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higher polarization is required to break this resistive layer. Indeed, after the polarization barrier is overcome, the resistance values decrease very rapidly but are still present until the beginning of the upper charge plateau. Then, it seems that Li2S stays until there, which is in agreement with XRD findings concerning the moment of complete Li2S consumption. In this case, also the capacitance values are shown (Figure 6-21a). However, it was noticed that for RPS//CPEPS semicircle fitting, very small error values were obtained (for resistance: ~ 1 – 2 %, for capacitance: ≤ 1 %). It again highlights the fact that our simplified model should be modified in order to precisely evaluate the MF responses, which are very small in the Nyquist plot, as compared with the polysulfide/carbon interface, especially at the end of discharge. For the capacitance representation (F cm-2), we used the estimated electrode surface of 471 cm2 (based on BET measurements of composite electrode, see Figure 3-15). It can be seen that the capacitance values are increasing until the middle of the lower discharge plateau. Right after that, when the resistance starts to increase rapidly (starting form x ~ 9 h), the capacitance is progressively decreasing until the end of charge. Moreover, we can notice that the capacitance values are in the range of ~ 10-6 F cm-2, and this value is usually consider as a double layer capacitance (Cdl), which in turn proves the fact that we correctly attributed this semicircle to the charge transfer resistance on the positive electrode. The resistance value (RPS) is the lowest at the moment where the concentration of the polysulfides is the highest, in accordance with an easier charge transfer reaction on the carbon surface, in the presence of soluble polysulfides species. This evolution is correlated with electrolyte viscosity (~ Rel), i.e. the highest concentration of polysulfides corresponds to the highest viscosity. The diffusion processes evolution is a complicated mechanism to follow, in order to understand it correctly. It is still under investigation and thus was not presented in this work. 6.6. Further cycles evolution Figure 6-22 shows a visual comparison of the Nyquist plot evolution recorded during 2nd and 3rd discharges, together with a comparison to the initial discharge, as extensively discussed previously. It can be noticed that during next cycles, the contribution of the Li/electrolyte interphase (MF semicircle) becomes smaller than in the initial state, reaching relatively stable (and small – almost negligible as compared to a large LF semicircle appearing) resistance value in the further cycles. It proves again that the EIS response during initial discharge is strongly dictated by the Li electrode. On the contrary, other contribution evolutions, like formation of Li2S (thus progressive increase of RPS), seem to be reversible. 218 Chapter 6: EIS and low temperature studiesPDF Image | Accumulateur Lithium Soufre
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