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(Figure 3-25b,c), may prove that sulfur reduction in TEGDME/DIOX-based electrolyte can proceed relatively easily, without necessarily good coating of each sulfur particle with carbon, when C/20 was used. However, the low voltage discharge plateau is much shorter in case of ‘S-melted’ and ‘catholyte’ than ‘S-on-NwC’. As the capacity of the low voltage plateau is not only/directly related to the electrode surface (as discussed in section 3.4.6), the use of coated composite electrode (‘S-on-NwC’) may only slightly increase the performances, while having the surface of about 60 times higher than pure NwC Ø 14 mm disk area (to recap: based on BET measurements, the NwC collector area is about 7.3 cm2, while the area of composite ink S/SuperP®/PVdF was found to be 471 cm2). It proves again, that higher surface area does not necessarily result in proportionally higher discharge capacity, and that we should rather talk about ‘available for polysulfides’ conductive area and porosity. When comparing capacity retention over several cycles (Figure 3-26b,c), in both practical mAh cm-2 and mAh g-1, it can be noticed that the ‘S-melted’ electrode does not behave as good as the ‘S-on-NwC’ one, this latter containing additional amount of SuperP® carbon. Indeed, even if the first discharge capacities are relatively high for the two electrodes (~ 3.5–5.0 mAh cm-2 or 800 – 1000 mAh g-1), the ‘S-melted’ one shows severe capacity fading, with only 25 % of the initial capacity remaining, while the ‘S-on-NwC’ electrode retains about 75 % of the initial capacity. To conclude, these results prove that the preparation method based on melting sulfur on NwC, even if offering simplicity and quite nice initial discharge capacity, is not an optimized solution for prolonged cycling. The most stable capacity retention was observed with ‘catholyte’ cell. Further tests must be perform to explain the large differences in cyclability behavior of ‘S- melted’ and ‘catholyte’ cell. In fact, after the reduction of sulfur, the two aforementioned configurations are the same. The only difference lies in their loading, much higher for ‘S- melted’ than the ‘catholyte’ cell. Therefore, in future studies the loading effect on the performances must be also taken into consideration. 3.4.10. ‘Insulated sulfur electrode’ coin cell concept We previously demonstrated that even if sulfur is melted onto a NwC disk, and it occupies all the volume between carbon fibers, without necessarily intimate connection with conductive surface of a carbon nano-material, sulfur can still be easily reduced. To give additional clues of that, a special coin cell was built as shown on Figure 3-27. 103 Chapter 3: S8 electrode on NwCPDF Image | Accumulateur Lithium Soufre
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