Preparation Of Coal-Based Carbon Anode Materials And Its Electrochemical Performance In Sodium Ion Batteries

Sodium-ion battery（SIB）is a promising candidate for large-scale electric storage devices due to their low cost and earth-abundance resources.As the core component,the electrode materials directly affect the performance of SIB.The main challenge for the commercialization of SIB is the preparation of high-performance and low-cost electrode materials.Carbon materials have become potential anode materials due to the advantages of low price,strong chemical stability,and controllable structure.Coals are regarded as the most promising precursors to prepare hard carbons as anode materials of SIB,owing to the advantages of high carbon content and low cost.In this paper,coal with different grades of metamorphism is used as the precursor.By studying the influence of ash removal methods and preparation conditions on the structure and morphology of carbon materials,we prepared high-capacity carbon anode materials with outstanding initial coulombic efficiency and excellent rate and cycle performance.The specific work includes the following two aspects:Anthracite with low cost and high carbon content is selected as the precursor to prepare highly purified anthracite-based carbon material.It is found that the method for acid ash removal using hydrochloric acid and hydrofluoric acid can effectively remove the ash with a high ash remove rate of 90%.The ash removal strategy improves the degree of graphitization and conductivity of anthracite,which effectively enhances the electrochemical performance of the material.In order to further optimize the sodium storage performance,carbon materials with high purity were carbonized at various temperatures.When carbonized at 1000℃,the anthracite-based carbon exhibits a capacity of 252.2 m A h g^-1at a current density of 0.02 A g^-1and delivers a capacity retention rate of 97.4%after cycling over 500 cycles at a high current rate of 1 A g^-1.Coal with different grades of metamorphism were selected as carbon precursors and a series of coal-based hard carbons were prepared through a deashing treatment and then high temperature carbonization.Moreover,the effects of the degree of metamorphism and carbonization temperature on the microcrystalline structure and superficial heteroatom composition of coal-based hard carbon were investigated,and the corresponding sodium storage behavior was systematically investigated as well.The results show that carbonization temperature has an effect on the layer spacing and graphitization degree of hard carbons for the same type of coal.As the grades of metamorphism increase,the carbon layer spacing,defect sites and heteroatom content gradually decrease,while the crystallite size and the sp² carbon content increases.When carbonized at 1400℃,the bituminous,sub-bituminous and lignite derived carbons exhibit capacities of 317.5 m A h g^-1,326.1 m A h g^-1,and 338.8 m A h g^-1 at a current density of 0.02 A g^-1.respectively.And the corresponding initial coulombic efficiencies were 80%,79.8%and 81.1%,respectively.By comparing the characterization results,we found that lignite derived hard carbon has the largest carbon layer spacing,rich reversible sodium storage defect sites and an appropriate heteroatom content,which provides more sites for the insertion and a adsorption of sodium ions.Lignite-based hard carbon delivers a higher capacity retention rate of 96%after cycling over 500 cycles at a high current rate of 1 A g^-1,showing excellent electrochemical performance and structural stability.