Mechanism Of Thermal Conversion From Coal-based Carbon Components To Carbon Materials And Enhancement For Sodium-ion Storage

Exploring the non-fuel utilization of coal is crucial to optimize China’s energy structure;high-efficiency electrochemical energy storage technology provides guarantee for the large-scale utilization of clean energy.Sodium-ion electrochemical energy storage systems have received widespread attention due to abundant sodium resources and low-cost.Developing high-performance and low-cost electrode materials is an important development direction at present.Therefore,developing the preparation technology of coal-based carbon materials is significant for promoting the clean,low-carbon,and high-value-added utilization of coal.Typical coal-based carbon components mainly include gas,liquid,and solid phase.Thermal conversion is a key step in the conversion of coal-based carbon components to carbon materials.Regulating the thermal conversion paths and optimizing the multiscale nanostructures of carbon products are the key to realizing high-efficiency sodium-ion storage.In addition,the quality-based utilization of coal-based gas/liquid/solid components is of great significance to achieve efficient use of coal and reduce carbon emissions.Therefore,this paper focuses on the mechanism of thermal conversion of coal-based gas/liquid/solid components into carbon materials,and the multiscale nanostructure optimization of carbon materials,and the enhancement of sodium-ion storage.Develop regulation methods of conversion paths of different coal-based carbon components into carbon materials,realize the collaborative optimization of the morphology,pores and microcrystalline structures of carbon materials,obtain high-performance low-cost sodium storage materials,and reveal the structure activity relationship between the multiscale structure and sodium-ion storage performance.To realize the in-situ conversion of coal pyrolysis gas into solid carbon materials,this paper proposes a strategy of catalytic deposition into carbon materials,which realizes the in-situ conversion of coal pyrolysis gas to carbon materials by coupling the chemical vapor deposition process.The medium/low metamorphic coal with high volatile content used as raw materials.Firstly,the composition and distribution of gas phases during the pyrolysis of coal were studied,and the effect of pyrolysis temperature on the catalytic deposition process was analyzed.Furthermore,the sodium-ion storage performance of the deposited carbon was evaluated.The results show that the deposition effect of bituminous coal is the best because the pyrolysis gas is rich in organic small molecular species.The formation of tar is inhibited due to the catalytic cracking and deposition of volatile molecules on the surface of templates.Carbon products inherits the structural features of templates and exhibits a rod-like hierarchical pore structure,which shows high capacity of 280 mAh g^-1for sodium-ion storage at the current density of 0.1 A g^-1.Aiming at the thermal conversion of coal tar to carbon materials,this paper adopts template-assisted liquid phase carbonization process to obtain carbon nano-plate/sheet materials with tunable pore structures,which realizes the collaborative improvement of capacity and rate performance of sodium-ion storage.Carbon materials with different morphologies were obtained by using two different templates.The effects of templates on the morphology and pore structure of carbon products were studied,and the effects on sodium-ion storage were analyzed.The results show that the introduction of templates directly determines the morphology of carbon products.Three-dimensional stacking structures of carbon materials provide space and surface for sodium-ion transport and storage.Porous carbon nano-plates/sheets exhibit high capacity and rete performance of sodium-ion.The sodium-ion capacitor based on the mesoporous carbon nanoplate anode and the hierarchical porous carbon nanoplate cathode shows a good kinetic matching,high energy,and power densities(122.4 Wh kg^-1 and 17.5 kW kg^-1),and long cycle life(71%capacity retention after7000 cycles at 1 A g^-1).Aiming at the problem that coal tar is prone to form long-range ordered carbon structures during liquid-phase carbonization,a pore-confined carbonization strategy is proposed to suppress the formation of a liquid-phase environment and the orderly development of aromatic structure.First,the effects of the mass ratio of coal tar to porous carbon and the carbonization temperature on the microcrystalline and pore structure of carbon products were investigated.The relationship between the nanostructures and the sodium-ion storage performance of carbon products was further analyzed,the enhancement mechanism of sodium-ion storage was explored,and the structure-activity relationship was established.Finally,the electrochemical performances were evaluated under high mass loading and and sodium-ion full battery systems.The results show that pore-confined successfully inhibits the formation of long-range ordered structures during the carbonization of coal tar,and the conversion rate of coal tar to carbon materials is increased by as much as two times.The coal tar-based hard carbon shows a high sodium storage capacity of 361.7m Ah g^-1 with an initial Coulombic efficiency of 81.6%due to the large carbon interlayer spacing,suitable microcrystalline size,high content of pseudo-graphite region,and abundant closed pores.When the active material loading is 11.08 mg cm^-2,it can provide a high areal capacity of 3.83 mAh cm^-2.The assembled sodium-ion battery exhibits high average operating voltage（3.19 V）and energy density(254.3Wh kg^-1).Aiming at the problem that ordered graphite microcrystals of coal are easy to form in the process of solid phase thermal conversion,a strategy of chemical activation coupled with secondary carbonization is proposed to suppress the development of graphite-like microcrystal.Firstly,the evolution of microcrystalline and pore structure of carbon products under two thermal conversion paths was studied,and the effects of activator dosage and secondary carbonization temperature on the structure of carbon products were analyzed.The sodium-ion storage performance of the carbon products under two thermal conversion pathways were further investigated,revealing the source of the improved sodium-ion capacity.Finally,the electrochemical performance of the materials in sodium-ion batteries and sodium-ion capacitors were evaluated.The results show that the strategy of chemical activation coupled with secondary carbonization successfully inhibits the development of graphitized structure in the carbonization process of high-rank coal.Chemical activation treatment can etch the microcrystalline structure and introduce large open pore structures,which further can convert to closed pore structures during the subsequent carbonization process.Benefiting from the abundant closed pores,the carbon products show a high sodium-ion storage capacity of 308.4 mAh g^-1 and the initial Coulombic efficiency is 82.3%.The assembled sodium-ion battery and capacitor exhibit excellent electrochemical performance.