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Chapter 5: In situ and operando XRD studies: insight into the structural changes of the active material 5.1. Motivation of the work As already mentioned in the bibliographic summary, before we performed this work, most of the previous XRD studies were done through ex situ analyses. However, the opinion was divided when concerning the most important questions related with sulfur and Li2S formation. Predominantly, crystalline Li2S was detected on the positive electrode just at the end of discharge28,205,206 or already in the middle of lower plateau117,207 and turned to disappear completely during following charge. Some other reports claimed that complete conversion of Li2S into soluble species does not occur37. Opinion about the moment of sulfur disappearance/reappearance during cycling was divided as well117,206,207. The debate was also concerning the end of charge, whether elemental sulfur is formed117,244 or soluble polysulfides never transform back to solid S837,207,245. Later, in situ and operando tests performed by Nelson et al.199 and Cañas et al.198 did not completely give a clear answer to the questions. Nelson’s results reported on complete disappearance of elemental sulfur during initial discharge and reformation at the end of charge, in agreement to Cañas’ statements. However, the main discrepancies lied in Li2S product. Nelson did not find any traces of crystalline Li2S (however, the presence of amorphous phase was not verified), while Cañas et al. reported that it starts to form at ~ 60% DOD, and disappears completely during following charge. Therefore, the aim of this work was to have clearer idea on the solid/soluble phase evolution occurring in our cells and to find the answers on still unresolved or debated questions. The fact of performing two independent synchrotron sessions allowed to design variety of measurements. Obtained results gave a deeper vision of the Li/S system performances, where cycle number and C-rate were also taken into consideration. The idea of using two current densities (C-rates) aimed at verifying how the kinetic and the moment of solid products formations/disappearances would be affected. 157 Chapter 5: In situ and operando XRDPDF Image | Accumulateur Lithium Soufre
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