Synthesis of Uniquely Structured Yolk–Shell Metal Oxide Microspheres Filled with Nitrogen-Doped Graphitic Carbon with Excellent Li–Ion Storage Performance

Novel structured composite microspheres of metal oxide and nitrogen-doped graphitic carbon (NGC) have been developed as efficient anode materials for lithium-ion batteries. A new strategy is first applied to a one-pot preparation of composite (FeO_x-NGC/Y) microspheres via spray pyrolysis. The FeO_x-NGC/Y composite microspheres have a yolk–shell structure based on the iron oxide material. The void space of the yolk–shell microsphere is filled with NGC. Dicyandiamide additive plays a key role in the formation of the FeO_x-NGC/Y composite microspheres by inducing Ostwald ripening to form a yolk–shell structure based on the iron oxide material. The FeO_x-NGC/Y composite microspheres with the mixed crystal structure of rock salt FeO and spinel Fe₃O₄ phases show highly superior lithium-ion storage performances compared to the dense-structured FeO_x microspheres with and without carbon material. The discharge capacities of the FeO_x-NGC/Y microspheres for the 1st and 1000th cycle at 1 A g⁻¹ are 1423 and 1071 mAh g⁻¹, respectively. The microspheres have a reversible discharge capacity of 598 mAh g⁻¹ at an extremely high current density of 10 A g⁻¹. Furthermore, the strategy described in this study is generally applied to multicomponent metal oxide–carbon composite microspheres with yolk–shell structures based on metal oxide materials.