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Condens. Matter 2020, 5, 75 20 of 28 7. XRD and Pair Distribution Function While XRD covers only Bragg scattering and provides long-range average structural information, PDF analysis utilizes the total scattering (Bragg and diffuse scattering) to investigate materials with short-range ordering. PDF can provide local information on the atomic pair distribution relating to chemical, structural, and morphological transformations that occur during electrochemical reactions. For this reason, it is particularly useful to study nano-sized materials or highly disordered materials. Applied to battery studies, PDF analysis can provide detailed insights into the local atomic structure, phase progression, and particle size/ordering in electrodes [112,113]. For instance, Sottman et al., combined operando PDF and XRD-CT to study the chemical structure of specific sodium ion battery components. Using the ID15A beamline at ESRF facility (Grenoble, France), they analysed the different mechanisms of sodiation and desodiation of phosphorus. Chapman’s group used operando PDF combined with nuclear magnetic resonance (NMR) spectroscopy to gain comprehensive insights into the electrochemical reaction mechanism of iron oxyfluoride electrodes using an AMPIX cell [114]. On the 11-ID-B beamline at the APS at the Argonne National Laboratory (USA), they observed complex behaviour during cycling, including a multistep sequence of structural transitions and formation of amorphous and nanoparticle phases. Figure 16a shows the PDF data obtained during the first charge-discharge cycle. The data seems reversible, implying reversibility of the electrochemical reaction. However, the atomic structure of the electrode formed after cycling differs from the pristine uncycled electrode material. Fits of the data reveal that upon cycling the electrode is a nanocomposite of an amorphous rutile phase and a nanoscale rock salt phase. Fe nanoparticles grow to ∼30 Å (at 1.3–1.2Li) during charge followed by the formation of a rutile phase. The rock salt phase does not react until late into charge, after all the Fe nanoparticles have transformed to amorphous rutile. Some rock salt remains in the fully charged material. The evolution of Fe phases during cycling is shown in Figure 16b. Condens. Matter 2020, 5, x 22 of 30 Figure 16. (a) PDFs obtained during the first discharge−charge cycle (for x = 0.60, 50 °C, C◦/30, ∼35 h, Figure 16. (a) PDFs obtained during the first discharge−charge cycle (for x = 0.60, 50 C, C/30, ∼35 h, 4.5−1.5−4.5 V). The arrows indicate distances characteristic of the rock salt intermediate. The colours 4.5–1.5–4.5 V). The arrows indicate distances characteristic of the rock salt intermediate. The colours reflect relative peak intensities; (b) evolution of Fe phases during cycling of x = 0.60 at 50 °C from full reflect relative peak intensities; (b) evolution of Fe phases during cycling of x = 0.60 at 50 ◦C from full profile fits to the PDF data. profile fits to the PDF data. 8. Conclusions and Perspective 8. Conclusions and Perspective In situ and operando experiments have led to significant progress in understanding complex In situ and operando experiments have led to significant progress in understanding complex battery materials and related processes. XRD has been revealed as a powerful tool in this context and battery materials and related processes. XRD has been revealed as a powerful tool in this context and this review reports the most recent in-situ XRD synchrotron-based experiments on battery materials. this review reports the most recent in-situ XRD synchrotron-based experiments on battery materials. The combination of different synchrotron characterization techniques, such as XRD combined with The combination of different synchrotron characterization techniques, such as XRD combined with neutron, CT, Raman, XAS, and microscopy has allowed for LIBs to be studied simultaneously at neutron, CT, Raman, XAS, and microscopy has allowed for LIBs to be studied simultaneously at multi-length scales and has achieved a more clear and comprehensive understanding of the mul(tei-vloelnvginthg)shcaetlersoagnednehitaiessawchitiheivnedLIaBmoartercialelsa.rEaxnpdercimomenptrsetheantshiavveeubnedeenrsctoannsdidienrgedofetxhtreem(eevloylving) hetedriofgfiecunletituinestilwrietlhatiinveLlIyBrmeceantetlryia,lnso.wExhpaevreimbecnotmsethraotuhtianvee,wbeitehnrcaopnidsimderaesdurexmtreenmtsealyndiaffilacrugletuntil number of available facilities. It is now possible to monitor changes in interfacial regions for high- relatively recently, now have become routine, with rapid measurements and a large number of available resolution primary particle analysis, probing redox reactions occurring to a single particle. The facilities. It is now possible to monitor changes in interfacial regions for high-resolution primary contemporary analyses of morphology and crystallography of battery materials are now becoming particle analysis, probing redox reactions occurring to a single particle. The contemporary analyses common on a single beamline. This is an enormous advantage, because it allows for the use of multi- of morphology and crystallography of battery materials are now becoming common on a single operando techniques, increasing the level of results and reducing the experimental time. In the future, several types of experiments should be carried out on the same sample sequentially, reducing the risk of exposure of the sample to the atmosphere and without the need for manual exchange, with automatic motion inside and outside the beamline. Nevertheless, the issues related to the measurements of amorphous materials should also be considered. Disordered materials still represent a challenge for XRD analysis because of the lack of sharp peaks deriving from no long-PDF Image | Synchrotron-Based X-ray Diffraction for Lithium-Ion Batteries
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