Facile construction of capsule-shaped Fe₇S₈@C with superior pseudocapacitance for sodium/potassium ion storage

Sodium-/potassium-ion batteries, due to abundant raw material resources and similar electrochemical behaviors to lithium-ion batteries, stimulated considerable attention. However, there is a lack of suitable anode materials that seriously limits their practical application. The capsule-shaped Fe₇S₈@C nanoparticles are successfully prepared via a simple metal-organic framework-derived method combined with a solid sulfidation procedure. Due to the unique structural design, this capsule-shaped Fe₇S₈@C displays a maintainable capacity (341.3 mAh g^-1 at 1.25 A g^-1 upon 150 cycles) and outstanding rate (292.6 mAh g^-1 at 2.0 A g^-1) for sodium ion batteries. As a potassium-ion batteries anode, the capsule-shaped Fe₇S₈@C also exhibits a maintainable capacity (186.0 mAh g^-1 at 1.0 A g^-1 upon 1000 cycles). The excellent electrochemical performance can be attributed to the distinctive capsule-shaped structure, which can moderate the pulverization of Fe₇S_8, inhibit the aggregation of Fe⁰, and offer a larger contact area with electrolytes. Furthermore, the carbon framework enhances electronic conductivity and can also offer abundant active sites for Na⁺/K⁺ storage. This rational designing strategy is available for fabricating other transition metal chalcogenides for rechargeable batteries.