Preparation And Properties Of Porous Life₅O₈ Powders And Their Composites

With the rapid development of science and technology,increasing energy consumption and electromagnetic interference?especially the microwave in GHz band?pollution have become a serious problem that is a threat to human health and the living systems.Therefore,it is extremely urgent to explore new green energy storage devices and effective electromagnetic wave absorbing materials.Lithium ferrite?LiFe₅O₈?is one of the most famous cubic ferrites,belonging to a soft ferrite materials with high Curie temperature,a square hysteresis loop and a high magnetization.It has been proved to have important applications in many fields,such as microwave devices,optical isolators,transformer cores and rod antennas.In this thesis,three-dimensional flower-like LiFe₅O₈ microspheres,LiFe₅O₈@PPy nanocomposites and rGO/porous LiFe₅O₈ composites were prepared by a novel two-step approach consisting of molten salt method and one-step etching method.The morphologies,mocrostructures,electrochemical properties and electromagnetic wave absorption properties of all the samples were characterized and investigated.The following is the main research content of the papers:?1?Preparation and properties of three-dimensional flower-like LiFe₅O₈microspheresThe bulk LiFe₅O₈ particles prepared by the molten salt method were used as precursors.Bulk LiFe₅O₈ was etched into the three-dimensional flower-like LiFe₅O₈ microspheres by a one-step etching process.The XRD patterns and SEM images revealed that the LiFe₅O₈ phase did not exhibit any changes after the etching process,and the crystal morphology evolved from blocks to flower-like three-dimensional porous LiFe₅O₈ spheres by controlling the experiment parameters.The change of morphology led to the significant increase of specific surface area,thereby the great changes in the performances.The results of electrochemical properties showed that the specific capacitance of the three-dimensional flower-like LiFe₅O₈ microspheres was 190 F·g^-1 at a current density of 0.5 A·g^-1.The maximum specific capacitance value for the flower-like LiFe₅O₈ microspheres was 20 as big as that of the unetched LiFe₅O₈ powder.Furthermore,the three-dimensional flower-like LiFe₅O₈ microspheres electrode exhibited a good rate capability and maintained 78.3%of initial capacity at a current density of 1 A·g^-1 over 2000 cycles.The result of electromagnetic wave absorption properties showed that the RL_max of the three-dimensional flower-like LiFe₅O₈ microspheres with a thickness of 3.2 mm can reach-30.1 dB at 16.3GHz in the frequency range of 2-18 GHz.?2?Preparation and properties of hierarchical LiFe₅O₈@PPy core-shell nanocompositesBulk LiFe₅O₈ paticles prepared by molten salt method and PPy nanotubes febricated by template method were used as precursor.we developed a facile etching strategy for core-shell LiFe₅O₈@PPy nanocomposites.The results of electrochemical properties showed that the specific capacitance of the LiFe₅O₈@PPy nanocomposites was 211 F·g^-1 at a current density of 1 A·g^-1,and its specific capacitance still remains 89 F·g^-1 at a high current density of 10 A·g^-1,43%of that at 10 A·g^-1.In addition,the LiFe₅O₈@PPy nanocomposite electrode exhibited a good rate capability and maintained 82.99%of initial capacity at a current density of 1 A·g^-1 over 10000 cycles,indicating that the LiFe₅O₈@PPy nanocomposite has superior high specific capacitance and good cycle stability than the LiFe₅O₈ electrode.The result of electromagnetic wave absorption properties showed that the RL_max of the LiFe₅O₈@PPy nanocomposite with a thickness of 2.2 mm can reach-51.6 dB,and its effective bandwidth?RL?-10dB?was up to 5.5 GHz,indicating LiFe₅O₈@PPy nanocomposite has good absorbing properties.?3?Preparation and properties of composites of reduced graphene oxide with porous LiFe₅O₈ microspheresThe LiFe₅O₈ powder prepared by the molten salt method and the graphene oxide?GO?prepared by the Hummers method were used as precursors.A novel three-dimensional composites of reduced graphene oxide and porous LiFe₅O₈microspheres were prepared by a facile etching method.The microstructure,composition,electrochemical properties and microwave absorption properties of rGO/porous LiFe₅O₈ composites were characterized and studied.The results of electrochemical properties showed that the the specific capacitance of the rGO/porous LiFe₅O₈ composite was 359.6 F·g^-1 at a current density of 1 A·g^-1.In addition,the rGO/porous LiFe₅O₈ composite electrode exhibited a good rate capability and maintained 84.31%of initial capacity at a current density of 1A·g^-1 over 5000 cycles,indicating that the rGO/porous LiFe₅O₈ composite has excellent electrochemical properties.The result of electromagnetic wave absorption properties showed that the RL_max of the rGO/porous LiFe₅O₈composite with a thickness of 2.2 mm can reach-53.4 dB,and its effective bandwidth?RL?-10 dB?was up to 3.5 GHz?from 10.4 GHz to 13.9 GHz?.