Structural Regulation And Electrochemical Performance Of Coal-Based Carbon Anode Materials For Lithium/Sodium Storage

Coal is a high-carbon solid combustible sedimentary rock and is one of the important fossil energy resources in China.Coal,a low-cost carbon precursor with high residual carbon and high porosity,can be used to prepare a variety of advanced carbon-based materials,thereby significantly reducing costs.Meanwhile,the material transformation of coal is also conducive to expanding its clean and efficient utilization.Lithium-ion batteries（LIBs）and Sodium-ion batteries（SIBs）have shown wide application prospects in electric vehicles and smart grids because of their high energy density,long cycle life,and low self-discharge rate.Anodes have a direct impact on the electrochemical performance of the batteries,such as energy density,power density,cycle life,and safety.Therefore,the design and optimization of anode materials are crucial for achieving high electrochemical performance and low cost of batteries.The carbonization mechanism of coal-based carbon anode materials and the correlation between structure of carbon materials and the electrochemical performance of batteries are crucial for the coal refining process to produce high-quality carbon materials.In this thesis,coal-based disordered carbon materials for the anode of LIBs/SIBs were prepared by one-step carbonization.The evolution of the microstructure of coal during the carbonization process was systematically investigated,and a two-stage carbonization mechanism for coal was proposed.Based on the study of electrochemical performance in lithium/sodium-ion batteries,the correlation between structural and electrochemical performance of coal-based disordered carbon materials is constructed.This study provides important clues for a deeper understanding of the fine preparation of coal-based carbon materials.The main research contents and conclusions are as follows:（1）The chemical structure and microstructure evolution process of bituminous coal during carbonization were studied,and a two-stage carbonization mechanism of coal was proposed.In stage I（<1000℃）,the structure of macromolecule fractures,with the original carbon skeleton destroyed,the migration of the carbon layer,and the release of small molecules.This manifests as an expansion of interlayer spacing and the generation of porous structures.Unstable chemical bonds are recoupled,thereby increasing the defect concentration.In stage II（1000-1600℃）,interlayer spacing and defect concentration both decreases owing to the orientation arrangement of the carbon layers and the repair of crystallites,respectively.By controlling the temperature,the structural properties of coal-based carbon materials can be effectively regulated.Coal-based disordered carbon materials prepared by carbonization at around 1000℃ have abundant pseudo-graphitic domains,high interlayer spacing,and defect concentration,while the prepared materials have abundant nano-microporous structures.（2）When using materials prepared by carbonization in LIBs systems,CC-1000exhibits the lowest internal impedance（38.17Ω）,which is attributed to the large interlayer spacing(d₀₀₂=3.82（?）)facilitating ion diffusion and storage into the carbon micro-crystals;The high defect concentration(A_D1/A_G=1.90)can serve as active sites for ion insertion,and its reversible capacity is as high as 384.3 m Ah g^-1 at 0.1 C and can maintain about 170 m Ah g^-1 even at 5 C.The good rate performance is attributed to its abundant pseudo-graphite domains;The nanopore structure provides sufficient space and diffusion channels for ion storage,thereby enhancing the kinetic properties of ion diffusion.CC-1200 has a large interlayer spacing(d₀₀₂=3.78（?）),high defect concentration(A_D1/A_G=1.73),abundant nanopores,and a low heteroatom content.In the SIBs,it exhibits the lowest internal impedance（88.73Ω）,a reversible capacity of 270.1 m Ah g^-1 at 0.1 C,and an initial Coulombic efficiency of 86.8%.Therefore,larger interlayer spacing,higher defect concentration,and rich nano-porous structural can promote rapid charge transfer,enhance ion diffusion kinetics,and reduce internal impedance of ion and charge transfer processes in the material,demonstrating high insertion and removal reversibility.In addition,heteroatoms can serve as active insertion sites in lithium-ion batteries,but have little effect in sodium-ion batteries.In summary,this thesis has prepared lithium/sodium-ion battery anode materials using cheap coal as carbon precursors,systematically investigated the carbonization mechanism of coal-based carbon materials,and constructed the structure-performance relationship.This work lays a certain research foundation for the subsequent refinement of coal-based carbon materials preparation.