Preparation Of Hollow Carbon Materials With Highly Graphitic Framework By Melting Coating Method And Their Electrochemical Energy Storage

Carbon materials have been widely used as anode materials in various energy storage devices.However,the energy density and rate performance of carbon materials are not good enough to meet the application requirements of electronic devices and new energy vehicles in the future.In order to obtain large-capacity,long-cycle,high-rate performance carbon-based electrode materials,a lot of effort has been devoted to morphology and structural design to improve the ion transmission and storage process.In the design of carbon morphology,shell carbon materials（such as hollow carbon materials）have a lot of application space,mainly because the outer surface of the shell can be used as a doped carrier or provide channels and storage sites for ions,and the hollow structure can provide a path for ions and a volume expansion space for the electrode,etc.In terms of carbon structure modification,high graphitization is a good choice because it has the effect of further increasing the conductivity of the electrode and improving the electrochemical stability of the energy storage device.In recent years,people have used various methods to make shell morphology,including chemical vapor deposition,hydrothermal method,template method,etc,among which template method has irreplaceable advantages due to its accurate morphology control and structure design capability.However,the current template method requires a complicated nano-casting or surface growth process,and the steps are cumbersome.At the same time,the carbon source used is usually sugar or thermosetting resin,which is difficult to obtain a high graphitization structure,which is not conducive to improving the conductivity of the material and improving its rate performance.Therefore,it is still extremely urgent to develop a simple hard template process and design highly graphitized hollow carbon materials.In response to the above problems,we have further improved the template method by selecting a low-melting precursor,which takes advantage of the property that the precursor can autonomously coat the template to form carbon in the molten state,which we call the"melt coating method".The melt coating method relies on the melting point property and composition property of the precursor,and the temperature is adjusted to simplify the preparation process and complete the change of the carbon material structure.In short,using the molten coating method to take advantage of the precursor itself,prepare a hollow graphite material with high graphitization and analyze the impact of its structure on the electrochemical energy storage device,which is ultimately used to improve electrochemical performance main target.In this thesis,two precursors,phthalocyanine and nickelocene,were used to prepare the hollow carbon materials suitable for sodium-ion batteries and lithium-ion batteries using the melting coating method,and the carbon materials were analyzed by physical characterization analysis of carbon material characteristics,and then select the most appropriate control method,explore the relationship between the carbon material structure and electrochemical performance.The main contents are as follows:（1）In order to prepare carbon materials suitable for sodium ion batteries,phthalocyanine is used as a nitrogen-containing precursor,silica balls are used as templates,and a simple mechanical mixing method is used to prepare hollow carbon hemispheres.Phthalocyanine has a low melting point and a suitable rate of carbonization.In the process of temperature rise,it will melt spontaneously and cover the surface of silica template.No special nano-casting or surface growth process is needed,and the process is simple.The hollow carbon hemisphere has an ultra-low carbon layer thickness（～10 nm）,a high nitrogen content（～6.5%）,a large specific surface area of 382 m²g^-1,and contains a large number of micropores and mesopores.At the same time,due to its typical soft carbon characteristics,the hollow carbon hemisphere has a highly graphitized carbon skeleton,and its layer spacing can be precisely adjusted.When used as a negative electrode material for sodium ion batteries,the hollow carbon hemisphere surface has excellent performance:its first coulombic efficiency can reach 80%,and it has excellent rate performance and cycle performance.（2）In order to prepare carbon materials suitable for lithium-ion batteries,nickelocene is used as a precursor,silica spheres are used as templates,and hollow carbon nano-hemispheres are prepared by a melt coating method.The nickel atoms carried by nickelocene can catalyze the graphitization of the carbon layer autonomously,thereby obtaining a thin-layer graphite carbon skeleton.Interestingly,we found that the catalytic graphitization of nickel atoms had an anomalous temperature effect due to the low melting point of the nanocrystalline nickel elements:at 800℃,the optimal graphitization structure could be obtained,while the increase in the carbonization temperature would lead to a reduction in the degree of graphitization.The pore structure test shows that the specific surface area of the sample can reach 791 m² g^-1.When used as a cathode material for lithium ion batteries,the graphitized carbon skeleton of the carbon nanosphere can provide a place for lithium ion intercalation reaction,while the hollow structure can serve as a storage pool for lithium ions and shorten the ion transport distance and improve the ion transport rate.Because of the above structural advantages,the hollow carbon material produced exhibits excellent lithium storage performance.For example,when the current density at 0.1 A g^-1,the lithium storage capacity can reach 466 m Ah g^-1.There was no dramatic capacity degradation during the 200-round cycle test.（3）In order to prove the universality of melting coating process and realize the morphology design of the hollow carbon material.In this chapter,we use zinc oxide nanowires synthesized by hydrothermal reaction as a template and phthalocyanine as a precursor,using the spontaneous coating properties of phthalocyanine during carbonization to construct hollow carbon nanotubes.This result proves the adaptability of phthalocyanine as a precursor and the superiority of melting coating method,and also provides the possibility of making supercapacitor carbon electrodes.