Vertical Co9S8 hollow nanowall arrays grown on a Celgard separator as a multifunctional polysulfide barrier for high-performance Li-S batteries

Lithium-sulfur (Li-S) batteries have been regarded as one of the most promising next-generation energy-storage devices, due to their low cost and high theoretical energy density (2600 W h kg(-1)). However, the severe dissolution of lithium polysulfides (LiPSs) and the fatal shuttle effect of the sulfur cathode seriously hinder the practical applications of Li-S batteries. To address such issues, we present here, for the first time, a novel metal organic framework (MOF)-derived Co9S8 nanowall array with vertical hollow nanoarchitecture and high electrical conductivity, which is grown in situ on a Celgard separator (Co9S8-Celgard) via a feasible and scalable liquid-reaction approach, as an efficient barrier for LiPSs in Li-S batteries. Benefiting from the direct in situ growth of vertical Co9S8 hollow nanowall arrays as a multifunctional polar barrier, the Co9S8-Celgard separator possesses large surface area, excellent mechanical stability, and particularly strong LiPS-trapping ability via chemical and physical interactions. With these advantages, even with a pure sulfur cathode with a high sulfur loading of 5.6 mg cm(-2), the Li-S cells with the Co9S8-Celgard separator exhibit outstanding electrochemical performance: the initial specific capacity is as high as 1385 mA h g(-1) with a retention of 1190 mA h g(-1) after 200 cycles. The cells deliver a high capacity of 530 mA h g(-1) at a 1C rate (1675 mA g(-1)) even after an impressive number of 1000 cycles with an average capacity fade of only 0.039% per cycle, which is promising for long-term cycling application at high charge/discharge current densities, and pouch-type Li-S cells with the Co9S8-Celgard separator display excellent cycling performance. When the optimized cathode with the sulfur loading in well-designed yolk-shelled carbon@Fe3O4 (YSC@Fe3O4) nanoboxes is employed, the cell with Co9S8-Celgard delivers a high initial capacity of 986 mA h g(-1) at a 1C rate with a capacity retention as high as 83.2% even after a remarkable number of 1500 cycles. This work presents a strategy to grow on the separator a multifunctional polar interlayer with unique nanoarchitecture and high conductivity to chemically and physically trap the LiPSs, thus significantly enhancing the performance of Li-S batteries.