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Chapter 4: Li2S electrode (a) (b) Figure 4-16. Linear relation between charge and discharge capacities, obtained for the Li/Li2S cells, presented in Figure 4-15 (a). Discharge capacity as a function of the capacity related with the upper voltage plateau, i.e. amount of sulfur reduced (b). However, we can also notice that, even if reaching the full charge capacity (yellow dot added on Figure 4-16a), a discharge capacity value is not any more on the straight extrapolation line. The reason for that may be related with other limitations appear, which prevent from obtaining full discharge capacity values in return, i.e. higher than 650 – 700 mAh gLi2S-1. This limitation seems to be very similar to the one already described in previous chapter, when working with sulfur cathode. Such linear evolution of charge-discharge relation also proportionally affects the irreversible capacity, calculated as capacitycharge – capacitydischarge. At that moment, a clear explanation cannot be given for this observation. Such proportionality seems to indicate that, whatever the amount of oxidized species is produced, only ~ 65 % is reduced afterward. This evolution cannot be related to some surface area problem, but perhaps more to the loss of active mass in the electrolyte, or to the large pulverization of the electrode. Other investigations must be performed in order to propose a full explanation. When comparing the shape of the discharge curves (Figure 4-15b), and more particularly the length of higher discharge plateaus, we can notice that cell #1, which presents the highest charge capacity, shows the longest upper discharge plateau, which is usually attributed to the reduction of solid sulfur to high-order polysulfides. We estimated the capacity corresponding to the upper discharge plateau (i.e. reduction of solid sulfur) for each of five cells presented and plotted vs. complete discharge capacity obtained. A very linear relation was found (Figure 4-16b), meaning that indeed, a final discharge capacity is proportional to the amount of sulfur being reduced during the upper discharge plateau. In order to confirm the hypothesis of sulfur formation at the end of charge, an in situ and operando XRD was performed on the Li2S electrodes, and obtained results are discussed in section 4.2.8. The necessity of applying higher voltage limit to perform Li2S activation, especially for the cells with higher initial overpotential, can also be shown by the CV experiments. Figure 4-17 131PDF Image | Accumulateur Lithium Soufre
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