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during further cycles anymore. Because of such particular behavior of initial charge, the cut- off voltage is usually set up to higher values (> 3.0 V), while the following charges are reaching typical potential limits of Li/S cells, i.e. 3.0 V (or 2.8 V). More detailed discussion concerning the cut-off voltage potential is presented in section 4.2.5. Obtained charge capacity is as high as 970 mAh g-1, which is 85% of the theoretical value (1166 mAh g-1). During subsequent discharge, typical behavior of sulfur electrode is seen. Two main kind-of- plateaus can be distinguished: a ~ 2.4 V sloping plateau corresponding to the sulfur reduction and formation of long chain polysulfides, and a ~ 2.1 V plateau related to the creation of shorter chain polysulfides and solid Li2S product. The capacity obtained in the initial discharge is 630 mAh g-1, which stands for 54 % of the theoretical value. It is also important to note here that the capacity values are presented with respect to the mass of Li2S active material (molar mass: 46 g mol-1). If converted per mass of sulfur (32 g mol -1), capacity value of 906 mAh gSulfur-1 would be obtained**, which is in line with the results reported in chapter 2.3.2. Further cycle profiles exhibit exactly the same features as the cell with sulfur electrode. Very large irreversible capacity loss is observed (340 mAh g-1), and possible explanation is discussed later. Figure 4-5 compares the voltage profiles of Li2S and sulfur-based electrodes during initial cycle, both cycled with metallic Li, where the capacity values are presented in respect to the sulfur mass (mAh gsulfur-1) for both cells. Initial charge of Li/Li2S cell, due to its particularity, was not taken into consideration, thus the second charge profile was used instead. The capacity values are not exactly the same and should not be compared quantitatively, since both cells were of different active material loadings, thus the performances might also differ (1.23 mgLi2S cm-2 ↔ 1.43 mAh cm-2, and 2.46 mgSulfur cm-2 ↔ 4.12 mAh cm-2 for these particular cells). It can be seen that the discharge profiles are very similar (Figure 4-5a) and both plateaus appear at the same potential††. During the charge process presented on Figure 4-5b, both voltage profiles display characteristic little overpotential dip just at the beginning (related with breaking the insulating barrier formed during preceding discharge), followed by two main plateaus, typically assigned to the oxidation of shorter lithium polysulfides to longer chains (1st plateau), then further oxidized to elemental sulfur. Chapter 4: Li2S electrode ** All along this chapter, the capacity values are presented with relation to Li2S weight and not sulfur. This needs to be kept in mind, since it is not a conventional way of reporting the capacity results in the Li/S batteries. †† Slight overpotential of upper discharge plateau of S8 electrode is due to the big particles of sulfur raw material (even up to 50 μm), which vanishes in the following discharges due to sulfur dissolution and changes in particles distribution and morphology. 116PDF Image | Accumulateur Lithium Soufre
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