Study On The Preparation Of Fe₃O₄-graphene Composite And Its Electrochemical Properties

Currently, lithium-ion batteries?LIBs? have become one of the most widely used and promising high-energy batteries. Carbon, as the commercial LIBs anode material, cannot meet the increasing demands for LIBs with both high energy and power density because of its poor specific capacity. Significant achievements have been gained in the research of anode materials with superior specific capacity. Fe₃O₄, one of the most important transition metal oxides, has been considered one of the most promising anode materials because of its high specific capacity, low cost and environmental friendly. However, its practical application in LIBs is restricted because the disadvantage of poor cycle stability and rate performance. Graphene?GN?, a new type of two-dimensional carbon materials, has drawn tremendous scientific interest for energy-storage application due to its excellent electrical conductivity, large surface area and high mechanical property. It is believed that such Fe₃O₄-GN composite could possess large reversible specific capacity, good rate capability and long cycling life. In this research, in the basis of exploring the technique conditions of preparing Fe₃O₄, Fe₃O₄-GN composite was prepared through two different methods, including electrostatic attraction and in-suit growth, and their electrochemical properties were analyzed respectively. The main contributions are listed as follows:?1?Fe₃O₄ was prepared by solvothermal method in the absence of ferrous chloride and hydrazine hydrate, SEM and TEM images show that the spherical particles are nearly monodisperse in size with an average diameter of ¹60 nm. Then the reaction mechanism and the effects of some reaction conditions on the morphology were studied. The results show that hydrazine hydrate plays a crucial effect on preparing single phase Fe₃O₄. Ethylene glycol can control the nucleation and growth of Fe₃O₄ particles. The particles prepared at 180 ?possess the structure of the porous, and PEG 1000 can effectively prevent the agglomeration of particles. As an anode material, the first discharge/charge capacities of Fe₃O₄ electrode at 0.1 C(1 C = 924 m A·g^-1) are 1342 and 991 m Ah·g^-1, respectively. But its cycling and rate performance are poor.? 2? Graphene oxide?GO? was prepared by modified Hummers method with graphite as raw material. In the process of reduction, the negatively charged GN was obtained by adding SDBS, and then combined with Fe₃O₄ by electrostatic attraction to obtained the Fe₃O₄-GN composite. The XRD pattern of the composite reveals the presence of face-centered cubic Fe₃O₄ and disordered GN. FT-IR and Raman spectroscopy demonstrate that the GO is reduced to GN by hydrazine hydrate. SEM results indicate that the Fe₃O₄ particles with an average diameter of ¹60 nm are uniformly anchored on GN sheets. As an anode material, the first discharge/charge capacities of Fe₃O₄-GN electrode at 0.1 C are 1847 and 1189 m Ah·g^-1, respectively. At high current densities of 5 C, the sustainities of discharge/charge capacities remain at 62.2% and 59.5%. After 50 cycles at 1 C, the discharge/charge capacities remain 67.5% and 73.2% of the initial capacities, respectively. The Fe₃O₄-GN composite exhibits excellent capacitive behaviour in terms of high specific capacity, good rate capability and cycling performance.? 3? In the basis of the technique conditions of preparing Fe₃O₄, Fe₃O₄-GN composite was prepared via a facile one-step solvothermal method directly from GO and ferrous chloride in the presence of hydrazine hydrate. Through this simple in situ method, the reduction of GO, the formation and the deposition of Fe₃O₄ particles on GN sheets by directly anchored way occur simultaneously without additional molecular linkers and further process. The XRD pattern of the composite reveals the presence of face-centered cubic Fe₃O₄ and disordered GN. FT-IR and Raman spectroscopy demonstrate that the GO is reduced to GN in the solvothermal process. SEM results indicate that the porous Fe₃O₄ particles are anchored on GN sheets with an average diameter of ¹60 nm. As an anode material, the first discharge/charge capacities of Fe₃O₄-GN electrode at 0.1 C are 1912 and 1450 m Ah·g^-1, respectively. At high current densities of 5 C, the sustainities of discharge/charge capacities remain at 41.9% and 31.6%. After 50 cycles at 1 C, the discharge/charge capacities remain 83.5% and 94.7% of the initial capacities, respectively. Compared with the Fe₃O₄-GN electrode prepared by chapter there, this electrode possesses better cycling performance and worse rate performance, which attribute to the distribute of Fe₃O₄ particles on GN sheets and the combination mode between Fe₃O₄ and GN.