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5.7. Conclusions This work brought new information and deeper understanding of the working mechanisms of Li/S batteries. We successfully analysed the structural changes of active materials inside a Li/S battery during few initial cycles using XRD technique. Our in situ synchrotron-based results clearly indicate formation of crystalline Li2S on the positive electrode at the very beginning of lower discharge plateau. We also propose a sequential discharge mechanism related to the low voltage plateau. First, reduction of S42- into Li2S occurs with an efficiency close to 1, whereas for higher DOD%, a competitive reaction appears, and the obtained data seem to be coherent with the formation of Li2S2 phase in the second step of lower discharge plateau. During charge, Li2S complete consumption was observed. The moment of Li2S disappearance is relatively late, probably due to the insulating nature of Li2S and its difficulty to be oxidized. We also confirmed that soluble polysulfides are oxidized into solid S8 at the end of each charge. It was found that, after recrystallization, sulfur does not come back to its pristine structure, but it appears as another allotrope: monoclinic β-sulfur. The same behaviour was obtained over the next cycles, with the formation of Li2S at the beginning of the lower discharge plateau in a two- step process, and with significant hysteresis between the charge and discharge processes. Similar evolutions were also observed at moderated C-rate (C/8). The XRD results allow to make a step forward regarding the understanding of discharge/charge mechanisms, but electrochemical impedance spectroscopy was additionally applied to the Li/S system and correlated with the previously discussed findings. The obtained results are presented in the following chapter. 190 Chapter 5: In situ and operando XRDPDF Image | Accumulateur Lithium Soufre
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