| Since the twenty-first century, energy and environmental issues have become increasingly prominent, so the development of clean energy has attracted more and more attention. New energy storage is the first problem to be faced. As new energy storage devices, supercapacitor and lithium ion battery are considered to be promising due to their high energy density, large reversible capacity, long cycle life, no memory effect, safety, environment-friendly and so on. Electrode materials are important parts of the supercapacitor and lithium ion battery, and they are the key factors to determine the performance of the devices. Therefore, the preparation of high performance electrode materials has become a hot research object.As an anode material for lithium ion batteries, magnetic oxide(Fe3O4) has potential applications and its theoretical capacity is up to 924 mAh/g, which is 2.5 times higher than that of traditional commercial graphite. Besides, owing to their advantages of abundant reserves, low cost, and no toxicity, Fe3O4 has attracted many domestic and foreign scientists to research it. Unfortunately, the structure of Fe3O4 will crack due to their intrinsically low conductivity, and large volume change during the repeated lithiation/dethiation process, which will lead to its rapid performance decline. In this thesis, in order to enhance the performance of Fe3O4 as an anode material for lithium ion battery, we prepared the novel Fe3O4@Fe3C-C yolk-shell nanospindles to improve the structural stability and electrical conductivity of the material.Hierarchical porous carbons are currently used as electrode materials for supercapacitors, due to their high surface area, multimodal pore structure, good conductivity, and adjustable pore size. Usually, hard template method is commonly used to prepare hierarchical porous carbon materials, but the synthetic process is complex and the templates need to be removed through wet chemical etching, which is against the synthesis idea of green chemistry. In addition, the morphology and pore structure of porous carbons have great influence on its electrochemical performance,so it is an important way to optimize the performance by adjusting the morphology and pore structure of porous carbons.The main contents of this thesis are as follows:(1) Fabrication of the heterogeneous Fe3O4@Fe3C-C yolk-shell nanospindles and their excellent performance for lithium-ion battery.An in-situ nanospace pyrolysis strategy was used for the synthesis of heterogeneous Fe3O4@Fe3C-C yolk-shell nanospindles, in which the Fe3O4@Fe3C core@shell nanoparticles act as yolks and carbon nanospindles as shells. Compared to Fe3O4@C core-shell nanospindles, Fe3O4@Fe3C-C exhibits better performance for lithium-ion battery application. It delivered a high reversible capacity of 1128.3mAh/g at even 500 mA/g, excellent high rate capacity(604.8 mAh/g at 2000 mA/g)and prolonged cycling life(maintaining 539.7 mAh/g at 2000 m A/g for 400 cycles).To the best of our knowledge, the present Fe3O4@Fe3C-C yolk-shell nanospindles are the most efficient Fe3O4-based anode materials for LIBs. Because the heterogeneous Fe3O4@Fe3C can not only maintain the structural stability of Fe3O4, but also store more Li+ in the oxide/carbide interface. At the same time, the porous carbon shells with high electron conductivity improved the speed of Li+ transport, and provided enough void space for the volume change of Fe3O4 during the charge/discharge process. This work is valuable for the design of high performance anode materials for LIBs.(2) One-step carbonization synthesis of hollow carbon nanococoons with multimodal pores and their enhanced electrochemical performance for supercapacitors.Hollow carbon nanococoons(HCNCs) were prepared through one-step carbonization process, and the HCNCs exhibit high specific capacity(220 F/g at 5mV/s, 3.5 times higher than that of HCSs) and excellent cycle stability(98% of the initial capacity after 1000 cycles). In our opinion, the non-spherical cavity plays an important role in enhancing the capacity of HCNCs. This work opens a new avenue for obtaining other non-spherical hollow capsules.(3) A dual templating route to three-dimensionally ordered mesoporous carbon nanonetworks and their optimized electrochemical performance.A dual-templating coassembly/hydrothermal approach was adopted to synthesize ordered mesoporous carbon nanonetworks with 3D cubic mesopores(OMCNW-c) bysimultaneously employing Pluronic F127 as the pore template and polyvinyl pyrrolidone(PVP) as the structure-directing agent. By adjusting the adding amount of PVP, the morphology and mesopore type of OMCs can be changed, and we can simultaneously achieve ordered mesoporous carbon nanonetworks with 2D hexagonal mesopores(OMCNW-h) and mesoporous carbon nanospheres(OMC-S). Owing to its3 D cubic mesopores and 3D porous network structure, which favor the electrolyte diffusion and electron conduction, OMCNW-c shows much better performance for supercapacitors and electrocatalytic activity for oxygen reduction reaction(ORR).The approach to mesoporous carbon materials in this work provides a new idea for new type OMCs. |
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