Na-ion Storage Performances of FeSe_x and Fe₂O₃ Hollow Nanoparticles-Decorated Reduced Graphene Oxide Balls prepared by Nanoscale Kirkendall Diffusion Process

Uniquely structured FeSe_x-reduced graphene oxide (rGO) composite powders, in which hollow FeSe_xnanoparticles are uniformly distributed throughout the rGO matrix, were prepared by spray pyrolysis applying the nanoscale Kirkendall diffusion process. Iron oxide-rGO composite powders were transformed into FeSe_x-rGO composite powders by a two-step post-treatment process. Metallic Fe nanocrystals formed during the first-step post-treatment process were transformed into hollow FeSe_xnanoparticles during the selenization process. The FeSe_x-rGO composite powders had mixed crystal structures of FeSe and FeSe₂ phases. A rGO content of 33% was estimated from the TG analysis of the FeSe_x-rGO composite powders. The FeSe_x-rGO composite powders had superior sodium-ion storage properties compared to those of the Fe₂O₃-rGO composite powders with similar morphological characteristics. The discharge capacities of the FeSe_x- and Fe₂O₃-rGO composite powders for the 200^th cycle at a constant current density of 0.3 A g^-1 were 434 and 174 mA h g^-1, respectively. The FeSe_x-rGO composite powders had a high discharge capacity of 311 mA h g^-1 for the 1000 th cycle at a high current density of 1 A g^-1.