First Introduction of NiSe₂ to Anode Material for Sodium-Ion Batteries: A Hybrid of Graphene-Wrapped NiSe₂/C Porous Nanofiber

The first-ever study of nickel selenide materials as efficient anode materials for Na-ion rechargeable batteries is conducted using the electrospinning process. NiSe 2-reduced graphene oxide (rGO)-C composite nanofibers are successfully prepared via electrospinning and a subsequent selenization process. The electrospun nanofibers giving rise to these porous-structured composite nanofibers with optimum amount of amorphous C are obtained from the polystyrene to polyacrylonitrile ratio of 1/4. These composite nanofibers also consist of uniformly distributed single-crystalline NiSe₂ nanocrystals that have a mean size of 27 nm. In contrast, the densely structured bare NiSe₂ nanofibers formed via selenization of the pure NiO nanofibers consist of large crystallites. The initial discharge capacities of the NiSe₂-rGO-C composite and bare NiSe 2 nanofibers at a current density of 200 mA g^-1 are 717 and 755 mA h g^-1, respectively. However, the respective 100 th-cycle discharge capacities of the former and latter are 468 and 35 mA h^-1. Electrochemical impedance spectroscopy measurements reveal the structural stability of the composite nanofibers during repeated Na-ion insertion and extraction processes. The excellent Na-ion storage properties of these nanofibers are attributed to this structural stability.