Unique structured microspheres with multishells comprising graphitic carbon-coated Fe₃O₄ hollow nanopowders as anode materials for high-performance Li-ion batteries

Multishell structured metal oxide microspheres with considerable structural stabilities during repeated cycling and higher volumetric capacities than those of hollow structured microspheres are popular as anode materials for lithium-ion batteries. However, the long-term cycling and rate performances of multishell metal oxide microspheres for lithium-ion storage require further improvement. Herein, novel and unique structured microspheres (Fe₃O₄-GC) with multishells comprising graphitic carbon (GC)-coated Fe₃O₄ hollow nanopowders are successfully synthesized. The reduction of pitch-infiltrated Fe₂O₃ yolk-shell microspheres yields metallic Fe yolk-shell microspheres filled with pitch-derived carbon. Oxidation yields yolk-shell Fe₃O₄-GC microspheres with multishells comprising hollow nanopowders coated with GC via the selective decomposition of amorphous carbon and oxidation of metallic Fe. Metallic Fe nanocrystals transform into hollow Fe₃O₄ nanopowders due to nanoscale Kirkendall diffusion. The primary hollow Fe₃O₄ nanopowders comprising yolk-shell microspheres and GC layer formed by pitch-derived carbon decrease the distance for lithium-ion diffusion and improve the electrical conductivity resulting in outstanding cycling and rate performances of Fe₃O₄-GC, respectively. The discharge capacity of Fe₃O₄-GC microspheres for the 1000^th cycle at a current density of 2.0 A g^-1 is 1018 mA h g^-1, and the microspheres exhibit a high reversible capacity of 649 mA h g^-1 even at 20 A g^-1.