Porous FeS nanofibers with numerous nanovoids obtained by Kirkendall diffusion effect for use as anode materials for sodium-ion batteries

Porous FeS nanofibers with numerous nanovoids for use as anode materials for sodium-ion batteries were prepared by electrospinning and subsequent sulfidation. The post-treatment of the as-spun Fe(acac)₃-polyacrylonitrile composite nanofibers in an air atmosphere yielded hollow Fe₂O₃ nanofibers due to Ostwald ripening. The ultrafine Fe₂O₃ nanocrystals formed at the center of the fiber diffused toward the outside of the fiber via Ostwald ripening. On sulfidation, the Fe₂O₃ hollow nanofibers were transformed into porous FeS nanofibers, which contained numerous nanovoids. The formation of porosity in the FeS nanofibers was driven by nanoscale Kirkendall diffusion. The porous FeS nanofibers were very structurally stable and had superior sodium-ion storage properties compared with the hollow Fe₂O₃ nanofibers. The discharge capacities of the porous FeS nanofibers for the 1^st and 150^th cycles at a current density of 500 mA·g^–1 were 561 and 592 mA·h·g^–1, respectively. The FeS nanofibers had final discharge capacities of 456, 437, 413, 394, 380, and 353 mA·h·g^–1 at current densities of 0.2, 0.5, 1.0, 2.0, 3.0, and 5.0 A·g^–1, respectively.