In situ-grown compressed NiCo 2 S 4 barrier layer for efficient and durable polysulfide entrapment

Modifying a polypropylene (PP) separator with a polysulfide barrier layer can improve the cycling performance of lithium–sulfur (Li–S) batteries. However, conventional slurry-coating- and vacuum-filtration-designed barriers usually show poor particle connection and require extra binder. Herein, we propose a facile in situ growth method and a subsequent compression strategy to design multifunctional NiCo2S4 (NiCoS) nanosheet arrays on a PP membrane for high-performance Li–S batteries. The in situ grown NiCoS nanosheet arrays are interconnected, conductive and closely adhered to the PP membrane without using any binder. After mechanical compression treatment, the overall NiCoS film is compacted, lightweight (0.148 mg cm−2) and ultrathin (0.8 μm). Density functional theory calculations combined with adsorption and diffusion tests prove that the NiCoS nanosheets have highly efficient physical/chemical entrapping capabilities for preventing polysulfide shuttling. Moreover, in situ electrochemical impedance spectroscopy demonstrated that the NiCoS barrier could efficiently suppress polysulfide diffusion and concurrently facilitate redox reactions. When applying this multifunctional separator, a sulfur/carbon nanotube (S/CNT) cathode with high sulfur content (75 wt%) delivers significantly improved long-term cycling performance, with 0.056% capacity decay per cycle over 500 cycles. This work opens up new opportunities to design multifunctional separators by an in situ growth strategy for high-performance Li–S batteries. Nanomaterials that retain their porous frameworks even after being crushed can help experimental lithium batteries avoid premature failure. While high-capacity lithium–sulfur batteries are promising for the electric vehicle market, their electrodes normally need to be wrapped in heavy barrier layers to prevent sulfur atoms from moving during recharging. Hui Ying Yang from the Singapore University of Technology and Design and co-workers have now designed a lightweight barrier with natural sulfur-capturing capabilities. The team grew thin nanosheets of nickel–cobalt–sulfur crystals directly onto a polypropylene membrane, and then mechanically compressed the film with rollers. Characterization experiments revealed the nanosheet film had abundant bonding sites for sulfur atoms within its pores, and a compact structure that inhibited diffusion. Batteries containing the new barrier had improved long-term cycling performance compared with unmodified lithium–sulfur devices. A facile in situ growth and subsequent compression strategy has been proposed to modify the separator. The in situ grown barrier layer is compact, lightweight and thin, exhibiting efficient physical/chemical entrapping capability on preventing the polysulfide shuttling. Applying this multi-functional separator, the CNT/S cathode with high sulfur content (75 wt%) delivers significantly improved long-term cycling performance with 0.056% capacity decay per cycle over 500 cycles.