Size-controllable synthesis of covalently interconnected few-shelled Fe3O4@onion-like carbons for high-performance asymmetric supercapacitors

Fe3O4 is considered as an appealing anode material for constructing high-energy-density asymmetric supercapacitors owing to its high theoretical capacitance, large potential window, natural abundance and eco-friendliness. However, the dilemma of inadequate practical capacitance, low rate performance and unsatisfactory cycling stability of Fe3O4-based electrode materials significantly plagues their progress in supercapacitor applications. To well utilize the advantage of Fe3O4, herein, Fe3O4@onion-like carbons (OLCs) with covalently interconnected and few-shelled graphitic coatings were synthesized in a size-controllable manner from uniform-sized monodispersed nanoparticles of Fe3O4@oleic acid ligands. The ultrasmall-sized Fe3O4 nanoparticles with a good size uniformity enable a high electrochemical activity and a pseudocapacitive-dominated electrochemical behavior. Meanwhile, the efficient ion/electron transportations are facilitated by the structural advantages including covalently interconnected graphitic layers, conformal and seamless graphitic coating with ultrathin thickness, and hierarchical pores. As a result of the synergistic cooperation of these merits, the Fe3O4@OLCs hybrid structure exhibits a high specific capacitance of 686.1 F g−1 at 1 A g−1 and a high rate capability with 71.3% capacitance retention at 10 A g−1, superior to most of the reported Fe3O4-based electrode materials. An asymmetric supercapacitor device assembled based on Fe3O4@ OLCs shows a high energy density of up to 63.1 Wh kg−1, which is still maintained as high as 39.1 Wh kg−1 at a high power density of 1.49 kW kg−1. Moreover, a high cycling stability (more than 80% of initial capacitance remained after 10,000 cycles) is obtained. This study offers a novel strategy for preparing high-performance Fe3O4/carbon hybrid structures for asymmetric supercapacitor applications.