3.2. Soft X-ray absorption spectroscopy (SXAS)

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

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

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].

capacity.

Soft XAS is an XAS technique that uses soft X-rays, with energies ranging from 150 to 1200 eV. This energy range covers the K-edge of light elements, for example, B, C, N, O, and F, along with the L2,3 edges of the first-row transition metal elements. In an XAS experiment, tunable X-rays hit the sample and 1s electrons are ejected when the X-ray reaches a specific energy, such as the Kedge energy of oxygen (532 eV). The resulting core hole is relaxed either by transfer of electron from higher levels into the core hole which leads to the emission of fluorescent X-rays or by releasing the Auger electrons. A schematic diagram of the core hole relaxation process is shown in Figure 8. Both the fluorescent X-rays and the Auger electron signals can be utilized to get XAS spectra as both the signals are proportional to the incident X-ray absorption. The fluorescent Xrays possess higher escape depth of about 2000 Å, contrary to the Auger electrons, which have an escape depth of only about 50 Å. Because of this difference in escape depths, different information can be collected from fluorescent X-rays and Auger electrons. The fluorescent X-ray signal is more sensitive about the bulk structure, whereas the Auger electron yield is responsive for the surface structure. By measuring fluorescent and electron yields simultaneously, information about both surface and bulk can be obtained in the same experiment [16].

Figure 8. Schematic diagram of principle of (a) absorption, (b) fluorescent, and (c) Auger electron-yield soft X-ray absorption spectroscopy.

#### 3.2.1. Case study

Thermal stability is a critical issue related to the safety of the rechargeable batteries. Traditionally, it is studied by using thermo-analytical techniques like TGA, or there are some studies by using in situ XRD. Yoon et al. utilized in situ temperature-dependent soft XAS measurements for the first time, in order to understand the role of different transition metals in thermal degradation of the charged LiNi0.8Co0.15Al0.05O2 electrode [17]. They monitored the elementselective structural changes in the charged cathode material on the surface and in the bulk during heating of electrode material. The findings of their study provide important guidelines to design new electrode materials with enhanced thermal safety.

Normalized Ni L-edge spectra of Li0.33Ni0.8Co0.15Al0.05O2 cathode using fluorescent yield (FY) mode at various temperatures are shown in Figure 9(a), and the partial electron yield (PEY) mode spectra are shown in Figure 9(b). Due to spin-orbit interaction of the core hole, the absorption spectrum splits into two energy bands, Ni 2p3/2 (L3 edge) and Ni 2p1/2 (L2 edge). Changes in energy position of these bands can indicate valence-state variations during the heating process as energy position shifts about 1 eV per oxidation-state change [18]. Ni L3 and L2 spectra obtained in the bulk sensitive fluorescent yield mode do not show energy position changes. Li0.33Ni0.8Co0.15Al0.05O2 material is based on layered structure with R3m space group, and the change into the Fd3m structure during heating would not involve a valence-state change or shift in energy position of L-edges. However, the energy position of Ni L3 and L2 spectrum moves to lower-energy values in case of surface-sensitive electron yield mode, and a rather prominent shift takes place at around 200°C that shows the presence of a NiO-type rock salt structure on the surface at this temperature. Figure 9(c) and (d) shows normalized Co Ledge XAS spectra at various temperatures using FY and PEY mode, respectively. Unlike the Ni L-edge spectra, the electron-yield spectra of the Co species do not show energy shifts. There are no visible changes in both the FY and the PEY spectra which show that cobalt ions have better thermal stability compared to the nickel ions. Partial substitution of nickel by cobalt in the cathode materials enhances its thermal stability.

Figure 9. Normalized XAS spectra of Li0.33Ni0.8Co0.15Al0.05O2 cathode material at different temperatures using (a) Ni L-edge FY mode, (b) Ni L-edge PEY mode, (c) Co L-edge FY mode, and (d) Co L-edge PEY mode [17].

