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Chapter 4: Into safer cells: metallic lithium- free Li-ion/S system 4.1. The goal of the work In the previous chapter, we saw several examples where lithium dendrites formation effectively shorten (or ended) the life of the battery. These finding, supported by the general trend of lithium metal safety issues, gave us the motivation to go into the direction of safer lithium- ion/sulfur (Li-ion/S) cells, by eliminating metallic lithium electrode and replacing it by other insertion or alloying type negative electrodes. Our first choice focused on the use of a silicon (Si) negative electrode. Indeed, Si active material offers the highest specific capacity among all negative electrode materials, and is usually foreseen to be combined with sulfur positive electrode189,190,233. Moreover, it is known that Si electrode can be cycled with ether-based electrolytes, the electrolyte of choice for sulfur- based materials189,234. Indeed, lithium polysulfides are strongly reactive with carbonyl groups29,155,157, thus classical carbonate electrolytes of Li-ion batteries would not be an optimized choice for Li-ion/S system. One should note that Si electrode is also known from its issue of cycle life and large volumetric changes233,235 occurring upon cycling, but optimization of these was not the goal of this study. In order to pair silicon with sulfur (S8) into a full lithium metal-free cell, one of these components requires pre-lithiation. Another solution consists in the use of an alternative cathode material, lithium sulfide (Li2S), known as a lithiated counterpart of sulfur. As already presented in the bibliographic study chapter (section 1.3.1.e), Li2S is lately receiving an increasing attention, mainly due to its high specific capacity (1166 mAh g-1) and the fact that it offers the possibility of direct combination with Li-free negative electrode: Si141,154, tin (Sn)193 or even graphite140. Another considered option for Li-ion/S cell was the use of graphite electrode, which is currently widely used in Li-ion systems, is well-known in terms of structure and electrochemical mechanisms, and which shows a stable electrode morphology and discharge capacity upon cycling. However, due to the reactivity of the polysulfide species with C=O groups, carbonates solvents are a priori not usable, while graphite electrode is getting exfoliated in the presence of ethers236,237. On the other hand, vinylene carbonate additive (VC) is well-known to improve the passivation layer (SEI) on the graphite electrode in carbonate-based electrolyte mixtures. Therefore, we investigated the effect of adding a low amount of VC to the ether-based 109 Chapter 4: Li2S electrodePDF Image | Accumulateur Lithium Soufre
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