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4.2.3. Typical electrochemical response 4.2.3.a) Galvanostatic cycling (GCPL) results The voltage profiles of two initial cycles (at C/20 and RT) of Li2S positive electrode (reference composition 70/20/10 wt% = Li2S/SuperP®/PVdF; active material loading of 1.23 mgLi2S cm-2) cycled with lithium as counter electrode, are as shown on Figure 4-4. Since the battery was assembled in the discharged state (lithium in the Li2S positive electrode), the cycling process started with a charge (oxidation of Li2S). It can be seen that the voltage profile of the initial charge significantly differs from the following charge, and a characteristic potential barrier just at the beginning is present, encircled in blue. This behavior is typical for Li2S positive electrode, and has been already reported in the literature139,141,143. Appearance of such ‘activation’ barrier is strongly related with the difficulties to oxidize Li2S active material, since it is a very poor electronic and ionic conductor (σion = 10-13 S cm-1)145, and as initially is present in the electrode in the form of micrometric particles. Furthermore, contrary to sulfur, Li2S is practically insoluble in most of the organic solvents45, which may also hinder the initial activation step. (a) (b) Figure 4-4. Typical cycling response of Li2S/SuperP®/PVdF positive electrode cycled vs. Li at C/20. Voltage profiles of two initial charges (a) and two initial discharge processes (b) are shown. Electrode loading of 1.23 mgLi2S cm-2. Blue circle marks the initial activation step. Once the cell starts to charge, the voltage increases rapidly up to ~ 3.25 V (almost 1 V overpotential), followed by a fast drop and stable charge plateau at ~ 2.5 V. Further discussion provides more detailed information concerning the origin of this barrier and its relation with cycling parameters and electrolyte additives. The end of charge is accompanied by a continuous increase of the potential along with an oxidation process at above 3.0 V, which is not observed Chapter 4: Li2S electrode 115PDF Image | Accumulateur Lithium Soufre
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