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The cells used for ESRF session were prepared from the electrodes with precisely known sulfur mass (optimized way of the electrode preparation). Each pouch cell was scanned in one position only (complete cell response; no ‘reference’ holes in the electrodes). Since the XRD data collection frequency was of about 18 minutes* for the same point, it would have been too long (not enough frequent) if recording on three different cell positions (like it was done for SOLEIL cells), i.e. risking to lose the important information of solid/soluble phase transition. 5.3. The initial cycle evolution at C/20 5.3.1. XRD and electrochemistry – results The following results show the complete cell evolution, where the beam penetrated through both electrodes (position [2] on Figure 5-1b). The results obtained at two other beam positions are discussed in further section. Figure 5-2 presents the XRD patterns recorded during initial galvanostatic cycle at C/20, and corresponding electrochemical data. The obtained cycling curve displays an expected voltage profile of a Li/S cell, together with high discharge and charge capacities of 980 mAh g-1 and 976 mAh g-1 respectively. The first spectrum recorded at the initial state shows peaks of elemental orthorhombic α-sulfur (PDF-2; no. 00-008-0247), which is fully expected from the raw material we used to prepare the electrodes. When the discharge starts to proceed, the intensities of sulfur peaks are gradually decreasing until they disappear completely with the end of the initial discharge plateau (Figure 5-2a; red line). This proves complete dissolution and reduction of sulfur already at the early stage of discharge process (i.e. at about 175 mAh g-1). In the ‘sloping’ region between the two discharge plateaus (2.3 V – 2.0 V), active species are present in the soluble form of high-to- mid order polysulfides, and no peaks are detected (up to about 380 mAh g-1). Right after the beginning of lower discharge plateau, the signal of crystalline Li2S (PDF-2; no. 00-023-0369) starts to appear, characterized by four peaks at 2θ = 11.6°, 13.4°, 19.0°, 22.3°, which correspond to the reflexions (111), (200), (220) and (311) respectively. The intensity of these peaks increases gradually, reaching a maximum at the end of discharge. interpretations (depth of discharge (%), moment of sulfur disappearance on the voltage profile), which were independent of active mass normalization. * During ESRF session, three cells were observed in parallel. A real frequency of recorded pattern between the cells was ~ 6 minutes. Thus, if scanning three cells in parallel, this time was elongated to ~ 18 minutes. Chapter 5: In situ and operando XRD 161PDF Image | Accumulateur Lithium Soufre
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