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diffusion of species increases31. In the same manner, more viscous electrolytes (based on more viscous solvents) also result in reduced shuttle mechanism45. Once the ‘infinite charge’ appears, the cell may have difficulties to come back to the normal electrochemical operation, and thus the battery life is shortened. Presence of ‘shuttle’ may also result in the deposition of solid species (Li2S) on the metallic lithium surface, leading to a capacity fade because of the irreversible active material losses. 1.2.2.c) Negative electrode issues Metallic lithium is commonly used as a negative electrode in Li/S system, and is also impeded by its own problematics. The dendrites growth (due to an uneven deposition of Li during plating) may cause several micro short-circuits during battery charging46, which may in turn provoke complete cell death and safety issues. Some studies reported that, in the presence of polysulfides, the growth of dendrites may be suppressed or slowed down, due to the strong reactivity of polysulfides with lithium47,48. Nevertheless, it still stays as an unresolved problem of metallic Li negative electrode, even if some solutions are proposed currently, as presented further. Another issue related to the use of metallic Li as negative electrode is its high reactivity towards air, water and organic species. In particular, a passivation layer (solid electrolyte interphase, SEI) can easily be formed on its surface during lithium lamination, cell assembly, when electrolyte is introduced in the cell, during aging etc. This SEI evolution in the cell may cause a remarkable irreversible capacity loss and lithium consumption, while also resulting in an increased internal cell resistance, if the passivation layer is too thick and/or resistive. Aforementioned system limitations lead to the situation that, despite of intensive research which has been conducted during last two decades, Li/S batteries still offer practical energy density values rather below the expectations, together with relatively short cycle life as compared with the mature Li-ion technology. The next paragraph presents briefly state of the art of today’s Li/S batteries, with some examples of the strategies addressing these particular issues. 1.3. State of the art and the strategies undertaken for Li/S system improvements Several different strategies were undertaken in order to mitigate abovementioned limiting problems, i.e.: • Low utilization of sulfur during initial discharge, due to low electronic conductivity 22 Chapter 1: Literature reviewPDF Image | Accumulateur Lithium Soufre
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