Okra-Like Fe₇ S₈ /C@ZnS/N-C@C with Core-Double-Shelled Structures as Robust and High-Rate Sodium Anode

Core-multishelled structures with controlled chemical composition have attracted great interest due to their fascinating electrochemical performance. Herein, a metal-organic framework (MOF)-on-MOF self-templated strategy is used to fabricate okra-like bimetal sulfide (Fe₇ S₈ /C@ZnS/N-C@C) with core-double-shelled structure, in which Fe₇ S₈ /C is distributed in the cores, and ZnS is embedded in one of the layers. The MOF-on-MOF precursor with an MIL-53 core, a ZIF-8 shell, and a resorcinol-formaldehyde (RF) layer (MIL-53@ZIF-8@RF) is prepared through a layer-by-layer assembly method. After calcination with sulfur powder, the resultant structure has a hierarchical carbon matrix, abundant internal interface, and tiered active material distribution. It provides fast sodium-ion reaction kinetics, a superior pseudocapacitance contribution, good resistance of volume changes, and stepwise sodiation/desodiation reaction mechanism. As an anode material for sodium-ion batteries, the electrochemical performance of Fe₇ S₈ /C@ZnS/N-C@C is superior to that of Fe₇ S₈ /C@ZnS/N-C, Fe₇ S₈ /C, or ZnS/N-C. It delivers a high and stable capacity of 364.7 mAh g^-1 at current density of 5.0 A g^-1 with 10 000 cycles, and registers only 0.00135% capacity decay per cycle. This MOF-on-MOF self-templated strategy may provide a method to construct core-multishelled structures with controlled component distributions for the energy conversion and storage.