Enhanced Catalytic Conversion of Polysulfides Using Bimetallic Co₇Fe₃ for High-Performance Lithium-Sulfur Batteries.

Abstract

Practical applications of lithium-sulfur (Li-S) batteries have been severely hindered by their low capacity, poor rate performance, and fast capacity degradation, which mainly originate from the notorious polysulfide shuttle effect. Herein, with density functional theory calculations, we show that the alloying of Fe into carbon-coated Co not only provides moderate binding interactions with the polysulfides to hinder their diffusion but also serves as an active catalyst in the spontaneous and successive lithiation of S₈ to Li₂S. Based on the fast migration of Li ions and the spontaneous lithiation of Li₂S₂ on the carbon-coated Fe-Co alloy, the entrapping-conversion processes of polysulfides are both thermodynamically and kinetically promoted in redox cycling. Experimentally, rationally designed Co₇Fe₃@porous graphite carbon-carbon nanotubes (Co₇Fe₃@PGC-CNT) electrocatalysts are introduced into Li-S batteries through separator functionalization. Consistent with theoretical predictions, Li-S batteries with Co₇Fe₃@PGC-CNT modified separators exhibit a dramatically enhanced rate capacity (788 and 631 mAh g^-1 at 10 and 15 C rates, respectively) and cycling stability (a slow capacity decay of 0.05% per cycle over 1000 cycles at 2.0 C), which are superior to those of most reported Li-S batteries coupled with state-of-the-art separators. Furthermore, it is shown that the excellent hindering of the shuttle effects enables a high areal capacity of 4.7 mAh cm^-2 after 90 cycles at a high sulfur loading of 6.7 mg cm^-2. Our work provides a feasible method for developing high-energy and long-life Li-S batteries, which might drive the commercialization of Li-S batteries.