Catalyzing polysulfide redox conversion for promoting the electrochemical performance of lithium-sulfur batteries by CoFe alloy

•CoFeCN@C nanoparticles are rationally designed via a simple pyrolysis strategy.•The adsorptive and electrolytic effect of CoFeCN@C for polysulfides is explored.•The electrochemical performance of Li-S batteries with CoFeCN@C is improved.•A high areal capacity of 6.2 mAh cm−2 is retained after 150 cycles. Lithium-sulfur (Li-S) batteries have aroused tremendous attention owning to the merits of high theoretical energy density, natural abundance, and environmental benignity. However, the notorious shuttle effect of polysulfides severely hampers the practical application of Li-S batteries. Herein, we have successfully developed N-doped-carbon-layer-enveloped CoFe alloy (CoFeCN@C) nanoparticles, which are employed to functionize the commercial polypropylene separators for Li-S batteries. In the hybrid structure, the N-doped carbon layer provides abundant conducting pathways for ion/electron transport, while CoFe alloy offers sufficient polysulfide adsorptive and catalytic sites, which can simultaneously accelerate the multiphase conversion of sulfur/polysulfide/sulfide and suppress the polysulfide shuttling. As a consequence, the Li-S batteries assembled with CoFeCN@C modified separators demonstrate a high sulfur utilization (high initial discharge capacity of 1264 mAh g−1 at 0.2C), high rate performance (720.1 mAh g−1 at 4C), and superior cycling stability (a low capacity decay of 0.05% per cycle after 1000 cycles at 1C). More encouragingly, even with a high sulfur loading up to 8.5 mg cm−2, a high initial areal capacity of 7.9 mAh cm−2 is achieved, with a capacity retention of 6.2 mAh cm−2 after 150 cycles, demonstrating the great potential of CoFeCN@C for high-energy-density Li-S batteries.