Abstract: Sodium-ion batteries are promising candidates for large-scale grid storage systems and other applications. Their foremost advantage derives from superior environmental credentials, enhanced safety as well as lower raw material costs than lithium-ion batteries. It is still challenging to explore desirable anode material. In this study, FeSe₂@CoSe₂/FeSe₂, with a yolk-shell structure was prepared by ion exchange and selenisation. The FeSe₂@CoSe₂/FeSe₂ prepared as anode material for sodium-ion batteries exhibits excellent rate capability due to the synergistic effect of bimetallic selenides and the interfacial effect of the heterostructure. Moreover, it delivers high performance (510 mAh g^-1 at 0.2 A g^-1), superior rate capability (90% retention at 5 A g^-1), and good long-time cycling stability (78% capacity retention after 1800 cycles at a high current density of 2 A g^-1). The optimized sodium-ion full cell with FeSe₂@CoSe₂/FeSe₂ as the anode and Na₃V₂(PO₄)₃ as the cathode still demonstrates excellent performance. Namely, a capacity of 272 mAh g^-1 (at 1 A g^-1) within the operating voltage from 1 to 3.8 V can be obtained. This work illustrates the potential of bimetallic selenides with heterostructures for performance enhancement of sodium-ion batteries.

Key words: Cobalt-iron selenide, Heterojunctions, Sodium storage, Anode material, Full battery