2.1.1. Case study

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 mate-

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

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

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 (2θ), the photon wavelength (λ), and an inter-planar distance (d) between

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 [2]. Synchrotron radiation covers a large range of energies and that allows for superior data acquisition. In the case of XRD, it enables

2d sin θ ¼ nλ (1)

2. Synchrotron radiation-based X-ray scattering techniques

rials.

along with structural studies.

parallel planes; see Eq. (1):

2.1. Wide-angle X-ray scattering (WAXS)

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 Lix(Ni0.5Mn1.5)O4 (0 < x < 1) shows only a small volume change (3%) for the two-phase region [3]. LiFePO4, however, displays excellent highrate performance when nanosized, despite undergoing a two-phase transformation to FePO4 during delithiation, along with small volume change of 6.8% [4]. The limited Li solubility in LiFePO4 and FePO4 indicates that (de)lithiation occurs via a two-phase reaction, where the relative LiFePO4 and FePO4 amount is changed by a moving phase boundary, and not via a solid solution. The Li solubility increases by decreasing particle size, as a result of the increased interfacial energy per unit volume. By considering this interfacial energy, ex situ diffraction studies of LiFePO4 nanoparticles suggest that once an energetically unfavorable LiFePO4-FePO4 interface is formed, this interface quickly moves through the particle so as to return to the most stable LiFePO4 or FePO4 state, so only LiFePO4 and FePO4 particles are observed by ex situ characterization techniques.

Recently published in situ XRD investigations performed on micrometer-sized LiFePO4 show the emergence of a metastable crystalline phase with an intermediate Li composition of Li0.6 <sup>−</sup>0.75FePO4 when cycled at high rates [5]. Whereas, studies on nanometer-sized LiFePO4 particles are limited to low [6] and moderate [7] current rates, and only small deviations in stoichiometry from LiFePO4 and FePO4 were observed during cycling. Due to the faster transport kinetics of nanoparticles, a high current rate is required to reach the kinetic limit of a phase transformation including an in situ XRD setup with high X-ray intensity and a fast read detector so that the reaction can be probed with high time resolution. By studying the nanoparticles under high current rates, Liu et al. [8] were able to force enough particles to transform simultaneously so that the reacting particles can be detected and the nature of the phase transformations that occur at an overpotential can be determined.

In situ diffraction patterns during the first five cycles of LiFePO4 with an average size of 186 nm at 10 C galvanostatic charge-discharge are shown in Figure 2(a). All peaks in the diffraction patterns can be indexed to either the Li-rich Li1<sup>−</sup>αFePO4 phase or the Li-poor LiβFePO4 phase. During charge, peaks representing Li1<sup>−</sup>αFePO4 phase disappear, and these peaks reappear on discharge; conversely LiβFePO4 peaks appear on charge and disappear on discharge. Interestingly, they observed the appearance of positive intensities between the 8.15–8.4, 13.95–14.1, and 15.15–15.4, 2θ ranges, which shows the formation of phases, in which the lattice parameters are different from those of LiFePO4 and FePO4. A closer view of individual diffraction patterns in selected 2θ regions is shown in Figure 2(b). All of the reflections exhibit highly symmetrical profiles at the beginning of the first charge; however, the LiFePO4 (2 0 0) and (3 0 1) reflections start to broaden asymmetrically toward higher angles with the charge. The most significant asymmetrical broadening is observed on discharge in patterns (f) and (g), where the (2 0 0) reflections from both phases are connected by a positive intensity band. Similar trend is observed in the second cycle as well. Neither the peak position nor the peak shape of LiFePO4 is restored to that of the original state by the end of the second cycle. All selected peaks shift toward higher angle and become broad, as shown in pattern (r). This peak shift shows a decrease in the unit cell volume, which will in turn reduce the accessible capacity of LiFePO4 at high rates. So, at the end of each cycle, the Li composition is not restored to stoichiometric LiFePO4, instead a solid solution (Li1 <sup>−</sup>αFePO4) is formed with a smaller unit cell volume than that of the stoichiometric LiFePO4. By using synchrotron radiation-based wide-angle X-ray scattering technique and doing further analysis like profile fitting by convoluting separate contributions from size and lattice-parameter variations with appropriate analytical functions, they further confirmed that phase transformations in nanoparticulate LiFePO4 proceed, at least at high rates, via a continuous change in structure rather than a distinct moving phase boundary between LiFePO4 and FePO4.

