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1.3.1.b) The effect of a binder The role of binder is very important and probably even more crucial than in Li-ion classical cells, due to the significant morphological changes appearing in the positive electrode, since repeating dissolution/deposition processes appear upon cycling. Binder is indeed responsible for maintaining the electrode integrity after the active material is dissolved, together with allowing a good adhesion to the collector. The important role of the binder is also to ensure a homogenous dispersion of sulfur and carbon particles. It should not undergo swelling when in contact with organic solvents111, and preferably being dissolved in non-toxic solvents. The most popular polymeric binders used in Li/S cell fabrication are poly(ethylene oxide) (PEO)107,109 and poly(vinylidene difluoride) (PVdF). Nevertheless, both of them are reported to possess some problems: PEO exhibits rather poor adhesion to the current collector and low ionic conductivity at room temperature112. Moreover, PEO can swell in some ether based electrolytes (TEGDME for example), which may induce a loss of contact113. On the contrary, according to Lacey et al.107,114, their PEO-based electrodes display the best performances in terms of capacity (~ 700 mAh g-1 was preserved at 1C over 200 cycles) over three studied binders: PEO, CMC/SBR (carboxymethylcellulose combined with styrene-butadiene rubber) and PVP (poly(vinylpyrrolidone)). However, the studies were performed on positive electrodes with only 0.5 – 0.8 mgSulfur cm-2. PVdF is known from being chemically stable in organic electrolytes for Li-ion batteries115. However, as a poorly-conductive polymer, even if some swelling can occur when in contact with the electrolyte, it may limit both electronic and ionic conductivities, and thus limit the performances of the cell. Another negative point is that PVdF needs to be dissolved in N- methyl-1-1-pyrrolidone (NMP), which is a toxic organic solvent, difficult to evaporate from the prepared electrodes. Risk of sulfur sublimation (when vacuum and higher temperature are used to evaporate NMP), eventual electrode contamination by residual solvent or finally not- environmentally friendliness (due to its toxicity), leads the researchers to look deeper into more friendly and cost-effective solutions. It was also reported by Lacey et al.116 that PVdF binder is blocking the electrode pores of the electrode (simply prepared carbon/sulfur composite using ball-milling, with eventual heat treatment applied further for sulfur infiltration inside the pores), which in turn results in decreased surface area accessible to the electrolyte. As ‘ecological’ alternative to PVdF, several binders which are water-soluble or water-dispersed in the form of latex, were also investigated in the literature, like carboxymethylcellulose combined with styrene-butadiene rubber (CMC+SBR)111, gelatin112,117,118, polyacrylic acid (PAA)119, Teflon108, Nafion®108. Schneider et al.108 demonstrated that binder may effectively influence the cycling behavior and the stability of the positive electrode. Their comparison studies were performed on three aqueous-based electrodes, using Nafion®, Teflon and PAN-CMC (polyacrylonitrile/ carboxymethylcellulose). Teflon-based electrode gave the best performances in terms of Chapter 1: Literature review 25PDF Image | Accumulateur Lithium Soufre
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