Effective Trapping of Lithium Polysulfides Using a Functionalized Carbon Nanotube-Coated Separator for Lithium–Sulfur Cells with Enhanced Cycling Stability

Abstract

The critical issues that hinder the practical applications of lithium-sulfur batteries, such as dissolution and migration of lithium polysulfides, poor electronic conductivity of sulfur and its discharge products, and low loading of sulfur, have been addressed by designing a functional separator modified using hydroxyl-functionalized carbon nanotubes (CNTOH). Density functional theory calculations and experimental results demonstrate that the hydroxyl groups in the CNTOH provoked strong interaction with lithium polysulfides and resulted in effective trapping of lithium polysulfides within the sulfur cathode side. The reduction in migration of lithium polysulfides to the lithium anode resulted in enhanced stability of the lithium electrode. The conductive nature of CNTOH also aided to efficiently reutilize the adsorbed reaction intermediates for subsequent cycling. As a result, the lithium-sulfur cell assembled with a functional separator exhibited a high initial discharge capacity of 1056 mAh g (corresponding to an areal capacity of 3.2 mAh cm) with a capacity fading rate of 0.11% per cycle over 400 cycles at 0.5 C rate.

Separator Membranes for Lithium–Sulfur Batteries: Design Principles, Structure, and Performance

“…To achieve efficient and working Li-S batteries, another nitrogen and sulfur co-doped porous carbon material was obtained from eggshell as starting material by Yuan et al [140] Ponraj et al reported the incorporation of MWCNTs partially functionalized with hydroxyl groups as a conductive coating on a commercial polyethylene separator (Asahi ND420). [141] Both theoretical predictions and experimental results affirmed the presence of a strong interaction between the hydroxyl groups and lithium polysulfides, effectively containing the polysulfides within the cathode side, while inherently conductive CNTs helped in the reutilization of active sulfur. SEM imaging revealed a tortuous intertwined morphology of the nanotubes both before and after functionalization with no visible change, while on the separator surface, the pore structure was not continuous.…”

“…Ponraj et al reported the incorporation of MWCNTs partially functionalized with hydroxyl groups as a conductive coating on a commercial polyethylene separator (Asahi ND420) . Both theoretical predictions and experimental results affirmed the presence of a strong interaction between the hydroxyl groups and lithium polysulfides, effectively containing the polysulfides within the cathode side, while inherently conductive CNTs helped in the reutilization of active sulfur.…”

Graphene/Carbon Black Co‐modified Separator as Polysulfides Trapper for Li‐S Batteries

“…The XPS spectrum of S 2p shows the presence of the polythionate and thiosulfate located at 169.5 and 168.4 eV, separately. The polythionate is formed by physical adsorption of polysulfides, while the formation of thiosulfate is attributed to the chemical bonding polysulfides with the hydroxyl group on rGO . Thus, it can be concluded that, after long‐term cycling, the polysulfides gradually localize at the rGO/CB (1:1) layer and stop them from passing through the separator to the lithium anode.…”

Mechanistic understanding of the role separators playing in advanced lithium‐sulfur batteries

“…Surface functionalization, which generates surface functional groups on carbon materials, can enhance the trapping effect through chemical interactions with polysulfide species . Ponraj et al prepared a hydroxyl functionalized‐CNT (CNTOH)‐coated separator via a vacuum filtration method. The hydroxyl groups generate a strong interaction with lithium polysulfides on the CNTOH surface, which is indicated by the color change of the Li 2 S 4 solution (Figure A,C).…”

“…F, Theoretical binding energies for Li 2 S n intermediates on CNT and CNTOH. Reproduced with permission: Copyright 2017, American Chemical Society…”