Synchrotron Radiation-Based X-Ray Study on Energy Storage Materials http://dx.doi.org/10.5772/67029 81

diffraction studies of LiFePO4 nanoparticles suggest that once an energetically unfavorable LiFePO4-FePO4 interface is formed, this interface quickly moves through the particle so as to return to the most stable LiFePO4 or FePO4 state, so only LiFePO4 and FePO4 particles are

Recently published in situ XRD investigations performed on micrometer-sized LiFePO4 show the emergence of a metastable crystalline phase with an intermediate Li composition of Li0.6 <sup>−</sup>0.75FePO4 when cycled at high rates [5]. Whereas, studies on nanometer-sized LiFePO4 particles are limited to low [6] and moderate [7] current rates, and only small deviations in stoichiometry from LiFePO4 and FePO4 were observed during cycling. Due to the faster transport kinetics of nanoparticles, a high current rate is required to reach the kinetic limit of a phase transformation including an in situ XRD setup with high X-ray intensity and a fast read detector so that the reaction can be probed with high time resolution. By studying the nanoparticles under high current rates, Liu et al. [8] were able to force enough particles to transform simultaneously so that the reacting particles can be detected and the nature of the

In situ diffraction patterns during the first five cycles of LiFePO4 with an average size of 186 nm at 10 C galvanostatic charge-discharge are shown in Figure 2(a). All peaks in the diffraction patterns can be indexed to either the Li-rich Li1<sup>−</sup>αFePO4 phase or the Li-poor LiβFePO4 phase. During charge, peaks representing Li1<sup>−</sup>αFePO4 phase disappear, and these peaks reappear on discharge; conversely LiβFePO4 peaks appear on charge and disappear on discharge. Interestingly, they observed the appearance of positive intensities between the 8.15–8.4, 13.95–14.1, and 15.15–15.4, 2θ ranges, which shows the formation of phases, in which the lattice parameters are different from those of LiFePO4 and FePO4. A closer view of individual diffraction patterns in selected 2θ regions is shown in Figure 2(b). All of the reflections exhibit highly symmetrical profiles at the beginning of the first charge; however, the LiFePO4 (2 0 0) and (3 0 1) reflections start to broaden asymmetrically toward higher angles with the charge. The most significant asymmetrical broadening is observed on discharge in patterns (f) and (g), where the (2 0 0) reflections from both phases are connected by a positive intensity band. Similar trend is observed in the second cycle as well. Neither the peak position nor the peak shape of LiFePO4 is restored to that of the original state by the end of the second cycle. All selected peaks shift toward higher angle and become broad, as shown in pattern (r). This peak shift shows a decrease in the unit cell volume, which will in turn reduce the accessible capacity of LiFePO4 at high rates. So, at the end of each cycle, the Li composition is not restored to stoichiometric LiFePO4, instead a solid solution (Li1 <sup>−</sup>αFePO4) is formed with a smaller unit cell volume than that of the stoichiometric LiFePO4. By using synchrotron radiation-based wide-angle X-ray scattering technique and doing further analysis like profile fitting by convoluting separate contributions from size and lattice-parameter variations with appropriate analytical functions, they further confirmed that phase transformations in nanoparticulate LiFePO4 proceed, at least at high rates, via a continuous change in structure rather than a distinct moving phase boundary between

phase transformations that occur at an overpotential can be determined.

observed by ex situ characterization techniques.

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

LiFePO4 and FePO4.

Figure 2. The variations of XRD pattern during the galvanostatic charge and discharge at a rate of 10 C. (a) The image plot of diffraction patterns for selected reflections during the first five electrochemical cycles. The corresponding voltage curve is plotted to the right. (b) Selected individual diffraction patterns during the first two electrochemical cycles stacked against the voltage profile [8].
