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Therefore, along the whole cycle, a relatively constant‡ contribution of RNwC is present, as shown on Figure 6-15b (in red). All the changes of MF semicircle upon cycling thus purely originate from modification of metallic lithium anode (RLi//CPELi), and more precisely of the passivation layer initially present on its surface. During discharge, the resistance value decreases and the specific relaxation time increases. Figure 6-15b (in green) shows the evolution of the Li/electrolyte interphase resistance upon initial cycle. We can clearly observe significant decrease of the resistance at the beginning of discharge, followed by quasi stable value until the end of discharge. Once the battery starts to discharge, oxidation of lithium is taking place, passivation layer is rearranged, and the surface of lithium is getting modified due to lithium stripping. Such observation was already reported in the Li-ion cell studies225. Surprisingly, in the Li/S domain, such decrease of the resistance was not correlated to the lithium response in literature, and it was rather attributed to the increase of positive electrode’s conductivity due to sulfur dissolution132,209. (a) (b) Figure 6-18. Comparison of Nyquist plot recorded at the initial state of Li/S (spectra # 1) and at the beginning of lower discharge plateau (spectra # 6) (a). Symmetric coin cells Li||Li with/without polysulfides left for storage (b). When comparing the Nyquist plot recorded at the initial state (spectra # 1) with the one at spectra # 6, it can be noticed that, apart from significantly decreased resistance, the characteristic frequency values are also shifted into higher range (Figure 6-18a). One of the hypotheses of such changes would lie in the presence of polysulfides in the electrolyte, which may affect the passivation layer on the Li surface in regard to both, composition, its resistive nature and thickness. Thus an additional experiment was performed to verify this hypothesis, i.e. if this resistance changes are strongly related with lithium modifications during cyclic in the presence of polysulfides. Symmetric coin cell composed of two lithium electrodes and ‡ In further discussion, more detailed information is provided. 213 Chapter 6: EIS and low temperature studiesPDF Image | Accumulateur Lithium Soufre
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