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As concerning the Li/electrolyte interphase, it can be seen that the resistance is very small (~ 4 Ω) and relatively stable. It again proves that strong variations of the MF semicircle in the 1st cycle are strictly related to the initial surface state of Li. After the 1st cycle, the surface of Li is stabilized. Very similar situation is observed for the capacitance values. A significant variation is observed rather at the end of discharge. Figure 6-25d shows the evolution of LF semicircle fitting parameters, i.e. the charge transfer resistance involving polysulfides species, and corresponding Cdl. Very similar trend is observed as was already presented in the initial cycle, meaning reversibility of the reactions, as concerning both parameters, capacitance and resistance evolution. To conclude, we saw that all the parameters (like RNWC, RLi, Rel, RPS and corresponding capacitances) in HF, MF and LF ranges are correlated together and their evolution is also depending one from the other. In order to have more precise interpretation of MF – LF semicircles, mostly when they got diminished to ~ 2 – 4 Ω barely, probably another model should be designed, with the addition of additional contribution in a frequency range between Li interphase response and polysulfides charge transfer reaction. However, already with a simple model we proposed, fruitful information could be extracted. 6.7. The influence of the low temperature 6.7.1. Changes of the cycling voltage profile The discharge voltage profiles of the Li/S cell cycled at different temperatures in each cycle (Figure 6-26) clearly show that upper discharge plateau is much less affected by temperature changes, since it corresponds to the fast kinetics processes. With decreasing the temperature, more pronounced plateau between the upper and the lower one is visible, since both the electrochemical reactions and the equilibriums between soluble polysulfides are slowed down. Very likely that the nature and the composition of polysulfide species are much more defined when the temperature decreases, which enables the appearance of intermediate plateaus (evolution from 0 °C to -40 °C, Figure 6-26a). Lower discharge plateau is very dependent on the temperature, since it is related with the formation of solid products, at least of Li2S, which precipitate on the electrode surface, thus inducing slower kinetics of the reaction. Even at the temperature of -30 °C, the lower discharge plateau displays a high polarization, but its shape is preserved, which also means that no other processes appear. At -40°C, the lower discharge plateau disappears. However, when decreasing the C-rate to C/200, the plateau was observed again. When the temperature decreases, it is also possible that the solubility limit of medium-to-low polysulfides (S42-, S22-) is significantly diminished, leading to the fast precipitation of solids onto the cell, as previously said. However, 222 Chapter 6: EIS and low temperature studiesPDF Image | Accumulateur Lithium Soufre
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