Preparation And Sodium Storage Mechanism Of Resorcinol Formaldehyde-based Hard Carbon Materials

With the continuous increase of the global population and the rapid development of technology,the demand for energy from various countries is constantly increasing.At present,a considerable proportion of energy still depends on the world’s limited fossil fuels.However,the combustion of these fossil fuels generates high levels of carbon dioxide,a major cause of global climate change.In addition,due to the shortage of resources which caused by the dependence on oil and natural gas,there is an urgent need to develop non-polluting or low-pollution renewable energy such as wind and solar power.However,there are some common problems with these energy sources: intermittent supply of energy,difficult to control,as well as cyclical,regional and so on.In order to be able to harness these energies,it is important to develop suitable large-scale energy storage technologies.Lithium ion secondary batteries in the new energy technology has an important application,can well meet the wind power,photovoltaic power generation and other new energy needs of power grid.The sodium and lithium are in the same main group,which also have similar chemical properties.Compared with lithium-ion batteries,sodium-ion batteries take advantages in rich resources and low costs,safety and cost-effective battery security,which make the sodium ion batteries very suitable for large-scale energy storage and have a very broad application prospects.As long as we can choose the appropriate anode and cathode materials,it is expected to develop more competitive sodium ion batteries than lithium ion batteries.The hard carbon materials as one of the anode materials show good overall performance,high reversible capacity and good cycle stability,have better research prospects.In this dissertation,the research focuses on hard carbon with controlled surface area and smaller specific surface area.By controlling different precursors and sintering processes,hard carbon materials with different microstructures and macrostructures are obtained,combining with in situ and ex situ structural testing to analysis the molecular structure and explore the sodium ions storage mechanism in hard carbon materials.Combined with the electrochemical test such as charge and discharge test and cyclic voltammetry,the influence mechanism of the microstructure changes on the macroscopic electrochemical performance was obtained.The details are as follows:1.Resorcinol formaldehyde(RF)resin was selected as the main research material of the study.RF resin precursor materials with different polymerization structures were obtained by changing the polymerization conditions and process parameters of the RF resin.The precursors were pyrolyzed to get various hard carbon materials with different structure and further discuss the relationship between structure of hard carbon and the sodium storage performance.The results show that with the increase of solvent polarity,the degree of cross-linking of RF resin increases,the sphericity of hard carbon material is improved and the defect degree gradually increases,the interlayer d-spacing of parallel graphene sheets almost constant and the length of the planar graphene sheets grow gradually into the bigger graphite-like microcrystallites.The hard carbon pyrolyzed from water-based RF resin showed good electrochemical performance with a reversible capacity of 310 m Ah/g at a current density of 20 m A/g and an initial coulombic efficiency of 80% with good cycling stability at this current density.In addition,a series of hard carbons were obtained by adjusting the time course parameters,the hydrothermal synthesis temperature and the p H value during the self-assembly of RF resin precursors,Sodium storage properties of these hard carbon were studied and found that hard carbon pyrolyzed from higher crosslinking degree of RF resin showed good electrochemical properties,is conducive to the improvement of sodium storage performance.2.In order to study the effect of the change of pyrolysis carbon temperature,RF resin precursors were pyrolyzed at different temperatures under the optimum conditions.We studied the effects of pyrolysis temperature,as well as relationship between hard carbon microstructure and macroscopic electrochemical performance.At lower pyrolysis temperature,of hard carbon show low reversible capacity,and the platform area is shorter.When the pyrolysis temperature reached 1300 oC,the capacity of the charge and discharge platform area reaches the highest and the coulomb efficiency is the highest.With the further increase of pyrolysis temperature,the microstructure of hard carbon tends to be graphitized,the interlamellar spacing of graphite crystallites decreases and the length of microcrystalline carbon layer grows,and the reversible capacity and the coulomb efficiency of the hard carbon reduce.The optimal pyrolysis conditions for hard carbon of RF resin were obtained,which laid a solid foundation for further research on hard carbon storage mechanism.3.Using the two series of materials,hard carbon of different solvents RF resin and hard carbon with different pyrolysis temperature,as the research object,the principle of diffusion kinetics of sodium ion intercalation was studied by GITT.PDF and ex-situ XRD are used to analysis the microstructure of hard carbon material,and we also obtained sodium ions three-stage process model for hard carbon material.With the deepening of the discharge depth,the process of intercalation of sodium ions into hard carbon is as follows: absorbed process-intercalation of sodium ions-pore filling process.