradually drops, and that of the Sn-Sn(Li) peak increases during this discharge region. The representative peaks for Sn-O bond disappear when the discharge capacity reaches 1500 mAh/g, which shows the completion of the conversion reaction. So, the remaining discharge capacity can be assigned to the alloying reaction only. Figure 6(c) shows the XAS data obtained from the mesoporous SnO2 electrode in the last discharge region of the first cycle. XANES data obtained in this discharge region show a shift toward high energy of the Sn K-edge. During the alloying reaction, charge redistribution takes place to minimize the electrostatic energy which results in shifts of Sn K-edge. In the EXAFS spectra, the amplitude of the Sn-Sn peak continuously decreases in this discharge region. Due to increase in the Li/Sn ratio, the amount of Li around Sn increases. Li has a much smaller electron-scattering cross section compared to Sn. So, the intensity of the Sn-Sn(Li) peak decreases when the Li/Sn molar ratio exceeds 3 [15]. This trend of XANES and EXAFS data suggests that the capacity in this deep discharge region is obtained only by Li alloying in the LixSn phase until it achieves its

material.

nominal composition of Li4.4Sn.

Figure 6. Sn K-edge XANES and EXAFS spectra with corresponding voltage profile taken in (a) the first discharge region, (b) the middle discharge region, and (c) the last discharge region of the first cycle [14].

Figure 7(a) shows XAS data taken from the mesoporous SnO2 electrode in the beginning of charge. The Sn K-edge XANES spectra shift reversibly toward lower-energy values, and EXAFS spectra show the rise of Sn-Sn(Li)-related peaks, suggesting that the dealloying reaction is taking place in this voltage region. After the cell was charged to 500 mAh/g, the intensity of the Sn-Sn(Li) peaks starts to decrease and that of Sn-O peak increases, as shown in Figure 7(b). Appearance of the Sn-O peak and damping of the Sn-Sn peak are only possible when Li2O formation is at least partially reversible, along with the formation of the SnO<sup>x</sup> phase. These results suggest that the reversible charge capacity at the end of the charge can be assigned to dealloying of LixSn phase as well as the conversion reaction of Sn into the SnO<sup>x</sup> phase. After achieving charge capacity of 900 mAh/g, peaks representing the Sn-O coordination shell grow with small change of the Sn-Sn peak, indicating that reversible charge capacity is mainly achieved by conversion reaction in this region. XANES spectra do not show noticeable shift in this charge region. The overall EXAFS data in the first cycle show that active material in SnO2 does not come back to its initial composition after one complete electrochemical cycle; it changes into metallic Sn with a small quantity of amorphous SnO<sup>x</sup> along with LixSn phase. In short, local structure analyses via hard XAS technique successfully demonstrated the origin of high capacity of mesoporous SnO2 beyond its reported theoretical capacity.

Figure 7. Sn K-edge XANES and EXAFS spectra with corresponding voltage profile taken in (a) the first charge region and (b) the last charge region of the first cycle [14].
