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electrolyte, and the charge maximum occurring always during semi-plateau after the complete oxidation of Li2S. Figure 6-27. Electrolyte resistance evolution recorded during cycling at different temperatures: 25 °C, 0 °C, -20 °C and -30 °C. As concerning the electrolyte resistance evolution, the appearance and disappearance of insoluble polysulfides seem to be independent of the temperature, in particular with the appearance of insoluble Li2S at the beginning of the second plateau. Especially, it looks that the decrease of the electrolyte resistance during the lower discharge plateau is less and less pronounced as the temperature decreases (especially between -20 °C and -30 °C). Indeed, the resistance obtained at the end of discharge at 25 °C is closed to its initial level; more than 90% of the resistance increase is recovered. Whereas at -30 °C, the resistance at the end of discharge is much higher than its initial level, i.e. 70 Ω instead of 35 Ω, which means that only ~ 50% is recovered. Indeed, at low temperature, the large polarization can induce a limitation on the length of the second plateau. The formation of insoluble Li2S should then be lower, thus more soluble polysulfides are still present in the electrolyte at the end of discharge, inducing a high Rel (~ 70 Ω at -30 °C). Moreover, the solubility of the short polysulfides can be lower at low temperature, inducing some earlier precipitation. The resistance at the end of charge is weakly higher than the value observed at the initial point for the all temperature investigated, indicating a good reversibility of the oxidation of soluble polysulfides to sulfur. Further studies need to be carried out to completely understand this phenomenon, in particular, conductivity measurement as function of temperature and polysulfides composition. Vogel-Tamman-Fulcher (VTF) equation252 can be used to correlate the variation of electrolyte viscosity with the temperature. In a first approximation, considering no modification of the ionic species concentration, viscosity can be proportionally correlated with the electrolyte resistance (conductivity), using Stoke-Einstein relationship. When plotting the maximum Chapter 6: EIS and low temperature studies 224PDF Image | Accumulateur Lithium Soufre
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