Due to its high theoretical capacitance, large potential window, natural abundance and eco-friendliness, Fe3O4 is considered an attractive anode material for constructing asymmetric supercapacitors with high energy density. However, the actual capacitance of Fe3O4-based electrode materials, low rate performance and unsatisfactory cycle stability seriously hinder their progress in supercapacitor applications. In order to take advantage of Fe3O4, this paper synthesized onion-like carbons (OLCs) with Fe3O4@covalent interconnection and less-shell graphite coating from uniformly sized monodisperse nanoparticles Fe3O4@ oleic acid ligands in a size-controlled manner. Ultra-small Fe3O4 nanoparticles with good dimensional uniformity enable high electrochemical activity and pseudocapacitance-dominated electrochemical behavior. At the same time, structural advantages including a covalently interconnected graphite layer, a conformal seamless graphite coating with ultra-thin thicknesses, and graded porosity facilitate efficient ion/electron transport. Due to the synergy of these advantages, Fe3O4@OLCs hybrid structure exhibits a high specific capacitance of 686.1 F g−1 at 1A g−1 and a high rate performance of 71.3% capacitance retention at 10A g−1, which is better than most reported Fe3O4-based electrode materials. The asymmetric supercapacitor device based on Fe3O4@OLCs assembly showed a high energy density of up to 63.1 Wh kg−1, and remained up to 39.1 Wh kg–1 at a high power density of 1.49 kW kg−1. In addition, high cycle stability (more than 80% initial capacitance after 10,000 cycles) is obtained. This study provides a new strategy for the preparation of high-performance Fe3O4/carbon hybrid structures for asymmetric supercapacitor applications.
来源：Xin Jiao , Biyu Li , Jian Wang , Yingchun Fan , Yongchang Ma , Zhihao Yuan , Chenguang Zhang，Size-controllable synthesis of covalently interconnected few-shelled Fe3O4@onion-like carbons for high-performance asymmetric supercapacitors，Carbon 203 (2023) 261–272，https://doi.org/10.1016/j.carbon.2022.11.053