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Li2S formation during discharge and moment of complete Li2S re-oxidation, at about 75 %SOC. These findings are in complete agreement with obtained in situ XRD results. It was also found that the initial resistance value was never obtained anymore upon cycling, even at full charge and discharge, which proves that some amount of active material may be lost in the electrolyte. The resistance of positive electrode bulk contribution was found to be weak and more or less stable during the cycling. However, changes of the MF semicircle upon cycling (and mostly during the initial cycle) are linked to the modification of the passivation layer on metallic lithium electrode. A strong impact of the polysulfides on the lithium passivation layer was also noticed, leading to the formation of a less resistive layer. The charge transfer reaction of the polysulfide species starts to be visible as a small loop just after the first discharge plateau. This semicircle increases largely by the end of discharge, and this phenomenon can easily be ascribed to the formation of Li2S insulating layer on the positive electrode surface. In addition, this evolution is accompanied by a strong increase of the transport limitation. During the charge process, the oxidation of Li2S results in a global decrease of the polysulfide charge transfer resistance. The evolution of the EIS spectra during further cycles is not so different from the evolutions obtained during initial one, except for the Li/electrolyte interphase. Indeed, initial resistance of the Li/electrolyte interphase is relatively high, since it is related with the initial state of metallic lithium surface. The contribution of lithium to the full impedance of the Li/S cell is then decreased after the first cycle. Last but not least, we were able to correlate the capacity retention as a perfect match with the electrolyte resistance evolution, which in turn gives an indication of the limiting factors. Regarding the cycling curve at low temperatures, more pronounced plateau between the upper and lower ones is visible, very likely due to the stabilization of polysulfides equilibria in solution (slower disproportionation/dissociation kinetics). Whatever the temperature, the shape of lower discharge plateau is preserved, indicating also that the formation of solid Li2S starts just at the same moment, while no other processes can be distinguished (single plateau is observed). At low temperature, less reversible evolution of the electrolyte resistance during discharge was observed, well-connected to the lower capacity obtained during the short plateau region. However, the resistance at the end of charge is near the initial value for all temperatures, indicating that the process of sulfur formation is much easier than Li2S one. Expected evolution of passivation layer response vs. temperature was found, i.e. increase of the resistance with a decrease of the temperature. The nonlinear evolution of the polysulfides charge transfer response, at the end of discharge and as a function of temperature, is well-correlated to antagonistic effects, i.e. lower kinetic at low temperature along with a lower coverage of the electrode surface by Li2S (lower capacity). 232 Conclusions & PerspectivesPDF Image | Accumulateur Lithium Soufre
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