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Trying to better understand this phenomenon, the main questions to answer were: • Which voltage profile should be considered as the ‘correct’ one (?) • If a standard profile exists (?) • What are the reasons for such high polarizations (?) In the first attempt, we tried to verify if a large overpotential might come from the lithium electrode, and its highly passivated surface at the initial state (more information in chapter 6). By performing different treatments on Li surface before closing in the coin cell (i.e. cleaning the surface with dimethyl carbonate (DMC) solvent, scratching it with Teflon® knife), we did not see any improvements, thus Li was eliminated from the suspected parameters. Also freshly prepared electrolyte solution indicated that such a lack of reproducibility was rather coming from the positive electrode. In the next step, we also eliminated SuperP® and PVdF 6020 powders from the suspected limitations, as well as cyclohexane and NMP, by simply replacing these compounds with freshly dried powders and new solvents or solutions. After preparing the electrodes with different Li2S powders (three bottles purchased from Sigma Aldrich and 1 from Alfa Aesar), we realized that the quality/purity of the active material was not necessary equal for all the batches, and that it significantly influenced the initial charge profile. As previously presented in section 4.2.2, XRD patterns recorded on Li2S powders showed mainly the pure Li2S phase, with small peaks of LiOH, which could affect the oxidation process of Li2S particles during initial charge. However, no clear relation between the amount of LiOH impurity and the first charge behavior could be noticed, and the presence of impurities did not allow to explain the lack of reproducibility during the first charge process (furthermore, the origin of the LiOH presence in the Li2S powder is not sure, and the observed impurity could be linked to the XRD analysis). In order to have more accurate information about the surface state of individual Li2S particles and electrodes, surface characterization techniques (like XPS) should be applied for further studies. Figure 4-10 shows the initial discharge profiles of the six cells, whose initial charges displayed different behaviors, as previously demonstrated on Figure 4-9. They all turn to show the typical features of a standard discharge curve, with two well-known plateaus, regardless the way the first charge has proceed, and capacities between 580 mAh g-1 and 660 mAh g-1 are obtained. It is worth noticing that the oxidation process occurring at high voltage (> 3.0 V) is not linked to the parasitic reactions, as the following discharge capacity is dependent on this charging activation step (more detailed discussion is provided in further parts). As a matter of fact, a deeper understanding of this first charge behavior was sought for, and Galvanostatic Intermittent Titration technique (GITT) was applied to this purpose. 122 Chapter 4: Li2S electrodePDF Image | Accumulateur Lithium Soufre
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