Sodium-ion storage performance of hierarchically structured (Co_1/3Fe_2/3)Se₂ nanofibers with fiber-in-tube nanostructures

Nanostructured multicomponent metal selenide materials and their carbon composite materials have been studied as anode materials for sodium-ion batteries (SIBs). Hierarchically structured (Co_1/3Fe_2/3)Se₂ nanofibers with fiber-in-tube nanostructures and (Co_1/3Fe_2/3)Se₂-C composite nanofibers with filled structures were prepared by electrospinning with subsequent selenization. Selenization of the CoFe₂O₄ nanofibers formed rod-type (Co_1/3Fe_2/3)Se₂ nanocrystals, and the tube-in-tube nanostructures of the nanofibers transformed into fiber-in-tube structures during this process. The discharge capacities of the hierarchically structured (Co_1/3Fe_2/3)Se₂ nanofibers and (Co_1/3Fe_2/3)Se₂-Se-C composite nanofibers were 594 and 512 mA h g^-1 (for the 60th cycle at a current density of 0.3 A g^-1), respectively, and their corresponding capacity retentions measured from the 2^nd cycle were almost 100%. The reversible discharge capacity of the hierarchically structured (Co_1/3Fe_2/3)Se₂ nanofibers decreased slightly from 585 to 497 mA h g^-1 as the current density was increased from 0.1 to 5.0 A g^-1. However, the reversible discharge capacity of the (Co_1/3Fe_2/3)Se₂-Se-C composite nanofibers decreased from 543 to 359 mA h g^-1 as the current density was increased from 0.1 to 5.0 A g^-1. The uniquely structured (Co_1/3Fe_2/3)Se₂ nanofibers with fiber-in-tube structures and featuring highly crystallized ultrafine nanorods (which have high electrical conductivity) showed superior rate performance compared to the (Co_1/3Fe_2/3)Se₂-Se-C composite nanofibers with filled structures.