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due to the missing XRD patterns (beam loss, detector disconnection, etc.), as indicated with a cross in the Table 5-2. ESRF session allowed to monitor the evolution of two Li/S cells as well. Cell 3 was monitored during two initial cycles at C/8. Cell 4 was cycled in the laboratory for 25 cycles (at C/20) and stopped at the discharged state prior being monitored with in situ XRD during its following 25th charge and 26th discharge at C/20. The goal of observing later cycle was to verify if solid phase evolution could be dependent or not from the prolonged cycling. SOLEIL-based results were used for both qualitative and quantitative interpretations, and allowed for a deeper look into the mechanisms. On the other hand, technical difficulties were encountered during the shifts at ESRF, which reduced the quality of the obtained data, for that reason ESRF-based results were mostly used for qualitative purposes. 5.2.2. Cells design All the experiments were performed in a pouch cell configuration, with sulfur positive electrode (3.5 x 1.8 cm) coated on non-woven carbon-based current collector (so-called ‘S-on- NwC’; sulfur loading ~ 4.5 mgSulfur cm-2), lithium foil (3.6 x 2.0 cm) and separators (Viledon® with Celgard® 2400; both 3.8 x 2.2 cm) soaked with standard electrolyte (1M LiTFSI + 0.1M LiNO3 in TEGDME/DIOX 1/1 vol). The pouch cells were prepared in a dry-room (dew point -40 °C), while final activation step with the electrolyte (600 μL were added, i.e. excess of ~ 250 %**) was done in an argon filled glove-box. The pouch cell was designed to allow for scanning the same cell in three different positions, as shown on Figure 5-1. The idea of having such ‘3-holes’ configuration was to observe complete cell evolution (where XRD beam penetrates both electrodes at the same time, position [2]) as well as each electrode separately (so called ‘reference’ holes; a Ø 3 mm hole in the opposite electrode permitted to scan only one of the two electrodes; position [1] for Li foil observation, position [3] for sulfur positive electrode observation). Indeed, the following questions were still unanswered: (i) if solid Li2S formation (if any?) appears on both electrodes, (ii) if having a hole in lithium in front of sulfur electrode will modify the kinetics of the observed reactions and in which extent. The pouch cell was then placed on a movable sample holder and was shifted every ~ 3 min, so that the XRD beam was positioned in another part of the cell. The use of a movable sample holder did not affect neither the quality (high resolution of scan), nor the quantity of obtained patterns. ** The volume of the electrolyte necessary to fulfill all available spaces was roughly calculated based on Viledon®, Celgard®2400 and NwC porosities, and was found to be ~ 220 μL. ‘Dead’ volume of a pouch cell was not taken into consideration, however, it should be low. One should also note that the effective amount of electrolyte was not known precisely, since a part of electrolyte volume was drawn off during sealing process with the vacuum machine. Chapter 5: In situ and operando XRD 159PDF Image | Accumulateur Lithium Soufre
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