Iron diselenide combined with hollow graphitic carbon nanospheres as a high-performance anode material for sodium-ion batteries

A scalable method for the synthesis of iron diselenide (FeSe₂) nanoparticles composited with hollow graphitic-carbon nanospheres (HGCNS) is presented. The resultant composite exhibits high sodium-ion-storage performance. A solution of iron acetylacetonate, polystyrene, and polyacrylonitrile dissolved in dimethylformamide is subjected to three continuous heat treatment steps. During this process, the amorphous carbon formed around the Fe species in the composite is selectively transformed into graphitic carbon by the catalytic action of the Fe. Residual amorphous carbon was selectively removed. Subsequent selenization of this carbonaceous material affords FeSe₂-HGCNS composite. The discharge capacity of this composite is 425 mA h g^-1 after 100 cycles at a current density of 0.5 A g^-1, and its capacity retention compared to that in the third cycle is 94%. The excellent sodium-ion-storage performance of the composite is attributed to both ultrafine FeSe₂ and HGCNS, which decrease Na⁺ ion diffusion length, increase electrical conductivity and allow easy penetration of the electrolyte.