Figure 10 shows the normalized O K-edge XAS spectra of Li0.33Ni0.8Co0.15Al0.05O2 cathode material at various temperatures, using FY mode and PEY mode. The first prominent absorption peak at 528.5 eV corresponds to transition from oxygen 1s orbital to a hybridized state of metal 3d-O 2p orbitals. The oxygen K-edge spectra contain information associated with transitions to hybridized states of O 2p-Ni 4sp and other empty orbitals in that energy region. Like the L-edge spectra, there is no significant change in the fluorescence-yield spectra, but the surface-sensitive electron-yield spectra show a significant decrease of the peak at 528.5 eV when temperature rises above 200°C. The PEY data show other distinct differences as well. The intensity of the distinct peak at ~534 eV is decreasing, whereas that of the peak at ~532 eV is increasing with rising temperature. The features at around 532 eV and 534 eV are associated with the presence of NiO and Li2CO3, respectively, as shown by the spectra of the standards in Figure 10(b). Upon heating, intensity of the features at 534 eV decreases which suggests that carbonate present on the surface is gradually decomposed. Conversely, the intensity of the 532 eV peak increases with temperature, particularly above 200°C, and the intensity of the 528.5 eV peak decreases. These observations indicate the formation of reduced divalent nickel oxide. This finding is in accordance with the Ni L-edge measurements. The presence of NiO-type rock salt structure and its increased formation at electrode surface with increasing temperature indicates nickel oxides tend to release oxygen at higher temperature. The oxygen K-edge spectra are consistent with the data obtained from the Ni L-edge and point toward the initiation of thermal reduction reactions around Ni sites on the surface of the cathode sample material.

mode spectra are shown in Figure 9(b). Due to spin-orbit interaction of the core hole, the absorption spectrum splits into two energy bands, Ni 2p3/2 (L3 edge) and Ni 2p1/2 (L2 edge). Changes in energy position of these bands can indicate valence-state variations during the heating process as energy position shifts about 1 eV per oxidation-state change [18]. Ni L3 and L2 spectra obtained in the bulk sensitive fluorescent yield mode do not show energy position changes. Li0.33Ni0.8Co0.15Al0.05O2 material is based on layered structure with R3m space group, and the change into the Fd3m structure during heating would not involve a valence-state change or shift in energy position of L-edges. However, the energy position of Ni L3 and L2 spectrum moves to lower-energy values in case of surface-sensitive electron yield mode, and a rather prominent shift takes place at around 200°C that shows the presence of a NiO-type rock salt structure on the surface at this temperature. Figure 9(c) and (d) shows normalized Co Ledge XAS spectra at various temperatures using FY and PEY mode, respectively. Unlike the Ni L-edge spectra, the electron-yield spectra of the Co species do not show energy shifts. There are no visible changes in both the FY and the PEY spectra which show that cobalt ions have better thermal stability compared to the nickel ions. Partial substitution of nickel by cobalt in

Figure 9. Normalized XAS spectra of Li0.33Ni0.8Co0.15Al0.05O2 cathode material at different temperatures using (a) Ni

L-edge FY mode, (b) Ni L-edge PEY mode, (c) Co L-edge FY mode, and (d) Co L-edge PEY mode [17].

the cathode materials enhances its thermal stability.

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

Figure 10. Normalized O K-edge XAS spectra of Li0.33Ni0.8Co0.15Al0.05O2 cathode material at different temperatures using (a) FY mode and (b) PEY mode [17].

These investigations demonstrated the capability of in situ soft XAS techniques to investigate thermal behavior of cathode materials and show that there is no valence-state change in the bulk despite the layered structure of the Li0.33Ni0.8Co0.15Al0.05O2 cathode material converts to spinel structure. The surface-sensitive PEY measurements reveal that this electrode material loses oxygen at high temperatures leading to a lower oxidation state of Ni and formation of NiO-like rock salt structure. No evidence of a surface reaction near Co sites in the investigated temperature range was found which shows that the Co is more stable at elevated temperatures compared to the Ni in Li0.33Ni0.8Co0.15Al0.05O2. The capability of soft XAS to discriminate the surface and bulk electronic structures with element specificity makes it a valuable addition to the advanced synchrotron-based characterization technique that help understand thermal behavior of battery electrodes.
