Zeolitic imidazolate framework-derived hierarchical quaternary nickel cobalt sulfoselenide materials for lithium-sulfur batteries: Se doping enhances bimetallic atomic synergies to improve cathode reaction kinetics.

The "shuttle effect" and slow reaction kinetics caused by the diffusive polysulfides (LiPSs) in lithium-sulfur batteries have seriously hindered the superiorities of high theoretical capacity and energy density of sulfur cathode. Metal compounds serve as cathode carrier materials for lithium-sulfur batteries can adsorb LiPSs and accelerate the sulfur redox reaction. Compared with monometallic systems, bimetallic components usually expose richer active sites, which facilitates the kinetics of LiPSs during the redox process. However, due to the limitation of morphology and structure, the catalytic capacity of single metal atom and the synergistic effect between bimetal atom are still not fully utilized, leading to the still poor reaction kinetics. Especially, their application effect in thick electrode with high sulfur load is significantly reduced. In this study, chemical etching reaction was utilized to convert zeolitic imidazolate framework (ZIF) 67 into a polyhedral hierarchical structured material stacked with layered double hydroxide (NiCo-LDH) nanosheets, and based on this, it was transformed into nickel cobalt sulfoselenide (NiCoSSe) with quaternary hierarchical structure. The bimetallic sulfoselenide obtained by this strategy retain the high specific surface area of the ZIF structure, thus providing more active sites for adsorption of LiPSs and catalyzing sulfur redox. Importantly, it was demonstrated by density-functional theory (DFT) calculations that, selenium doping can further enhance the interaction between Ni and Co atoms in NiCoSSe, which promotes the conversion of LiPSs and significantly improves the redox kinetics of the battery. An initial specific capacity of up to 1093.4 mAh g at a current density of 0.1C as well as a capacity retention of 98.2 % after 500 charge-discharge cycles at a current density of 2C were obtained based on the cathode with NiCoSSe carrier. By systematically investigating the synthesis of bimetallic sulfoselenide compounds and their enhancement effects on the reaction kinetics of lithium-sulfur battery cathode, this study will provide a detailed reference for the conformational relationship between the structure of quaternary NiCoSSe materials and electrochemical performance.