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2.3.2 Separator The separator is composed of a single porous polypropylene membrane (type 2400; Celgard, Charlotte, NC, USA). A large number of suitable separators is commercially available for lithium batteries [90]. While there are some interesting developments in the separator regime lately [91, 92], the material chosen for this work is known to work well from previous studies [77, 93, 94]. Also, assuming it can be wetted properly, the choice of separator does not affect the performance of the cell much — at least for low to medium discharge/charge rates, as can be told from the resistances reported in [95]. Therefore, the optimization of the separator membrane is beyond the scope of this work. 2.3.3 Electrolyte materials and preparation The separator as well as the porous electrode are filled with a liquid electrolyte. Un- fortunately, carbonate based electrolytes, which are commonly used for Li-ion cells [96, 97] are not suitable for Li/S batteries because of their reactivity with dissolved polysulfides [98]. Instead, popular choices have been glymes, ethers and dioxolane- based electrolytes [99–103]. Only recently, the first review on electrolytes for Li/S batteries was published by Scheers et al. [103]; a generic scheme for the classifica- tion of various electrolytes can also be found in Ref. [104]. For this work, electrolytes based on room-temperature ionic liquids (RTILs) are used, which are known to work well with Li/S batteries [104–107]. By selecting and chemically modifying the anion or cation, RTILs can be designed to have properties matching the requirements of a specific application. Various combinations have been used for energy applications [12, 108]. In general, RTILs are known for their good ionic conductivity as well as their excellent thermal and electrochemical stability. Additionally, they offer technical and environmental benefits including a very low vapor pressure, nonflammability, low toxicity and synthesis methods which are both scalable and environmentally friendly [108, 109]. Recent reports indicate that RTILs can even contribute to a better reversibil- ity of lithium metal electrodes by dynamically forming a favorable, protective SEI layer at the lithium|electrolyte interface [87]. If required, these electrolytes can also be turned into gels easily, as reported by [110]. For this work, several different compositions of liquid electrolytes were tested, all of them based on 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI) with 1.0mol/kg LiTFSI dissolved. Tab. 2.2 lists the compositions tested and Tab. 2.3 shows the chemical structure of the components used. Liquids are stored in sealed vials together with activated type 4 Å molecular sieves (TRICAT, Hunt Val- ley, MD, USA) in order to reduce the amount of trace water. Salts are dried under 24PDF Image | Lithium-Sulfur Battery: Design, Characterization, and Physically-based Modeling
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