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experiment is commercial sulfur powder without any special engineered properties, so the new design of the AFG binder can greatly promote the practical application of Li-S batteries. Jiao et al. synthesized another multi-functional binder (AHP), with abundant amide groups as an efficient binder for Li-S batteries.[91] The polar AHP binder prepared by polymerization of HDI with ethylenediamine (EDA) contains a large amount of amino groups, which can strongly anchor the polysulfides, thus suppressing the shuttle effect. Figure 7d shows the design concept and a schematic illustration of the synthesis of the polar AHP binder, with a series of polar amide groups introduced into the AHP binder during polymerization. Compared to the commercial binders, the amide polar groups in the novel binder could strongly anchor Li2Sn species to effectively keep them within the cathode region, improving the electrochemical stability of the Li-S batteries. A schematic illustration of the evolution of AHP and commercial PVDF binder during cycling, as shown in Figure 7e, indicates that the AHP binder not only can tolerate the massive volume change of sulfur cathode during the discharge process, but also maintains the strong interactions between the polysulfides and the binder even during the charge process. All these advantageous features of AHP binder endow the C/S composite (60 wt% S) cathodes with excellent capacity stability. 3.2 Copolymers of vinylidene difluoride (VDF) with different monomers Our group used copolymers of VDF with other monomers, e.g. poly(vinylidene difluoride- trifluoroethylene) (P(VDF-TRFE)), as binders to improve the electrochemical performance of Li-S batteries.[92] Figure 8a shows the molecular structure of PVDF and its copolymers with other monomers. From the point of view of chemical structure, there are two fluorine atoms in the monomer unit of PVDF but five fluorine atoms and one hydrogen in P(VDF-TRFE). For P(VDF-co-CTFE), the hydrogen in the trifluoroethylene is replaced by a chloride atom. The modification observed in the monomer unit leads to enhanced molecular polarity of the VDF based copolymers, which have a strong affinity to lithium polysulfides. In addition, the ab- initio simulations performed in the framework of density functional theory clearly confirm the 15PDF Image | Advances in Polar Materials for Lithium-Sulfur Batteries
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