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Figure 1-5. Schematic diagram of a Li/S cell. Elemental sulfur as an active material can be used as-received for the electrode preparation. A conductive carbon additive is always added to the electrode, in order to provide an electronic percolation of the whole electrode. Polymeric binder is used for bonding all the electrode components together, and provides sufficient adhesion to the current collector. Sulfur is a yellow non-metallic solid, a cyclic molecule composed of eight atoms, so-called S8. Sulfur forms more than 30 different allotropes19, but the most thermodynamically stable one at room temperature (RT) is orthorhombic alpha-sulfur (α-S8), with molecular mass of 32.066 g mol-1 and a density of 2.07 g cm-3. Sulfur has relatively low melting temperature of 115 °C, and it can get easily sublimed. Orthorhombic α-sulfur form is the one used for the sulfur electrodes preparation. Another allotropic form, monoclinic beta-sulfur (β-S8) is rather known from being stable at the temperatures higher than 95.5 °C, and it is likely formed when slowly cooling down the melted sulfur solution20-22. Latest reports demonstrated on the unusual formation of this allotropic sulfur form in the Li/S system at the end of charge23-25, or as a starting material in the positive electrode (obtained through infiltration of elemental sulfur into CNT structure26). Li/S system is based on the electrochemical reaction of sulfur with lithium to form lithium sulfide (Li2S, final reaction product). Since sulfur is in the charged state (no Li+), the cell operation starts from discharge, while metallic lithium is getting oxidized at the negative electrode and produces lithium ions and electrons. Li+ ions diffuse to the positive electrode through the separator/electrolyte, while electrons go through external electrical circuit. The oxidation reaction occurring at the negative electrode is as follow: 16Li0 →16Li+ +16e- (1) On the positive electrode, sulfur is getting reduced by accepting Li+ ions and electrons, according to the equation (2): 17 Chapter 1: Literature reviewPDF Image | Accumulateur Lithium Soufre
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