Cationic Polymer Binder for Simultaneously Propelling Ion Transfer and Promoting Polysulfide Conversion in Lithium–Sulfur Batteries

Abstract

Lithium−sulfur (Li−S) batteries, possessing substantial capacity, present a promising successor to current lithium-ion batteries, but the "shuttle effect" during cycling and deficient lithium-ion conductivity in Li−S batteries limit their practical applications. One potential remedy to these issues lies in the use of functional binders. In this study, building on the exceptional electrochemical stability of poly(vinylidene fluoride) (PVDF), we strategically grafted the cationic monomer 1-butyl-3-vinylimidazole with bis(trifluoromethanesulfonyl)imide (TFSI − ) coordination from the chain of PVDF, thereby engineering the ionomer binder PVDF-g-(1-butyl-3-vinylimidazolium bis((trifluorompropyl)sulfonyl)imide) (BVIM). Density-functional theory (DFT) calculations affirmed that these cationic polymer branches, possessing a high binding energy with lithium polysulfides (LiPSs), are effective in trapping the LiPSs generated at the cathode. Moreover, while adsorbing LiPSs the TFSI − originally coordinated with the branched chain will be displaced, forming a dynamic small molecule pathway in the cathode that promotes lithium-ion conduction. As a result, Li−S batteries with BVIM binders deliver a persistent reversible capacity of 792.5 mAh g −1 over 250 cycles at a rate of 0.5C. Concurrently, at a high sulfur loading of 5.5 mg cm −2 , a specific capacity of 3.3 mAh cm −2 was maintained after 50 cycles at 0.2C.