Understanding the electrochemical processes responsible for energy storage in batteries is critical for designing of next-generation batteries. The conventional laboratory-scale characterization instruments provide limited information required for better understanding of electrochemical reaction mechanisms. Synchrotron radiations have very high brilliance and broad energy range extending from far-IR through the hard X-ray region. The availability of synchrotron radiation is driving technical and theoretical advances in scattering and spectroscopic techniques from last couple of decades. These advances in synchrotron radiation-based characterization techniques have made it possible to study the underpinning issues of energy storage materials. An electrochemical road map based on much more knowledge-driven approach can be drawn by utilizing synchrotron-based element-specific spectroscopic as well as scattering techniques. Herein, we introduce various scenarios where synchrotron radiation-based characterization methods provide inherent advantages and flexibility in obtaining detailed mechanistic information along with structural studies.
- synchrotron radiation
- energy storage material
- Li-ion battery
- local structure
Synchrotron radiation is emitted by charged particles, traveling at speeds relative to the speed of light when accelerated by magnetic fields. The major advantages of synchrotron radiation include very high intensity, tunable energy range, and inherently linear polarization . However, one major drawback is the limited availability of the national and international synchrotron radiation facilities. The availability of synchrotron radiation, with its characteristics of high brilliance, particular collimation, and multi-wavelength accessibility, continues to drive technical and theoretical advances in scattering and spectroscopy techniques. An exciting area being developed is the exploitation of these advances in synchrotron radiation surface and bulk-specific probe techniques to study the underpinning issues of energy storage materials.
The long-term endurance of batteries and other electrochemical devices, used in highly demanding applications like electric vehicles, is closely related to the ability of the cathode and anode materials to accommodate and release guest ions without any structural damage. A challenge in developing the understanding of energy storage process in batteries is in the direct study of the electrochemical reactions involved during battery operation. The characterization tool required needs to provide element-specific as well as overall structural information with high resolution. Synchrotron radiation-based measurements under operating conditions of batteries are critical in order to map the mechanistic causality between the local and atomic structure of functional components of batteries and their electrochemical characteristics. In this chapter, we will examine various scenarios where synchrotron radiation-based X-ray methods provide inherent advantages and flexibility in obtaining detailed mechanistic information along with structural studies.
2. Synchrotron radiation-based X-ray scattering techniques
2.1. Wide-angle X-ray scattering (WAXS)
X-ray photons interact with matter in different ways including coherent scattering, Compton scattering, photoelectric interaction, and pair production. If the interaction of the X-ray photons is coherent and elastic, the interaction is called X-ray diffraction (XRD) or Bragg scattering. A distribution of electrons in matter will interact with a photon wave to produce an interference-modulated scattering pattern, called a diffraction pattern. If multiple identical electron distributions are periodically placed in space, the scattering from each of them will interact with that from the others and will result in destructive interference in most of the direction other than a few allowed directions. These allowed directions can be calculated by considering the lattice, and hence a crystalline structure can be fully resolved by using diffraction pattern. Bragg’s law is a useful model to describe the relation between the allowed scattering angles (2
The recorded diffraction peak from a sample will have an angular width due to the broadening from the instrument. Additionally, the peaks can be broadened by the finite size of the crystallites. The peak broadening does not correlate with the particle size, but with the coherent domain length where long-range order is preserved . Synchrotron radiation covers a large range of energies and that allows for superior data acquisition. In the case of XRD, it enables the ability to probe many different crystallographic planes at the same time, resulting in fast and rich data acquisition. The use of high energy, sometimes referred to as hard X-rays, is advantageous because these X-rays are not absorbed well in a solid material and therefore allow for deep penetration. These properties of synchrotron radiation, coupled with overall high intensities, allow for rich data collection and experimentation that were previously not possible. Experimental setup for synchrotron radiation-based high-energy XRD is shown in Figure 1 . As the high-energy photons are able to fully penetrate the cell, these measurements are conducted in the transmission mode in order to obtain 2D diffraction patterns. This also means that the cathode and anode can be investigated simultaneously.
2.1.1. Case study
Rechargeable Li-ion batteries are electrochemical energy storage devices of choice in portable electronics due to their high-specific energy density and now becoming increasingly popular for grid storage and electrical vehicles. In Li-ion battery, electrodes operate by reversible Li-ion insertion and extraction during charge and discharge. High-rate Li-ion battery electrode materials usually make solid solutions with Li over a large composition range in order to avoid phase transformation during (de)lithiation. Phase transformations, during cycling, are associated with small or negligible volume changes. For example, the layered compound LiNi1/3Mn1/3Co1/3O2 makes a solid solution and shows moderate volume changes; however, the high-voltage spinel Li
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.  have developed an
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 order to get more insight of the
3. Synchrotron radiation-based X-ray absorption techniques
3.1. Hard X-ray absorption spectroscopy (HXAS)
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. Sample preparation for XAS is very simple, and experiments can be performed
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 incoming photon beams. Other phenomena occurring are heat, X-ray fluorescence, production of unbound electrons, and of course the scattering of X-rays. The absorption of X-rays can be measured quantitatively, and it follows exponential decay given by the Beer’s law .
The EXAFS phenomenon arises from the quantum mechanical interference of the scattering of a photoelectron by the potential of the surrounding atoms. A photoelectron emitted by the photo-absorbing atom propagates as a spherical wave and spreads out over the solid. The amplitude of all the reflected electron waves adds either constructively or destructively to the spectrum of the absorbing atom as shown in Figure 5(a) and 5(b) respectively, and hence the X-ray absorption coefficient exhibits a typical oscillation. A crucial issue is the recognition that the photoelectron is not infinitely long lived; it decays as a function of time and distance, and thus the EXAFS cannot probe long-range distances. EXAFS can give only local structural information of about several angstroms around the selected atomic species.
The recent availability of high-brightness synchrotron radiation sources has resulted in a prosperous development of XAS technique, and it is finding wide application area including the energy storage material field.
3.1.1. Case study
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 . 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]:
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.  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-Li
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 Li
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
3.2. Soft X-ray absorption spectroscopy (SXAS)
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 K-edge 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 X-rays 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 .
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
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 . 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 space group, and the change into the
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.
These investigations demonstrated the capability of
The application of synchrotron-based characterization techniques to investigate energy storage materials is playing a major role in the fundamental understanding of the electrochemical reaction mechanism of energy storage materials. This chapter provides an overview about various X-ray synchrotron-based characterization techniques and their applications to electrode materials to characterize the nano- and mesoporous phase dynamics, long-range crystal order, and local and electronic changes during the electrochemical cycling of electrode materials. The combination of these techniques can provide critical information to reveal the electrochemical reaction mechanism and functional properties of electrode materials in order to better understand the existing energy storage systems and help design modern electrode materials for future applications. Given the recent developments, it can be expected that application of synchrotron-based characterization techniques will become increasingly important in development of high-performance and stable energy storage materials.
The authors acknowledge the financial support from Human Resources Development Program (No. 20124010203270) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP).