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charge, whether performing it to higher potential limits (like 3.8 V) or applying relatively low currents, in order to possibly activate all the Li2S material present in the electrode. 4.2.9. Conclusions Li2S/Li based cells exhibited the same behavior and the same limits as the Li/S cell. However, the electrochemical process of the first charge is largely different from the one obtained in Li/S cells, or during the following cycles of a Li/Li2S cell. This large difference is attributed to the micrometric size of Li2S starting material, in parallel of its low ionic and electric conductivities, which makes the first oxidation process very difficult. Indeed, Li2S seems to be present during all charge period, which limits the overall oxidation reaction and induces large polarization in the main part of the charge curve. In accordance to the easy oxidation of soluble polysulfides, sulfur seems to be obtained early during charge, then Li2S and S8 may be present together on the electrode upon charge. 4.3. Complete metallic Li-free cells We have seen previously (section 3.5) that metallic lithium causes severe short circuits due to the dendrites growth. Development of Li2S electrode allows to replace metallic Li by safer lithium metal-free negative electrodes, like Tin (Sn), graphite (C) or Silicon (Si). In this study, we aimed at combining the Li2S cathode with both graphite and/or Si anodes. Choosing to work with graphite - as the mostly used anode material in commercial Li-ion cells, very well-known for years and presenting a relatively stable discharge capacity upon cycling, brings additional challenges to be solved, i.e. electrolyte compatibility. It is commonly known that graphite material is getting exfoliated in ether-based electrolytes, the electrolytes of choice for classical Li/S batteries237. On the other hand, sulfur-based electrodes cannot be cycled with carbonate-based electrolytes, since polysulfides react with carbonyl groups. Vinylene Carbonate (VC) is an additive commonly known for its beneficial impact on the formation of a stable SEI on the graphite electrode, and is usually added to carbonate-based electrolytes (such as in EC/PC) in the popular amount of ~ 2 vol%. We were therefore interested to test our ‘reference’ electrolyte composition (1M LiTFSI + 0.1M LiNO3 in TEGDME/DIOX 50/50 vol%) with this additive designed for graphite, and evaluate how it would affect the cyclability of both sulfur and graphite electrodes. Obtained results are presented further. As previously said in the introduction of this chapter, silicon, being one of the most promising materials for negative electrode, was selected for the prof-of-concept of complete cell, i.e. Li2S vs. Si. 148 Chapter 4: Li2S electrodePDF Image | Accumulateur Lithium Soufre
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