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(a) (b) Figure 3-15. Schematic illustration of the surface area contribution coming from NwC and/or electrode ink (S/SuperP®/PVdF = 80/10/10 wt%) coated on the collector, for both electrodes: ‘S-on-Al’ (a) and ‘S- on-NwC’ (b). Surface area of the electrode ink was found to be ~ 471 cm2, calculated with respect to the coating weight (8.56 mg on the Ø 14 mm disk) and BET value (5.5 m2 g-1). The area of solely NwC disk, with its BET of ~ 0.05 m2 g-1, was found to be ~ 7.3 cm2. It is clear that a direct correlation between the surface area and the discharge capacity could not be evidenced. Indeed, in this particular case, differences in terms of surface area do not allow to explain the beneficial impact of NwC collector to the overall capacity and cyclability. It may be more relevant to talk about accessible surface area, when trying to correlate the electrode surface area with the practical capacity, as largely discussed in the literature104. We already mentioned that carbon fibers offer more efficient electronic percolating network over classical Al foil, due to its 3D structure. In order to go deeper into understanding of the polarization phenomena, Electrochemical Impedance Spectroscopy (EIS) measurements were performed on symmetric coin cells (cell preparation and EIS conditions previously described in section 2.2.1), composed of two identical electrodes. The EIS results show (Figure 3-16) that the presence of NwC in the electrode structure is accompanied by a significant decrease of the resistive response of the system, as compared with the Al-based electrode, with characteristic frequencies in the range of few kHz for both electrodes. As previously described, such large resistance value of ‘S-on-Al’ electrodes in symmetric cells, may be related to the poor homogeneity of the electrode, and attributed to its bulk resistance, resulting in poorly efficient ionic and electronic pathways, and polarization due to the charge accumulation. Since NwC is a non-woven electronic conducting tissue with a large porous structure and internal resistance of 0.8 Ω216, the electronic and even the ionic conductive pathways are more efficient than in a 1D electrode, and induce a neat decrease of the electrode response (~ 2 Ω instead of ~ 40 Ω). Therefore, the reason for such remarkable decrease of the MF semicircle resistance might relate to the fact that the carbon/sulfur clusters (cluster: sulfur particles bound together by the carbon particles and binder) are directly connected with the NwC carbon fibers, and the fact of having a non-homogenous ink (manual incorporation of a PVdF binder) does not necessarily give a rise of electrode bulk impedance, as electronic conductive pathway inside the electrode is very efficient. Then the response at the MF region seems to be well connected with the electronic pathway of the electrode. 91 Chapter 3: S8 electrode on NwCPDF Image | Accumulateur Lithium Soufre
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