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The Preparation Of Core-shell Structure Transition Metal Oxides And Their Electrochemical Performance Test

Posted on:2017-03-12Degree:MasterType:Thesis
Country:ChinaCandidate:Y ZhangFull Text:PDF
GTID:2271330503979839Subject:Materials Science and Engineering
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As the lithium-ion batteries is finding wider and wider application in the field of high-power electrical appliances, such as electric vehicle and energy storage device,consumers have a high requirement for commercial lithium ion battery energy density,stable voltage and good cycle performance. And battery performance is closely connected with the structure of the electrode materials. Transition metal oxides as the cathode material has higher specific capacity and low lithium embedded platform, has attracted more and more attention. But their large volume change during the process of charging and discharging has limited the service life and performance ratio. Therefore, in recent years a core-shell structure of the electrode material is widely used in lithium ion battery, which can realize the function of core with shell complementary and compound,effectively improved the comprehensive performance of the battery. At the same time,nano size of electrode materials can reduce the Li+ and electronic transmission path,which can improve the reversibility and cycle stability. Based on the above two points, in this paper, we apply a simple hydrothermal method, precipitation method and ion exchange method to prepare nano-sized core-shell structure transition metal oxide as lithium ion battery cathode.(1) In this paper, using solvent hot method synthesis small nanometer ball stack of Fe3O4 nanometer microspheres, which has high purity, good dispersion and structure controllable. Then with ultrasound depositing in Fe3O4 surface coating a layer of Mn3O4 nanoparticles, core-shell structure formed of Fe3O4 @ Mn3O4 nanometer microspheres.Using the X-ray diffraction(XRD), scanning electron microscope(SEM), transmission electron microscopy(TEM) and electrochemical tests on its structure and properties have been characterized. After electrochemical testing, under the current density of 0.1 C, the initial discharge capacity of Fe3O4@Mn3O4 nanometer microspheres is 1018 mAhg-1,after 200 times circulation reversible capacity is 639 mAh/g. Throughout the process capacity decline first and then pick up, capacity rising phenomenon has appeared in the transition metal oxides. the reason may be that the lithium ions in the electrolyte and the interaction of core-shell nanoparticles gradually from surface to interior, this is a progressive process to activate the electrode materials. And the initial discharge capacity of separate Fe3O4 nanoparticles microspheres is 940 mAhg-1, capacity retention rate is only 21.4% after 200 cycles.(2) The previous chapter has succeeded in synthesis of synergy Fe3O4@Mn3O4core-shell structure materials, which greatly improves the electrochemical properties. Inorder to further enhance the electrical conductivity, bovine serum albumin as medium material in this chapter, using the histidine for certain metal ions adsorption properties to connect two kinds of transition metal oxides, after heat treatment bovine serum albumin as a carbon layer, prepared Fe3O4@C@Mn3O4 multistage core-shell structure. In order to research the formation mechanism of multilevel core-shell structure, change the amount of the addition of bovine serum albumin to morphology observation. It is concluded that the formation of core-shell structure are relevant to the active site of amino acids, metal ionic radius and the valence bond theory. Fe3O4@C@Mn3O4 multistage core-shell structure as lithium-ion battery cathode materials, the electrochemical tests show that at0.1C current density the initial discharge capacity is 1261 mAhg-1. After 200 times cycles,there are still 987 mAhg-1 retention capacity, which showed good cycle performance.Improved the electrochemical performance boils down to two points: firstly, carbon materials can together relieve stress and active substances reunion by control volume change; secondly, carbon has good electrical conductivity, which can improve the electronic conductivity of the electrode.(3) This chapter is based on the concept of metal organic framework(MOF), using the method of ion exchange replace K+ of K3[Fe(CN)6] with Mn2+ to synthesis Mn3[Fe(CN)6]2 precursor. After calcining heat treatment the size of MnFe2O4 mesoporous nanometer microspheres were about 500-800 nm, average mesoporous size was 18.38 nm, BET specific surface and mesoporous volume were 55.1 m2 /g and 0.36m3/g. Large specific surface area gives the electrode electrolyte with large contact area and more lithium storage space, and hierarchical mesoporous can better transfer Li+ and electrolyte molecules, shorten the transportation path of electrode materials, retard the volume change in the process of charging and discharging. Then using chemical bath deposition coat a layer of TiO2 on MnFe2O4 mesoporous microspheres surface, it has good structural stability and long cycle life, both the composite compound the MnFe2O4@TiO2 core-shell structure mesoporous microspheres, After 200 times cycles,there are still 456 mAhg-1 retention capacity.
Keywords/Search Tags:Lithium ion battery anode, Core-shell structure, The surface coating, Transition metal oxides, Mesoporous structure
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