Synthesis,Characterization And Lithium-ion Battery Performance Of Fe₃O₄ Nanometer Microstructure

Fe₃O₄ nanoparticles are a novel functional material and applied in many fields.They have the small diameters,large specific surface areas and excellent magnetic properties.In addition,Fe₃O₄ also has the high theoretical specific capacity（924mAh/g）as electrode material.Therefore,it has the widely potential applications in magnetic recording,resistivity,absorbing,catalysis,biological medicine and other fields.In this thesis,Fe₃O₄ nanosheet,microsphere and flower nanometer microstructures are synthesized by a complex co-precipitation method at 100℃.On this basis,Fe₃O₄ microsphere/MWCNT（MWCNT:multi-walled carbon nanotubes）,Fe₃O₄ nanosheet/MWCNT and Fe₃O₄ flower/MWCNT nanocomposites are synthesized.Transmission electron microscope（TEM）,scanning electron microscope（SEM）,powder X-ray diffraction（XRD）,vibrating sample magnetometer（VSM）,pore specific surface analyzer,and Fourier transform infrared spectrometer（FTIR）are used to characterize the Fe₃O₄ nanomaterials and corresponding composite materials,including morphology,structure,magnetic,specific surface area,surface bonding conditions,etc.The study of electrochemical performance shows that the MWCNTs improve the electrical conductivity of Fe₃O₄ electrodes,and reduce the volume variation during the delithiation/lithiation processes.The Fe₃O₄/MWCNTs nanocomposites show the excellent electrochemical performance in lithium ion battery anode material.We firstly synthesize the Fe₃O₄ microspheres by one step reaction at 100℃.The microspheres have a particle size of 80-120 nm with polycrystalline nature and internal mesoporous microstructure.The reaction mechanism studies indicate that the microspheres experience a self-assembly-dissolve-recrystallization growth process of nanoparticles.VSM measurements show that the sample has high saturation magnetization（87.96 emu/g）,low coercive force（27.12 Oe）and remanent magnetization（4.67 emu/g）.The electrochemical performance results of Fe₃O₄microsphere/MWCNT composites indicate that the different amounts of MWCNTs have a great effect on the discharge specific capacity,charge/discharge stability and discharge platform.For the samples composed of different amounts of MWCNTs,the discharge specific capacities in 0.2C after 50th cycle are presented as follows:40%MWCNTs（842.2 mAh/g）>30%MWCNTs（790 m Ah/g）>20%MWCNTs（672.5mAh/g）>10%MWCNTs（566.4 mAh/g）>single-phase Fe₃O₄ microspheres（461.2mAh/g）.Fe₃O₄ nanosheets are synthesized by a complex co-precipitation method at low temperature（100℃）.The sample shows the single crystal nature and superparamagnetism.Fe₃O₄ nanosheets/MWCNT nanocomposites are synthesized by introducing the different amounts of MWCNTs in the solvent.FTIR spectra show that MWCNTs and Fe₃O₄ nanosheets are bonded with C-O-Fe bonds.For the electrochemical properties,the cycling rate of Fe₃O₄ nanosheets/MWCNT nanocomposites remains over 94%,which is higher than that of Fe₃O₄ nanosheets（69.3%）at discharge specific capacity and cyclic retention.In addition,the samples are calcined after introducing the glucose,the discharge specific capacity of Fe₃O₄/C composites in 0.2C after 50^th cycles is 615 m Ah/g and its cycling rate is 93.2%.Compared with the single-phase Fe₃O₄,Fe₃O₄/C composites also show a great enhancement of the ratio performance.Fe₃O₄ peony microstructure is also synthesized by self-assembly mechanism at100℃.The samples are superparamagnetism and indexed to a face-centered cubic（fcc）structure.Fe₃O₄/MWCNTs nanocomposites are preparedd by mixing the MWCNTs and as-prepared Fe₃O₄ peonies.The electrochemical performance of Fe₃O₄peony/MWCNTs nanocomposites is better than Fe₃O₄ peony sample.Its discharge specific capacity is 640.4 mAh/g（first cycle）and 535.5 mAh/g（second cycle）,and it remains above 510 mAh/g afterwards,the capacity retention rate remains above 95%and the coulomb efficiency remains above 99%.