Construction of Soft Base Tongs on Separator to Grasp Polysulfides from Shuttling in Lithium–Sulfur Batteries

Abstract

finally to the poor performance of Li-S batteries. [1][2][3][4][5][6][7][8][9][10] The state-of-the-art separators used in Li-S batteries are commonly porous polypropylene (PP) films, but their pores are too large to restrict polysulfide shuttling (Figure 1a). Lots of efforts have been made to mitigate this shuttle effect by physical repulsion or chemical adsorption routes such as filling or covering the pores with polymers, [11][12][13][14] metal-organic frameworks (MOFs), [15] metal oxides, [16,17] graphene, [18][19][20] modified carbon nanotubes, [21,22] etc. (Figure 1b). Unfortunately, the filled or covered pores also restrict the transportation of Li + ions, increase the inner resistance, and finally deteriorate the performance of Li-S batteries.In this report, we propose a new strategy to selectively coordinate high-order polysulfides with "tertiary amine layer (TAL)" by which to simultaneously keep the pores of the separator still open to lithium ion's transportation. The construction of the polysulfide tongs is based on the classic "soft and hard acid-base (SHAB) theory." Different from the literature chemical interaction strategies, the anchoring groups to trap polysulfide are chemically grafted onto the PP separator, which is tough and stable. Also, the dissolved high-order Li 2 S x (x = 4, 6, and 8) can be more efficiently grasped when they try to diffuse across the narrowed but opening complex pores (Figure 1c). Li 2 S x species are relatively "soft" acids as the positive charges are mainly dispersed on the surface of the agglomerated polysulfide mole cules. Moreover, after the lithium ions are coordinated by sulfur, there are still "electron cloud holes" in Li 2 S x molecules to accept electrons from other molecules ( Figure S2, Supporting Information). In this regard, if we construct a "soft" base on the separator, the high-order Li 2 S x (x = 4, 6, and 8) soft acid can be grasped when they are further reduced and released in the cathode. On the contrary, lithium ion, which is commonly recognized as "hard" acid because of its small size and concentrated charge density, will be repulsed away by the "soft" base on the separator according to the SHAB theory. The chosen "soft" base for grasping polysulfide tong is tertiary amine (TA) group, which has a dispersed electron cloud and large head volume. TA group do not possess N-hydrogen atom, so we do not need to worry about the potential side reactions between active hydrogen and lithium-related compounds. Also, because the largest binding energy (E b ) between Li 2 S x Rechargeable lithium-sulfur batteries, which use sulfur as the cathode material, promise great potentials to be the next-generation high-energy system. However, higher-order lithium polysulfides, Li 2 S x (x = 4, 6, and 8), regardless of in charge or in discharge, always form first, dissolve subsequently in the electrolyte, and shuttle to the cathode and the anode, which is called "shuttle effect." The polysulfides shuttle effect leads to heavy loss of the active-sulfur materials. Literatu...

MoN Supported on Graphene as a Bifunctional Interlayer for Advanced Li‐S Batteries

“…As one of the strategies, inserting an interlayer between the separator and sulfur electrode is widely applied due to the minimal changes to existing applications. [16][17][18][19] Various materials such as metal oxides, sulfides, and metal-organic frameworks have been proposed as functional interlayer to trap LiPSs through chemisorption, and this strategy has been demonstrated to be effective to block the migration of polysulfide species. [20][21][22][23][24] The interlayer serves as both adsorbent and current collector, where polysulfides can be reduced to insoluble products.…”

“…Significant efforts have been dedicated to the research of suitable approach to address polysulfide shuttling. As one of the strategies, inserting an interlayer between the separator and sulfur electrode is widely applied due to the minimal changes to existing applications . Various materials such as metal oxides, sulfides, and metal‐organic frameworks have been proposed as functional interlayer to trap LiPSs through chemisorption, and this strategy has been demonstrated to be effective to block the migration of polysulfide species .…”

Multifunctional Batteries: Flexible, Transient, and Transparent

“… 53 , 55 The main challenge of polymer electrolytes is their low ionic conductivity at room temperature. Gel electrolytes are reported, among others, for Zn 56 − 58 and Li/S batteries, 27 , 59 − 61 and they have demonstrated good compressibility, 56 which makes them good interlayer materials for stress accommodation. 62 …”

Sb nanosheet modified separator for Li–S batteries with excellent electrochemical performance

“…[9][10][11][12][13] In addition, sulfur, which is environment-friendly and of low cost, is used as the active material in Li-S batteries. 14,15 However, the practical application of Li-S batteries are still restricted by some problems, such as the low conductivity of sulfur and the shuttle effect of lithium polysuldes (LiPSs). [16][17][18][19] Many strategies have been proposed to overcome these issues, including modifying the sulfur molecule, 20,21 optimizing the pore structure of matrix materials, [22][23][24] and designing new electrolytes.…”