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second step of charge, the efficiency of Li2S oxidation decreases notably. Indeed, as previously mentioned, oxidation of Li2S4, leading to the formation of long chain polysulfides, can also occur in addition to the oxidation of Li2S2 and Li2S, in accordance with the potential increase, as probably of faster kinetics than the solid phase reaction of Li2S/Li2S2. 5.4.4. Conclusions To summarize, an original approach based on quantitative interpretation of in situ and operando XRD results, supported by ex situ XRD measurements (conducted on different S8/Li2S ratios), allowed us to propose a mechanism of solid products creation during discharge. At the beginning of lower voltage plateau, formation of Li2S occurs with efficiency close to 1, whereas afterwards, the efficiency clearly decreases, which could indicate the formation of intermediates species such as Li2S2, even if not detected by XRD. 5.5. Further cycles evolution at C/20 It is commonly known that initial cycle of Li/S batteries is not necessarily the most representative one. Therefore, in order to have more complete vision of the active material structural changes occurring in the system, it was important to record the XRD response of sulfur electrode during further cycles. 5.5.1. The second cycle The evolutions recorded during the second cycle are very close to the initial one. Figure 5-15 shows the XRD patterns obtained during 2nd discharge and 2nd charge, together with the experimental potential curves. The electrochemical performances result in expected voltage profile together with satisfying capacity values (892 and 923 mAh g-1 for discharge and charge, respectively). Higher capacity value for the charge can be associated with the shuttle mechanism. Despite of the technical issues (beam loss), leading to the loss of some XRD patterns, still valuable conclusions can be withdrawn. 181 Chapter 5: In situ and operando XRDPDF Image | Accumulateur Lithium Soufre
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