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often assigned to the Li2S formation, is of about 280 mAh g-1, which corresponds to the formation of 1.25 mg of Li2S, and which is not so far from the amount determined based on the SEM image. Moreover, we point out the fact that Li2S may not be the only product formed during the low voltage discharge plateau, which is further discussed in chapter 5, and a part of the current is dedicated to the S22- formation. Furthermore, the presence of the precipitates between the fibers may indicates that the amount of solid phase inside the electrode is larger than the one calculated using only the dense layer. Further investigations must be performed to have a clear idea about the nature of precipitates presented in the volume of the whole electrode. 3.4.8.b) The effect of pressing To follow this direction and to confirm that, indeed, the empty volume inside such porous carbon collector dictates the discharge capacity, we pressed the NwC T10-types disks on the pressing machine with: 1, 1.5, 2 and finally 5 tons‡‡, in order to decrease the thickness, thus the electrode porosity. ‘Catholyte’ type coin cells were prepared and cycled galvanostatically at C/10. Figure 3-23 shows the SEM photos of the pressed NwC T10 materials, with 0, 1 and 2 tons. It can be clearly seen that not only the carbon fibers are more packed, thus influencing the effective porosity, but they are also destroyed when pressed with too high load (2 tons). Figure 3-23. SEM photos of T10-NwC collectors: not pressed (a), pressed with 1 ton (b) and 2 tons (c). Figure 3-24 shows the initial cycle of five ‘catholyte’ cells, and their capacity retention upon few cycles (prolonged cycling was not obtained due to the “problematic” charge). During coin cell preparation, 75 μL of catholyte solution (0.25M Li2S6 + 1M LiTFSI + 0.1M LiNO3 in TEGDME/DIOX) was poured onto NwC disks. Cells were cycled at C/10, while the current was calculated in respect to the mass of sulfur introduced with the catholyte solution (i.e. 3.6 mgsulfur ↔ 6 mAh). ‡‡ When pressing the NwC Ø 14mm disk (1.539 cm2) with 1 ton, a pressure of 63.7 MPa (MN m-2) is applied. Proportionally, when pressing with 1.5, 2 and 5 tons, the pressure of 95.5 MPa, 127.4 MPa and 318.5 MPa is applied, respectively. Nevertheless, for clarity, the unit ‘ton’ instead of ‘Pa’ (or N m-2) was kept when referring to the pressed electrodes. Chapter 3: S8 electrode on NwC 99PDF Image | Accumulateur Lithium Soufre
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