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4.2.3.c) Cycling voltammetry profile Typical CV profile (Figure 4-8), recorded at a scanning rate of 0.03 mV s-1 is very coherent with GCPL curves previously shown (Figure 4-4a). While the potential was swept from the open-circuit voltage (~ 2.23 V) up to 3.2 V, a large oxidation peak related with Li2S oxidation appeared. The position of this first oxidation peak is shifted to higher potential values (~ 2.6 – 2.9 V), as compared with the other oxidation peaks recorded during next cycles. As previously discussed, oxidation of large Li2S particles induces high polarization. Once Li2S gets activated in the first charge, a characteristic response of a Li/S cell (when starting with a sulfur electrode) is observed, i.e. two cathodic peaks at 2.4 V and 2.0 V, corresponding to the reduction steps: (i) sulfur → high-order polysulfides and (ii) formation of low-order polysulfides153. During charging, a broad oxidation peak at ~ 2.35 V is observed, i.e. reflecting the oxidation of shorter polysulfides to longer chain polysulfides, together with a shoulder peak at ~ 2.5 V, interpreted by Manthiram et al.153 as the oxidation of an active material in confined structure. On the contrary, we believe that this peak should rather be attributed to the final oxidation of high- order polysulfides into elemental sulfur, in regard to the potential of sulfur formation by galvanostatic experiment, and supported by in situ XRD tests (see chapter 5), and the fact that in our electrodes no confinement of the active material was performed. The area of the peaks can be directly correlated with the practical capacity, and is relatively stable during 10 recorded cycles. Most of the changes occur during the initial cycle, which is in complete accordance with the capacity fading observed by GCPL technique. Moreover, the peaks are very sharp, probably because 20 wt% of carbon provide sufficient electron’s pathway for the reaction to occur at the applied scan rate. Figure 4-8. Cycling voltammetry curves of Li2S/SuperP®/PVdF electrode (with Li2S loading of 0.68 mg cm-2) at 0.03 mV s-1 scan rate, starting at OCV of ~ 2.23 V. Chapter 4: Li2S electrode 120PDF Image | Accumulateur Lithium Soufre
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