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S8 +16e- →8Li2S (2) The overall reaction occurring inside the Li/S cell, at the voltage of ~ 2.15 V, is as described by equation (3): 16Li+S8 →8Li2S (3) Based on the molecular weight of sulfur (which is a relatively light element as compared with transition metal oxides used for insertion positive electrode materials), and the fact that 2 electrons can be accepted per one atom of sulfur, very high theoretical specific capacity of 1675 mAh gsulfur-1 can be expected. A particularity of this system mostly lies in its working mechanism, which is very complex. The overall reaction between element sulfur and lithium to form lithium sulfide is not an intercalation reaction. It is neither a direct one, as the reduction of solid sulfur is accompanied by the formation of a family of species, so called lithium polysulfides (Li2Sx, 3 < x ≤ 8), which are easily soluble in the organic electrolyte. The properties of the different polysulfides (i.e. their solubility, color, etc.) are strongly related with their chain length27. We can distinguish thus the following polysulfides groups: (i) ‘high-order’ or ‘long chain’ polysulfides: these are the species with the longest chain, i.e. Li2S8, Li2S6; (ii) ‘medium-order’ is mainly Li2S4, still relatively easily solubilized; (iii) ‘low-order’ or ‘short chain’ polysulfides are the species: Li2S3, Li2S2 and Li2S. The shorter is the chain, the less soluble the species are. As a matter of fact, the final Li2S product is a solid material. A typical cycling profile of the Li/S cell is shown on Figure 1-6. Figure 1-6. Typical voltage profile of Li/S cell5. Chapter 1: Literature review 18PDF Image | Accumulateur Lithium Soufre
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