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4.2.8. In situ and operando XRD – Li2S structural changes upon cycling In the previous section, we had an overview on the electrochemical behavior of Li2S electrodes, and how different parameters/modifications could be tuned in order to improve the electrochemical response of Li2S electrodes. The literature concerning observation of Li2S electrodes evolution during cycling by XRD technique is not as abundant as for S8 electrode. Scrosati et al.151 performed in situ XRD tests during initial charge, and reported on almost complete vanishing of Li2S material. No evident proof of S8 formation at the end of charge was given, but they ‘reasonably assumed’ that indeed, sulfur got created. On the contrary, Cui et al., in their in situ XRD studies141, showed complete Li2S oxidation but no S8 appearance. Manthiram et al.153 reported that some un-reacted residuals of Li2S were still present at the end of charge, however, they did not focus on observing sulfur formation. Inspired by the literature data and trying to understand the behavior of Li2S active material upon cell operation, we focus in this part on the observation of the structural changes of Li2S material upon initial charge. The following points were questioned: • If all Li2S active material is consumed during initial charge (?) • If crystalline solid sulfur appears at the end of charge (?) • If finally higher charge potential limit may influence any of these changes (?) 4.2.8.a) Experimental details The measurements were performed in synchrotron facility (ESRF-Grenoble). Li2S electrodes (3.0 x 1.8 cm) were prepared on NwC collector inside the glove box, according to the procedure described in 4.2.1.a). The mass of each NwC collector was precisely verified before electrode ink casting. The final electrodes composition was 70/20/10 wt% (Li2S /SuperP®/PVdF 6020) with average Li2S loading of ~ 2.2 mgLi2S cm-2. Preparation of pouch cells is precisely explained in chapter 5 (fully devoted to in situ XRD studies of Li/S cells). However, in case of Li2S-based pouch cells, all the fabrication steps were conducted in the glove box. Two similar pouch cells were monitored in parallel during initial charge, thanks to the use of a movable sample holder. Such solution allowed displacing the cells so that the beam could penetrate through the two cells with the time interval of ~ 18 minutes. Two different charge cut-off voltage values were applied: 3.2 V and 3.6 V. Both cells were cycled at C/20 rate (i ~ 0.13 mA cm-2). XRD patterns were recorded at the wavelength of λ = 0.495936 Å. Chapter 4: Li2S electrode 144PDF Image | Accumulateur Lithium Soufre
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