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Condens. Matter 2020, 5, 75 8 of 28 radiation in 1947, and more precisely from its larger utilization in the 1980s [59], a large improvement in electrochemical in situ studies has been achieved. Synchrotron radiation provides a very bright and highly coherent source and can produce a well-defined, monochromatic beam [59]. The associated use of high performance detectors enables the recording of high quality data (i.e., high sensitivity and signal-to-noise ratio) with exposure times per point amounting to ms or less [60]. Masquelier’s group demonstrated the importance to have good angular resolution and high intensity data to understand the exact material behavior and properties of battery materials. They used both laboratory diffractometer and synchrotron radiation to analyze the stoichiometric Na3V2(PO4)2F3 powder [61,62]. A PANalytical Empyrean diffractometer using Cu Kα1,2 in Debye−Scherrer geometry radiation was used to obtain a starting model of the material structure; high-angular resolution synchrotron radiation diffraction was adopted using the CRISTAL diffractometer of the SOLEIL synchrotron facility (Saint-Aubin, France) to gain further insights into the crystal structure. They analysed the Na ions distribution within the available crystallographic sites, revealing a subtle orthorhombic distortion with unit-cell never observed before. Moreover, they analysed the crystal structure at high-temperature (up to 400 K) at ALBA synchrotron facility (Barcelona, Spain) observing a totally disordered distribution of Na ions at high temperature, in contrast to the partially ordered one of the room-temperature phase. Both the results were not observable with the use of a laboratory diffractometer. The research of compatible and high performance in-situ cell designs for synchrotron-based XRD experiments has led to the development of many different cell configurations. There are several factors which have been considered when preparing a beamline experiment. First of all, an XRD beamline can operate in reflection or in transmission mode. In reflection experiments, data is acquired from the material nearest the cell window; this is the most used configuration, in particular for conventional diffractometers in which the penetration depth of X-rays is limited [63]. Transmission measurements are instead the most practiced for synchrotron experiments, thanks to the higher penetration depth of the X-rays, which allows the extraction of information from all of the cell components simultaneously [38]. However, interference from inactive cell components can overlap with signal from the material of interest, and should be avoided. Cell components such as windows, separator, and current collector, have to be carefully selected, and the consequent background interference has to be removed. It is noticeable that an in-situ cell has to be designed carefully and all the conditions optimized. The generated background signal from undesired cell components has to be subtracted from the total signal for quantitative data analysis. This is particularly important for pair distribution function (PDF) technique, for instance, where the diffuse scattering is difficult to deconvolute without subtraction, compared with Bragg scattering. More information about this aspect will be provided in Section 7. A solution could be to measure the signal from an empty cell without the active sample, which can then be carefully subtracted. It is important that the empty cells have the same windows, current collector, separator and electrolyte used in the cell of interest. Moreover, geometric factors relating to the beamline of choice should also be examined, and the cell must be designed with respect to the sample holder that will host the sample. In addition, the cell has to be easy to assemble with the active materials, easy to align with the diffractometer and easy to disassemble after the experiment. Moreover, when performing an XRD synchrotron experiment, the beam interaction with the materials of interest also has to be considered. X-ray interaction can influence reaction during cycling and this can alter the material’s behavior [64]. The interaction is proportional to the material’s X-ray absorption, which is minimized with higher X-ray energies. Thus, in general, high energy X-rays should be used where possible, or continuous acquisition during electrochemical cycling avoided, for instance probing the samples with regular intervals during cycling. Borkiewicz et al. demonstrated that a combination of X-ray beam interaction, non-uniform stack pressure on electrodes, and non-conductive character of cell windows can influence the electro chemical reactivity in the batteries, altering the electrode activity by up to 20% [12]. It is therefore essential to carefully think about the in-situ cell design and to optimize the experiment parameters in order for the results to be relevant to commercial cell set-ups.PDF Image | Synchrotron-Based X-ray Diffraction for Lithium-Ion Batteries
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