Preparation And Study Of Fe₂O₃/graphene Freestanding Electrode By Solvothermal Method

The iron oxide based anode materials for lithium ion batteries have potential applications because of their good safety,high theoretical specific capacity,low cost and environment friendliness.The freestanding electrode is a new type of electrode that can be used directly without the collector.Its appearance saves the use of the material,simplifies the preparation process,shortens the preparation time,improves the energy density of the battery,and has practical application significance.In this paper,Fe₂O₃ was prepared by solvothermal method under the environment of water and ethylene glycol,and Fe₂O₃/graphene freestanding electrode materials were prepared by ultrasonic mixing method and solvothermal one-step process respectively.The crystal morphology and morphology of the freestanding materials were studied by XRD and SEM.The electrochemical performance of the materials were studied by CV,EIS and galvanostatic test.Take FeCl₃·6H₂O as iron salts.Firstly,the amount of iron salt,solvothermal reaction time and the ratio of iron salt to urea were explored and optimized in the solvent thermal process.The optimum conditions of water and ethylene glycol system were as follows: the amount of iron salt was 4 g,the reaction time was 8 h,the molar ratio of iron salt to urea was 2:3.In this condition,the corresponding material of the water system showed the most regular petaloid morphology with smaller particle size and less agglomeration.Its capacity achieved 650 m Ah/g after 100 cycles at the current density of 0.1 A/g with the capacity retention of 86.2%,maintaining 473 m Ah/g after 200 cycles at 1 A/g,331 m Ah/g and 232 m Ah/g after 10 cycles at the current density of 2 A/g and 5 A/g,respectively.The corresponding materials of ethylene glycol system showed spherical morphology with the smallest particle size and less agglomeration,and its capacity maintained 758 m Ah/g after 100 cycles at the current density of 0.1 A/g with the capacity retention of 90.3%,maintaining 547 m Ah/g after 200 cycles at 1 A/g,404 m Ah/g and 253 m Ah/g after 10 cycles at the current density of 2 A/g and 5 A/g,respectively.Based on the process optimization of solvothermal method for the preparation of Fe₂O₃,the Fe₂O₃/graphene freestanding electrode was prepared by ultrasonic mixing method.The best performance is obtained by studying different mixing ratios and the results are as follows: Under water system,the freestanding electrode with the ratio of Fe₂O₃ to graphene 6:3 showed 386 m Ah/g after 30 cycles at the current density of 0.1 A/g,maintaining 297 m Ah/g after 100 cycles at 1 A/g.Under ethylene glycol system,the freestanding electrode with the ratio of Fe₂O₃ to graphene 6:4 showed 499 m Ah/g after 100 cycles at 0.1 A/g,maintaining 268 m Ah/g after 100 cycles at the current density of 1 A/g.In order to further improve the properties,a one-step method for preparing Fe₂O₃ /graphene freestanding electrode material was performed.The results showed that in water system,Fe₂O₃ showed microspheres morphology and is uniformly distributed between the graphene sheets when the ratio of Fe₂O₃ to graphene is less than 6:3.The ratio of Fe₂O₃ to graphene 6:3 reached 548 m Ah/g after 100 cycles at the current density of 0.1 A/g with the capacity retention of 85.4%,maintaining 454 m Ah/g after 200 cycles at 1 A/g,236 m Ah/g after 10 cycles at the current density of 2 A/g and 131 m Ah/g at 5 A/g in the first cycle.The electrode under ethylene glycol system is easy to pulverize and the electrochemical performance are poor due to the Fe₂O₃ is coated on the graphene sheets.Even though the ratio of Fe₂O₃ to graphene 5:4 exhibit the best electrochemical performance,the capacity is only 172 m Ah/g after 100 cycles at 0.1 A/g and 155 m Ah/g after 200 cycles at the current density of 1 A/g.