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It can be seen that all available conductive area is cover by a dense layer. The solid reduction product deposited on the conductive surface of the composite electrode (i.e. on the SuperP®/PVdF matrix, found on the top of NwC) as well as on the carbon fibers. It means that the polysulfides formed during progressive discharge were able to penetrate throughout the whole electrode thickness, reaching also the bottom side of the collector. However, it can be also seen that two types of deposits got formed on the fibers’ surface (Figure 3-5d-f). A very dense but thin nano-layer (estimated to be around ~ 150 – 200 nm) was created in close contact with the ‘fresh’ carbon fibers. In addition, less dense precipitate can be seen on the surface of this thin nano-layer, as well as more in the volume of the electrode, i.e. between the fibers (marked with the green arrow). As the final discharge product (expected to be solid Li2S) is known from its insulating nature, it is surprising to see that it was observed in the whole volume of the electrode, and at a large distance from the conductive fibers (few μm). Therefore, this second but less dense layer may also arise from precipitated polysulfide species as Li2S2. Indeed, some literature studies proposed an insoluble behavior (or poorly soluble) of Li2S2 in the electrolyte37,38,194. Further discussion concerning this observation is included later in this chapter. 3.4. Electrochemical performances of ‘S-on-NwC’ electrode 3.4.1. Typical galvanostatic cycling response (C/20) An example of charge/discharge voltage profiles of a Li/S cell with ‘S-on-NwC’ electrode (loading of 4.42 mgSulfur cm-2), measured by galvanostatic cycling at C/20, is shown on Figure 3-6a. During the initial discharge, two characteristic plateaus are visible: (i) at 2.32 V, which is commonly attributable to reduction of solid elemental sulfur and formation of longer chain polysulfides, and (ii) the lower plateau at 2.02 V, related with the reduction of shorter chain polysulfides and formation of final solid products Li2S2/Li2S. A discharge capacity of 1116 mAh g-1 was obtained practically, which is about 67 % of the theoretical value (1675 mAh g- 1). If we thus consider the areal specific capacities, a relatively high value of 4.96 mAh cm-2 was obtained, which is much higher as compared to what is usually reported in many publications, and what we already presented for the aluminum based composite electrodes (see 2.3.2). For the same loading (i.e. 4.5 mgsulfur cm-2), areal capacity of 2.5 mAh cm-2 was obtained for Al-based electrode, a half of what was reached with NwC-based one. 78 Chapter 3: S8 electrode on NwCPDF Image | Accumulateur Lithium Soufre
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