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Electrolytes for Lithium-Sulfur Batteries
Published in Władysław Wieczorek, Janusz Płocharski, Designing Electrolytes for Lithium-Ion and Post-Lithium Batteries, 2021
Li-S batteries are based on the principle of reduction of sulfur (the cathode material) to polysulfide species (PS), which by interacting with lithium cations form lithium sulfide (Li2S). During discharge of the battery on the anode (built with metallic lithium) the process of lithium oxidation takes place: Li = Li+ + e−. At the same time, on the cathode, sulfur reduction takes place: S + 2e− = S2−. The overall reaction is as follows: 2Li + S = Li2S. Because sulfur in nature exists in the form of octasulfur+(S8),=it is necessary to modify the overall reaction equation to the following form: 16Li + S8 = 8Li2S. A mechanism of sulfur compound propagation in the electrolyte is described very well by Barghamadi et al. in a review paper [7]. In Li-S cells, various lithium polysulfides with a general formula Li2Sx (2 < x < 8) are formed during the cell discharge process. It is assumed that the elemental sulfur in the solid phase S8(s) is first dissolved in the electrolyte as S8 (solvated) and gradually reduce. During cell discharge, lithium polysulfides soluble in common organic solvents (Li2Sx, 4 < x < 8), insoluble in common organic solvents (Li2Sx, 1 < x < 2), and finally lithium sulfide (Li2S) appear one after another [8–13].
Lithium-Based Battery Systems
Published in Muhammad Asif, Handbook of Energy Transitions, 2023
C. M. Costa, J. C. Barbosa, R. Gonçalves, S. Ferdov, S. Lanceros-Mendez
Li-S batteries are composed of a metallic Li anode, which leads to safety concerns, but the main challenge lies in the sulfur cathode used in this battery. Sulfur is an insulating material (electrical conductivity: 1 × 10−15 S/m), and this material undergoes 80% volume change during charge/discharge behavior. Further, sulfur is not compatible with carbonate-based solvents commonly used in LIBs (Rana et al. 2019) and dissolves in most solvents compatible with the Li anode (Zhao et al. 2018). Presently, ether-based solvents are employed for this battery system, where sulfur has moderate solubility (Li et al. 2019). An assembled Li-S cell is charged and ready to be discharged once after its assembly. During the discharge process, sulfur undergoes reduction in several stages. Sulfur has over 30 solid allotropes, and the most common form is the cycle octasulfur (S8), which is the thermodynamically stable form followed by the cyclic S12 allotrope. During the discharge process, sulfur forms reduction products such as Li2S8, Li2S6, Li2S4, Li2S2, and Li2S (Zhao et al. 2018, Li et al. 2019). Li2S is the final discharged product, which is also an insulating material. On recharge, Li2S gradually oxidizes back to form S8 (Li et al. 2018). The reduction products of sulfur, such as Li2S8, Li2S6, and Li2S4 are soluble in the liquid electrolyte employed (LiSO3CF3 and LiN(SO2CF3)2 in dimethyl ether (DME) and 1,3-dioxolane (DOL)), while the other products (Li2S2 and Li2S) are insoluble in this electrolyte (Zhang 2013). Further, the high-order polysulfide reduction products dissolved in the electrolyte cross the separator and get reduced at the Li anode, reducing either as a passivating Li2S deposit on the Li anode or as a low-order polysulfide, which again diffuses back to the cathode, where it gets reoxidized to high-order polysulfides. In this way, the dissolved polysulfides shuttle between anode and cathode, undergoing reduction and oxidation, respectively. This polysulfide shuttle phenomenon causes the cell to undergo a kind of self-discharge. The use of dry solid electrolytes in Li-S cells is considered to be the ultimate approach to prevent sulfur and polysulfide dissolution into the electrolyte. In addition, solid polymer electrolytes (SPEs) are considered to suppress the dendritic growth of lithium and enhance safety.
Rheology, morphology and phase behavior of SBS/sulfur modified asphalt based on experimental assessment and molecular dynamics
Published in International Journal of Pavement Engineering, 2022
Sulfur exists stably in the form of octasulfur (formula: S8) in nature (Griebel et al. 2016). The main reason why sulfur can be used as a crosslinking agent can be ascribed to the reaction of ‘ring-opening polymerisation’, and polysulfides (R-Sn-R) or disulfides (R-S2-R) will generate along with the reaction. Due to the worse thermal stability of polysulfides (Wang et al. 2018), disulfides, as the ultimate form, participate in the vulcanisation reaction. The vulcanisation reaction in SBS-modified asphalt involves sulfide reacted with asphalt binder and SBS copolymer. There is currently no consensus on the vulcanisation reaction with asphalt binder. Some studies pointed out that the vulcanisation reaction depends on the reaction temperature (Syroezhko et al. 2003). Asphaltene and naphthene aromatic compounds can react with sulfur by forming the C–S bond at 140°C, and sulfur mainly reacts with naphthene-aromatic to generate asphaltene when the temperature exceeds 240°C. This report can also be found elsewhere (Zhang et al. 2022). Another research revealed sulfur tends to replace the hydrogen of the aromatic nucleus (Ye 2020). When the reaction temperature exceeds 140°C (Zhang et al. 2019), the thioether will be generated by replacing hydrogen in the aromatic nucleus or side chain. Thus, this paper mainly considered the compounds of naphthene aromatic nature in the sulfur-asphalt reaction. Combined with the substitute position, the vulcanisation products of aromatic are given in Figure 5. Regarding the small fractions of sulfur used in this paper, the aromatic products do not involve changing numbers and asphalt components.