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It is thus probably related to the fact that Li2S did not get activated because of too low potential limitation at the initial step. To avoid such problems, a slight modification of cycling program had to be carried out, without limiting the potential of the cells at the beginning. Another practical solution could also be the systematic addition of very little amount of polysulfides during cell fabrication, to completely eliminate the activation barrier. This alternative, however, was not implemented as a standard solution in this study. We have previously seen that the voltage shape of the initial charge (Figure 4-9) does not impact the cycling performances, and further cycle curves show the typical feature of a typical Li/S cell. GITT experiments proved that a large overpotential is not related with parasitic reactions, but is due to a difficult process of solid Li2S oxidation. Following this direction, we compared several coin cells’ results, in which different voltage profiles as well as variety of initial charge capacity values were obtained (Figure 4-15). The results presented here come from the Li2S electrodes punched out (Ø 14 mm disks) from two batches of electrode sheets (with reference composition 70/20/10 wt%), cycled at C/20 or C/10 rates, and using different potential limits. (b) Figure 4-15. Examples of various different initial charge (a) and corresponding discharge (b) voltage profiles obtained when cycling Li2S/SuperP®/PVdF = 70/20/10 wt% electrodes vs. Li at C/20 either C/10 rate, with a different initial charge voltage limits. All selected electrodes come from two batches of the electrode sheet, aimed to be identical. Active mass loading is varying between 0.77 to 1.73 mgLi2S cm-2. The numbers are given for simplicity, indicating the highest (1) to the lowest (5) capacity values obtained, both for charge and discharge. A linear relation was found between the charge vs. discharge capacity, as shown on Figure 4-16a, which proves (again) that the initial charge effectively impacts the following discharge capacity. Chapter 4: Li2S electrode (a) 130PDF Image | Accumulateur Lithium Soufre
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