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expected, since the lower plateaus are assigned to the formation of insulating species which cover the electrode surface. This process is thus mostly governed by both diffusion phenomena and decrease of the surface area due to Li2S and/or Li2S2 precipitation. At a very fast current density (1C), a unique sloping plateau is observed. Probably, the current is too fast to reduce all sulfur through the multi-step process, and the reaction takes place at the extreme surface of the electrode only, which is rapidly blocked and polarizes. The other explanation could be a limitation by mass transport, which induces a large polarization. Rate capability tests were also performed. The C-rate was gradually increased every five cycles in following sequence: C/20 → C/10 → C/5 → C/2 → 1C → C/5 → C/10 → C/20. As first trials, the charge and discharge currents were chosen to be the same. But since the electrodes were highly loaded (4.66 mgsulfur cm-2 ↔ 7.18 mAh cm-2), a current density as high as 14 mA cm-2 was used at 2C during charge, and caused a severe dendrites growth on lithium electrode and short circuit. Therefore, the cycling procedure was slightly modified (so called ‘asymmetric’ cycling), and charging rate was set up to C/20 for each cycle. Thanks to such slow recharge, dendrites formation was minimized and the cells could be discharged at the different C-rates. Figure 3-9 shows comparison of discharge capacities obtained in classical power rate tests and ‘asymmetric’ one. Coulombic efficiency values are not presented, since they do not have a significant meaning (charge always performed at C/20). Very stable capacity retention is obtained. Figure 3-9. Rate capability tests done on ‘S-on-NwC’ electrodes (sulfur loading of ~4.7 mgsulfur cm-2) in two cycling configurations: ‘symmetric’ i.e. the same current applied during both discharge and charge processes (in red), ‘asymmetric’ i.e. slow charge at C/20 applied for each cycle, while varying only the discharge rate (in black). Moreover, such ‘asymmetric’ cycling procedure results in better discharge capacities as compared with the standard ‘symmetric’ one, as the charge process allows for more complete re-oxidation of the active species. From these power rate tests, it can be seen again that the cell cannot really perform well at faster C-rates (1C, 2C), resulting in very low capacity (< 200 Chapter 3: S8 electrode on NwC 82PDF Image | Accumulateur Lithium Soufre
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Sulfur Deposition on Carbon Nanofibers using Supercritical CO2 Sulfur Deposition on Carbon Nanofibers using Supercritical CO2. Gamma sulfur also known as mother of pearl sulfur and nacreous sulfur... More Info
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