Superior electrochemical properties of micron-sized aggregates of (Co_0.5Fe_0.5)₃O₄ hollow nanospheres and graphitic carbon

Morphology-controlled micron-sized aggregates consisted of hollow nanospheres and graphitic carbon are considered to be efficient electrode materials for lithium-ion batteries because the advantages of hollow nanospheres are combined with those of micron-size powders with easy processability. In this study, carbon microspheres with extremely large surface area of 3350 m² g⁻¹ are successfully used as templates to synthesize (Co_0.5Fe_0.5)₃O₄-graphitic carbon (CoFeO-GC) composite microspheres, which in turn, are composed of hollow nanospheres. The CoFe alloy nanospheres act as catalyst in formation of graphitic carbon during reduction process and transform into metal oxide hollow nanospheres after oxidation by nanoscale Kirkendall diffusion. Owing to their unique structure, CoFeO-GC composite microspheres show lithium-ion storage performances superior to those of the CoFeO-amorphous carbon composites with ultrafine nanocrystals and dense structure. The CoFeO-GC composite microspheres have extremely high capacities of 1072 and 681 mA h g⁻¹ at current densities of 1 and 3 A g⁻¹, respectively, after 350 cycles. This hybrid structure employs synergistic effect of the hollow nanosphere aggregate and high content of graphitic carbon with high electrical conductivity, resulting in superior cycling and rate performances, when tested as anode materials for lithium-ion batteries.