Preparation And Applications Of Carbon Micro/Nano Composites Based On Iron Oxide (Sulfide)

Iron oxides Fe₂O₃?1007 mAh/g?and Fe₃O₄?926 mAh/g?,sulfides FeS₂?894 mAh/g?and Fe₇S₈ are good negative electrode materials due to their characteristics of high theoretical capacity,abundant natural resources,safety and environmental protection.However,the disadvantages of poor conductivity and volume expansion are also obvious.This paper is aimed at preparing micro-nano material with special structure and their composites with carbon materials to overcome these problems.Besides,the synthesis mechanism and electrochemical performance are also the important study points.Firstly,we synthesized Fe₃O₄ sub-microspheres with micro/nano size by one pot solvothermal method.According to the XRD test of materials at different reaction stages,we know that the precursors react to form Fe?OH?₃ with colloidal structure.Fe?OH?₃ react with EG or DEG to form mesophase and then mesophase decomposes thermally to form Fe₃O₄nuclei,which gradually grow into magnetic particles.Acetic acid can stabilize nanocrystals in the solution system.The soft template PVP can inhibit the savage growth of nuclei and promote the formation of uniform and stable magnetic sub-microspheres.Secondly,a N doped coaxial CNTs@?-Fe₂O₃@C composite was synthesized by solvothermal method and its electrochemical performance was tested.?-Fe₂O₃ nanoparticles with small particle size about 40-50 nm possess high surface area;The special structure of CNTs@?-Fe₂O₃@C can improve the conductivity to accelerate charge transfer and maintain the stability of the structure;Doped N element can provide more sites for Li binding.The discharge capacity of CNT@?-Fe₂O₃@C-10 was 1218.8mAh/g after 60 cycles and 1.1%of the discharge capacity decreased compared to the second cycle.The discharge capacity of CNT@?-Fe₂O₃@C-5 was 306.3 mAh/g after 60 cycles.Compared to CNT@?-Fe₂O₃@C-5,CNT@?-Fe₂O₃@C-10 has higher capacity and stability and at the same time its rate performance is also excellent.Finally,two different synthesis methods were used to synthesize FeS₂and Fe₇S₈ with special structure in hydrothermal/solvothermal environments.Accumulation body with single hole was synthesized by method I and cube structure with surface gully was synthesized by method II.XRD test indicates that the two kind of materials are composed of two phases,FeS₂ and Fe_1-xS.When the temperature decreased to 180?,FeS₂spherical product with pure phase was synthesized.Material with the smallest size was synthesized when the amount of PVP was 2 g.We got FeS₂@CNT composite in EG solvent system.The FeS₂ nanoparticles were uniformly distributed along the CNTs and the diameter of the particles was about 7-10 nm and the CNTs aggregated into microspheres-like clusters.If the dried CNTs were used,a flower-like structure formed.The main body of the material is 3D space structure with a flower-like structure and the crystal structure of the material is poor.After sintering,Fe₇S₈ with good crystal structure was obtained.