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(a) (b) Chapter 4: Li2S electrode Figure 4-25. Initial charge (a) and discharge (b) voltage profiles obtained through galvanostatic cycling of Li2S/SuperP®/PVdF electrode, made from ‘energetically’ ball-milled mixture (in red), and cycled vs. Li at C/20 rate ( potential window 3.8 V – 1.7 V). For comparison, electrode made from as-received Li2S grinded manually with SuperP® (in black). Both electrodes were prepared with the same active material loading: 1.14 mgLi2S cm-2. It can be seen that similar charge capacities are obtained for both electrodes, even if the voltage profiles are somehow different. After the ‘activation step’, the ball-milled sample exhibits a progressive increasing in the polarization of the ‘pseudo plateau’ of charge. The drastic difference, however, and rather coherent with SEM observations, is visible in discharge values (Figure 4-25b), i.e. ‘ball-milledenergetical’ powders exhibit very low capacity. As no additional carbon additive was incorporated to the electrodes during the ink preparation, and if carbon powder got agglomerated during the ball milling process, there may not be enough carbon nano-particles to create an efficient conductive network in the electrode, and to assure the charge process and the cohesion of the electrode, which is based on the binder adsorption on carbon particles. BET should be performed in the future for confirmation of this failure mechanism. To conclude, ball milling of commercial powder, even if decreasing the particles size, did not provide any enhancement of the electrochemical response. On the contrary, too energetic ball- milling provokes a strong carbon agglomeration, and gives the opposite from expected effect, i.e. no practical discharge capacity can be recuperated. 4.2.7.b) Carbon-based current collector (NwC) In this part, we investigate the beneficial effect of NwC current collector, and how it can affect the cycling of Li2S electrodes. For best comparison, the same slurry (70/20/10 wt% = Li2S/SuperP®/PVdF; Li2S used as-received) was prepared, and two electrodes were casted 141PDF Image | Accumulateur Lithium Soufre
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