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It can be observed that even 2 wt% of VC significantly affects cyclability of sulfur electrode. Very small differences between pure-ethers and enriched with 0.24 wt% VC are noticed. Further explanations can be given when comparing the cycling profiles of each cell. Figure 4-32b-d shows the feature of the voltage profiles of three cells at 1st, 5th and 40th cycles. It can be seen that during the initial sulfur reduction process, the upper discharge plateau is practically identical for the three cells (Figure 4-32b). Sulfur may indeed be relatively stable in the presence of little amount of VC. Once the polysulfides (and resulting S3-• radical, formed due to the disproportion of S62-) are the dominant species in the cell (transition region between two discharge plateaus), significant changes start to appear in the system. Especially, these changes are more pronounced when 2 wt% of VC is used. The reaction between carbonyl (C=O) groups of VC with the radical S3•- induces the formation of precipitated products, a large polarization and the consumption of active material. These changes are not directly correlated with the initial discharge capacity, but they strongly affected the capacity retention during the following cycles. After 40 cycles ‘0.24 wt% VC’ and ‘2 wt% VC’ cells display higher overpotential and lower capacity as compared with the ‘reference’ cell (no addition of VC). Parallel to the cycling tests done on sulfur electrodes, graphite electrodes were cycled (vs. Li) in three different electrolytes, for comparison: (i) classical carbonates LP10 (1M LiPF6 in PC/EC/DMC (1/1/3 in vol) + 2 wt% of VC, (ii) ‘reference’ ether-based electrolyte, (iii) ‘reference’ ether-based electrolyte enriched with 2 wt% of VC. Graphite electrodes (with composition 94/2/2/2 wt% = SLP30/SuperP®/CMC/NBR) were kindly provided from SRGE laboratory (CEA-Grenoble), and they were of average active material loading ~ 7.34 mg(SLP30) cm-2 (~ 2.64 mAh cm-2), coated on Cu foil. Cycling procedure applied to the three cells was: GCPL at C/10 with a cut-off potential equal to 10 mV, completed by a constant voltage step limited by a current value of C/100 C-rate in order to complete the discharge process. Figure 4-33 shows capacity retention of the three cells. 150 Chapter 4: Li2S electrodePDF Image | Accumulateur Lithium Soufre
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