| The growing demand for energy around the world is one of the major challenges facing human society in the 21st century.At the same time,due to the increasingly serious environmental problems,the requirements for efficient clean electricity resources such as solar and wind power,especially the need for sustainable energy storage systems,has led to continuous research aimed at improving the existing battery technology.Rechargeable lithium-ion batteries that have been widely used in portable electronic devices are now being gradually extended to large-scale energy storage systems such as electric vehicles and smart grids.While the current commercial applied lithium-ion batteries can not meet the needs of such a large-scale application.Therefore,the electrochemical performance of rechargeable lithium ion batteries must continue to be improved in terms of capacity,rate performance,cycle life,and energy and power density.The key to improve these performances is the research and development of safe and environmentally friendly new electrode materials with high-capacity,high-rate performance,and high cycle life.In this thesis,in order to achieve these objectives,several coating structrural composite electrode materials were prepared through a variety of synthetic methods.The composite electrode materials were characterized and tested the electrochemical properties as the anode or cathode of rechargeable lithium ion batteries.The main research contents are shown as following:(1)Antimony@carbon(Sb@C)composite fibers with Sb nanoparticles uniformly dispersed in carbon matrix have been successfully fabricated by a facile spinning approach and the subsequent calcination in argon flux.As the anode materials of lithium-ion battery,experimental results reveal that the increase of Sb concentration enhanced the lithium-ion storage capacity and worsened the cycling performance.Carbon matrix coated outside the Sb particles could efficiently buffer the volume change and maintain the integrity of electrode during the lithiation/delithiation,improving the cycling capability.Among the Sb@C composite fibers,the sample(Sb5)with appropriate carbon content of ca.50.1 wt%achieves a reversible capacity of 315.9 mAh g-1 after 100 cycles at a current density of 100 mA g-1 and the capacities of 464.4,395.4,328,246.2 and 149.8 mAh g-1 at 100,200,400,800 and 1600 mA g-1,respectively.Electrochemical impedance spectroscopy(EIS)results indicate that Sb5 shows more superior charge transfer rate and Li-ion diffusion ability than the samples with high Sb concentration(Sb6)or carbon concentration(Sb4),finally resulting in better capacity retention and rate performance.(2)Carbon coated composite fibers with Fe3O4,Fe3C,and TiO2 nanoparticles(Fe3O4/Fe3C/TiO2@C)had been successfully fabricated through a facile centrifugal dry-spinning approach,followed by annealing under argon flux.When used as the anode material of lithium-ion battery,the Fe3O4/Fe3C/TiO2@C composite fibers achieved a reversible capacity of 702.1 mAh g-1 after 400 cycles at a current density of 100 mA g-1 and ca.200 mAh g-1 at the current density of 1000 mA g-1 in 350 cycles.The superior cycling performance and high rate capability were attributed to the high theoretical specific capacity of Fe3O4,catalytic activity of Fe3C,structural stability of TiO2,and buffer function of carbon fiber.Compared with the corresponding Fe3O4/Fe3C@C and TiO2@C composite fibers,the Fe3O4/Fe3C/TIO2@C product exhibited higher cycling and rate performances due to the synergetic effect among Fe3O4,Fe3C,TiO2 and carbon components.(3)Composite organic cathode material of aromatic polyimide(PI)coated highly conductive graphene has been prepared via a facile in-situ polymerization method for application in lithium ion batteries.The in-situ polymerization generates intimate contact between PI and electric conductive graphene,resulting in conductive composites with highly reversible redox reactions and good structure stability.The synergistic effect between PI and graphene enables not only a high reversible capacity of 232.6 mAh g-1 at a current rate of C/10 but also exceptional high-rate cycling stability,that is,a high capacity of 108.9 mAh g-1 at an ultrafast current rate of 50 C with capacity retention of 80%after 1000 cycles.These superior electrochemical performance results from the elaborate combination of stable redox reversibility of PI and high electronic conductivity of the graphene additive.It is further demonstrated that the graphene based composite also exhibits much better performance than those based on multiwall carbon nanotube and C45 in terms of specific capacity and long-term cycling stability under the same current rates.(4)Organic composite electrode material of electron conductive polythiophene(PT)coated aromatic polyimide(PI)have been prepared by a facile in-situ chemical oxidation polymerization method.The common aromatic structure possessed in both electroactive PI and electron conductive PT allows intimate contacts,resulting in conductive polymeric composites with highly reversible redox reactions and good structure stability.It has been demonstrated that the PI composite material with 30 wt%PT coating(PI30PT)has the optimal combination of good electronic conductivity and fast lithium reaction kinetics.The synergistic effect between PI and PT enables a high reversible capacity of 216.8 mAh g-1 at a current rate of C/10,as well as a high-rate cycling stability,that is,a high capacity of 89.6 mAhg-1 at a high current rate of 20 C with a capacity retention of 94%after 1000 cycles.The elaborate combination of high electronic conductivity of the PT coating and fabulous redox reaction reversibility of the PI matrix offers an economic way to prepare high performance lithium ion batteries for sustainable energy storage applications. |
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