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electrolyte containing dissolved polysulfides was left for aging over several days. EIS was recorded every 1 hour. The results are presented on Figure 6-18b in comparison with the symmetric Li||Li cell without polysulfides in the electrolyte and left for storage. Almost identical behavior, as the one observed in the full Li/S cell during discharge can be noticed. The resistance of semicircle decreases, while the characteristic frequency increases of an order of magnitude. This confirms the strong impact of the polysulfides on the lithium passivation layer that probably change its composition and thickness, leading to a less resistive layer. Moreover, in regard to Li/S Nyquist plot evolution upon cycling, the most significant Li modifications occur at the moment where S42- concentration is the highest (little dip at the discharge curve). It may thus indicate that mid-order polysulfides like S42-, S3•- (a radical species), have a stronger influence on Li surface than the higher-order polysulfides. During following charge, the resistance of the lithium-attributed loop stays very small, and slightly increases by the end of charge. This evolution may be related to the passivation film evolution (increase of the thickness of the layer). However, when comparing the values obtained for the bulk response of the electrode (RNwC//CPENwC) and Li/electrolyte (RLi//CPELi) interphase, further information can be extracted. Figure 6-19 presents the resistance values (RNwC) and corresponding capacities (calculated based on Q and α parameters extracted from fitting the spectra). For better clarity, voltage profile and electrolyte resistance are shown as well. It can be seen that actually the resistance of the electrode bulk contribution (RNwC) is not so stable, but is varying between 0.5 – 2.0 Ω. Slight increase is visible just at the beginning of discharge, and increasing again at the end of discharge. The corresponding capacitance is also varying, having the higher values in the region where solid Li2S was detected by XRD (along lower discharge plateau, up to 75% SOC during charge). However, the fitting error is also very high in this region. Nevertheless, in the global view, the value of RNwC seems to be stable. 214 Chapter 6: EIS and low temperature studiesPDF Image | Accumulateur Lithium Soufre
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