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on the voltage profile at the beginning of lower discharge plateau (sometimes called as ‘supersaturation’ point), is the point where the concentration of soluble polysulfides anions is the highest (middle to short soluble polysulfides)106,246,250 and formation of Li2S starts (see in situ XRD, section 5.4.2). very poor solubility of Li2S210 results in the nucleation of solid phase. Once the progressive formation of Li2S occurs, soluble species are consumed, inducing the decrease of both viscosity and the resistance of the electrolyte. During charge, in opposite manner, progressive oxidation of solid Li2S is accompanied by the formation of mid-to-high order polysulfides, which in turn provokes the increase of viscosity. The maximum point (R”max) is in accordance with the moment of complete Li2S disappearance, as observed by XRD (75 % SOC). Afterward, formation of solid sulfur requires further consumption of soluble species (S62-, S82-), thus the electrolyte resistance decreases. These results are indirectly coherent to the results obtained through in situ XRD measurements (see chapter 5), and interpretation is in agreement with the one proposed by Kolosnitsyn et al.213 and other research teams132,209. It is interesting to notice that the electrolyte resistance never comes back to the initial value (red dashed line), which means that even at the end of charge/discharge, some soluble polysulfides are still present in the electrolyte, which is clearly visible when opening the coin cell for post mortem analysis. The color of the electrolyte solution is governed by the polysulfide species present in it, and it never become transparent again, neither at the end of charge nor discharge. This points out that some part of active material may be ‘lost’ in the electrolyte, and does not participate in the further reactions. This is rather coherent with the typical capacity retention behavior, where drastic capacity fade is mainly observed after the initial cycle. According to the literature, such capacity retention is mostly characteristic for the cells with large excess of electrolyte, which is actually our case. Another important point to mention is that the electrolyte resistance evolves in a different manner during discharge and charge. The maximum of the resistance during discharge (R’max = 10.2 Ω) corresponds to the highest soluble polysulfides concentration and is higher than during charge (8.3 Ω). As already discussed before for in situ XRD studies (chapter 5), a significant hysteresis between charge/discharge processes is observed for Li2S formation/re- oxidation. As demonstrated previously, solid Li2S product is detectable until 75 % SOC during charge. Since oxidation of Li2S to the soluble species is a difficult process, it may then explain the slower increase of the electrolyte resistance measured by EIS, i.e. slower increase of the soluble polysulfides concentration. In the charge process, the region where only soluble species are present (according to the XRD findings) is very short, and it was previously reported by Barchasz et al.194 that longer polysulfides (Li2S6, Li2S8) should be predominant in this region. It would mean that the composition of the electrolyte is not the same at the ‘maximum resistance points’ during discharge (R’max; where predominant species are S42-) and charge (R”max; with mostly S62- and S82-). Literature also reports on differences between charge and discharge, whatever the technique used: in situ and operando UV-Vis172 or in situ XAS202. 211 Chapter 6: EIS and low temperature studiesPDF Image | Accumulateur Lithium Soufre
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