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to 1.7 V (oxidation peak at ~ 2.35 V; Figure 4-20b). Thus, cycling tests down to 1.5 V or even lower should be avoided, because of the LiNO3 irreversible decomposition and the enhanced passivation of the Li2S electrode at the end of discharge, as well as the lack of stability and reversibility. (a) (b) Figure 4-20. CV curves of Li/Li2S cells containing LiNO3 as an electrolyte additive, and scanned at 0.03 mV s-1 with different potential windows: 3.2 V – 1.3 V (a) and 3.2 V – 1.7 V (b). Li2S electrodes have the same reference composition (70/20/10 wt%) and an active material loading of 1.35 and 0.86 mgLi2S cm-2. 4.2.6. C-rate influence To study the evolution of cycling performances at different currents, Li2S electrodes were carefully selected to have the most similar weights (~ 1.14 – 1.27 mgLi2S cm-2). They were then cycled at following C-rates over prolonged cycling: 1C, C/5, C/10, C/20 and C/100, using the same potential window: 3.8 V – 1.7 V and 2.8 V – 1.7 V, for initial and following cycles, respectively. Also extreme currents (equivalent of C/200 and C/1000) were applied in order to observe the evolution of initial activation barrier with the regime. Finally, rate capability tests were also performed. Figure 4-21 shows the voltage profiles obtained during initial cycles. When increasing the current, the polarization obviously increases, since Li2S is a very poor electronic and ionic conductor. As previously proposed, the oxidation reactions that occur along the ~ 2.5 V plateau (clearly visible at low C-rate) is associated with the oxidation of both Li2S and high-order polysulfides. At high current rate (i.e. 1C), oxidation reactions induce a large polarization and the cut-off voltage is reached easily. Moreover, the characteristic potential barrier, visible at the beginning of charge (Figure 4-21c), is reduced with the decreased current, as expected. At extremely low current (~ C/1000), oxidation reaction is no more limited by the kinetics, and the thermodynamic potential is obtained. In a large C-rate range (up to C/10), a similar potential plateau is obtained, very close Chapter 4: Li2S electrode 135PDF Image | Accumulateur Lithium Soufre
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