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As concerning the disappearance of Li2S, two steps are also noticed, however, the slopes and the capacity at the transition point (B) are different from those observed during the discharge process. It can also be noticed that complete Li2S disappearance moment is much later than when the formation starts, probably due to the insulating nature of Li2S and its difficulty to be oxidized (as discussed previously). This additional observation clearly indicates that there is a significant difference between discharge and charge processes, which is in agreement with the potential evolution. Recent studies of Petel et al.172 and Cuisinier et al.202 also reported on remarkable differences occurring between the processes involved in the discharge and charge, whether investigated by UV-Vis or XAS techniques, both applied via in situ and operando methodology. The surface area of all four Li2S peaks was integrated and then normalized to the same maximum value. The same formation/re-oxidation behavior was found for all reflections. It may mean that nucleation and growth of the Li2S particles are isotropic, since the increase is similar for all directions, and there is no preferential orientation of the crystallites in the electrode. Since Li2S peaks are very broad, they may be considered as nano-size particles. Therefore, Scherrer equation (12) was applied to calculate the crystal size (d), where k (instrumental factor) was assumed to be 0.9. = *+ (12) , !- . Only the most intense peak (111) was taken into consideration, and the results are presented on Figure 5-10. It seems that crystallite size increases very rapidly just at the beginning of Li2S formation, followed by very slow increase of size, reaching the maximum value of ~ 8.8 nm. The rapid increase of the crystallite size is concomitant with the large efficiency of Li2S formation. This evolution may be related to the control of Li2S crystallization by nucleation process. This parallel evolution may be associated with a slow growth of particles size, really likely due to the decrease of the electrode active surface by insulating Li2S coverage. Quite similar values of Li2S crystal sizes at the end of discharge were reported by Abruña et al.200 (7 nm), and Cañas et al.198 (6.5 nm). During the charge process, a two-step evolution can be also noticed regarding the crystallite size. First, in large part of Li2S dissolution, the crystallite size decreases really slowly, which may indicate that only smaller nano-particles of Li2S may contribute to the charge process. In the case of smaller particles consumption, calculated crystallite size may not evolve, while the total amount of Li2S is actually decreasing. This can easily be understood, as the smaller Li2S particles may be oxidized more easily as compared to the larger ones (as analogy to nano-Li2S and commercial micro-size Li2S, see section 4.2). In a second part, a neat decrease of the crystallite size occurs, which may be explained by the consumption of bigger Li2S nano-crystallites at this step. Chapter 5: In situ and operando XRD 172PDF Image | Accumulateur Lithium Soufre
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