A gradient hexagonal-prism Fe3Se4@SiO2@C configuration as a highly reversible sodium conversion anode

Abstract

Metal selenides have attracted great interest for high-efficiency sodium storage owing to their remarkable advantages of physicochemical features and electrochemical activity. However, substantial issues (e.g. poor intrinsic conductivity, severe interfacial side reactions and electrode structural destruction) in the electrochemical storage process restrict their practical feasibility and commercial prospects. Herein, a gradient hexagonal-prism configuration was exquisitely designed and developed via in situ conformal growth of double-shell silicon dioxide/carbon (SiO₂@C) reactors on highly uniform Fe-based MIL-88A nanorods and subsequent selenization–carbonization treatment (Fe₃Se₄@SiO₂@C). In the ingeniously designed Fe₃Se₄@SiO₂@C nanorods, the outer double-shell SiO₂@C reactors as charge-transfer and deformation-defense layers provide excellent ionic permeability and efficient charge-transfer channels and meanwhile endow superb resistance of volume variation, whereas the inner Fe₃Se₄ nanorods as a sodium-storage layer feature ample active sites for enhancing reversible capacity and satisfying surface-driven capacitive behaviors. Motivated by its distinctive structure and favorable properties, Fe₃Se₄@SiO₂@C is tremendously conducive to efficient sodium storage, as corroborated by its excellent rate capability and extraordinary stability with an ultrahigh reversible capacity of 272 mA h g⁻¹ at 20.0 A g⁻¹ after 4200 cycles; these results obviously outperform those of the contrast samples Fe₃Se₄@SiO₂ and single Fe₃Se₄@C as well as other reported Fe-based selenides. The conjunct exploration of ex situ structural characterizations strongly revealed a combination mechanism of intercalation and conversion from Fe₃Se₄ to the intermediate Na_xFe₃Se₄ and the final states Na_xSe/Fe. More interestingly, a Fe₃Se₄@SiO₂@C//Na₃V₂(PO₄)₃@C full cell configuration enables a considerably high capacity of 241 mA h g⁻¹ at 1.0 A g⁻¹ after 100 cycles, revealing great practical feasibility in energy storage systems.