| Iron-based fluoride cathode material is a novel electrode material based on traditional insertion/extraction reaction and chemical conversion reaction.It is considered as a promising cathode material for lithium-ion batteries and sodium ion batteries,because of its high specific capacity,environmental friendliness and low cost.However,due to its inherent disadvantages,such as strong electronegativity of fluoride ion,cause the wide band gap between the iron and fluorine,which leads to low electronic conductivity and serious polarization phenomenon.In addition,during the process of repeated conversion reaction,the structure of electrode material is seriously changed,resulting in poor cycling performance and rate performance,as well as severe first-time capacity irreversibility.This paper is aimed at the above problems of iron based fluorides,adding conductive carbon materials to enhance electronic conductivity of electrode materials,the surface coating,particle nanocrystallization and morphology design are used to enhance the structure stability,so as to improve the rate and cycle performance of electrode materials.The main research contents of the paper are as follows:(1)The octahedral morphology FeF3·0.5H2O/graphene nanocomposite electrode materials were prepared by solvothermal method and ball milling method.The particle size of the nanocomposite electrode was about 400 nm.The structure,morphology and electrochemical effects of the doping amount of different graphene on the nanosized octahedral FeF3·0.5H2O electrode materials were studied systematically.The results show that the nano-sized octahedral FeF3·0.5H2O electrode material will not damage the morphology during the ball milling process,and the nano-sized octahedral FeF3 0.5H2O electrode material is randomly distributed on the surface and inside of the conductive graphene.Moreover,the addition of graphene has significantly improved the rate performance and cycling performance of FeF3·0.5H2O electrode materials.When the addition amount of graphene is 5 wt.%,the performance is best,and the first discharge specific capacity is up to 410.0 mAh/g at a rate of 0.1 C.After 200 cycles,the capacity retention rate is 68.7%.Even the current rate as high as 5 C,the discharge specific capacity still has 86.3 mAh/g after 200 cycles.Graphene can not only improve the electrical conductivity of materials,but also has an elastic constraint effect on the octahedral morphology of the FeF3·0.5H2O electrode material,which ensures the integrity of its morphology in repeated charging/discharging processes.(2)First,the spherical FeF3 ·0.33H2O cathode material with uniform morphology and size was prepared by solvothermal method.Then a TiO2 layer was coated on the surface of spherical FeF3·0.33H2O material by gel sol-gel method.Compared with the raw materials,the fine TiO2 particles are coated on the surface of the spherical FeF3·0.33H2O material,which increases the specific surface area and enhances the stability of the structure.The test results show that there is no effect on the morphology of the original material during the process of coating TiO2.In addition,the coating of TiO2 results in a significant improvement in the electrochemical performance of the spherical iron fluoride materials,especially the cycling performance.After 200 cycles,the capacity is still 264 mAh/g at a current density of 0.2 C in the voltage range of 1.5-4.5 V.(3)In order to improve the electronic conductivity of iron-based fluoride electrode materials,FeF3·0.33H2O@3D-OMCs nano-composites with fast electron transport and three-dimensional pore structure were prepared by nano casting and mesoporous silica KIT-6 template method.The FeF3·0.33H2O nanoparticles are trapped inside the mesoporous KIT-6 of the mesopore,and the both tightly connected to constitute a fast conducting network.The aggregation phenomenon of FeF3·0.33H2O nanoparticles during the growth process is effectively restricted through the mesoporous structure of 3D ordered mesoporous carbon.Nano-scale FeF3·0.33H2O material can effectively promote the transmission of electrons and sodium ions,thereby improving the electrochemical performance of the material.In addition,a large specific surface area of ordered mesoporous carbon can ensure sufficient contact between the active material material and the electrolyte.The combination of these favorable factors and their synergistic effects make FeF3·0.33H2O@3D-OMCs nano-composites exhibit excellent rate performance and cycle performance in sodium ion batteries.At a current density of 20 mA g-1,the first discharge capacity is up to 386 mAh/g,and when the current density increased to 60 mA g-1,the discharge specific capacity still remains 183 mAh/g after 100 cycles. |
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