| Hard carbon has been considered as one of the most promising anode materials for sodium ion batteries because of large interlayer distance and radius of nanopores.However,its disordered structure makes low reversible capacity,poor rate performance and cycling performance.The microstructure of hard carbon is mainly influenced by the precursors,which largely determines the performance of anode.Therefore,the selection and modification of appropriate precursors is a feasible way to enhance the electrochemical performance of anode materials.In this paper,hard carbon anode materials were prepared by changing the carbonization conditions and nitrogen doping using modified epoxy resin as the precursor,and the intrinsic connection between the structure and performance was investigated in depth.（1）The effect of carbonization conditions on the structure and performance of hard carbon.The carbonization conditions include pre-carbonization step,pre-carbonization temperature and high temperature carbonization temperature.First,the pre-carbonization process increases the specific surface area and porosity of hard carbon,reduces impurities and defects,and enhances the electrochemical properties;the reversible capacity of SHC at a current density of 50 m A g-1 is 232.2 m Ah g-1,while PHC reaches 349.8 m Ah g-1,and the capacity retention rate after 300 weeks of cycling at a current density of 500 m A g-1 reaches 87.4%,which is significantly higher than the 77.8%of SHC.Secondly,rising the pre-carbonization temperature also increased the specific surface area and graphitization of hard carbon;the performance of PHC-500 was optimized when the pre-carbonization temperature was 500 oC,with a reversible capacity of 349.8 m Ah g-1 at a current density of 50 m A g-1 and a capacity retention rate of 87.4%after 300 weeks at 500 m A g-1.Finally,adjusting the high-temperature carbonization temperature can significantly change the crystal structure and electrochemical performance of hard carbon.As the temperature increases,the interlayer distance,defects,oxygen content,specific surface area and pore volume of hard carbon continue to decrease,while the radius of nanopores and graphitization continue to increase.When the temperature reaches 1800 oC,EPNHC-1800 has the optimal electrochemical performance with a reversible capacity of 480.3 m Ah g-1 at a current density of 50 m A g-1（74.8%of the plateau capacity）and initial coulombic efficiency of 84.6%;the reversible capacity after 1000 weeks of cycling at 500 m A g-1 is stable at 389.2 m Ah g-1,with a capacity retention rate of 92%.We suggest that the sodium storage mechanism of hard carbon is"adsorption-insertion-pore filling",the slope capacity at high potential comes from the adsorption of sodium ions on the surface of carbon layers,while the plateau capacity at low potential is provided by the combination of sodium ion embedding in the interlayer and filling in the nanopores.（2）The effect of nitrogen doping by PECVD on the structure and performance of hard carbon.The use of PECVD technology to dope nitrogen introduces additional nitrogen-containing active sites,which facilitates the adsorption of sodium ions on the surface of the carbon layers,and the formation of C-N bonds enhances the structural stability of hard carbon,improves the reversible capacity and cycling stability of NHC.NHC delivers reversible capacity of 397.7 m Ah g-1 at a current density of 50 m A g-1(improved 47.9 m Ah g-1);the capacity retention rate achieves 91.9%after 300 weeks of cycling at a current density of 500 m A g-1.Changing the nitrogen doping time can achieve the purpose of regulating the amount of nitrogen doping,and the defect content increases significantly with the increase of time,which improves the adsorption storage of sodium ions.The electrochemical performance of NHC@30 was optimal when the nitrogen doping time was 30 min,with a reversible capacity of 397.7 m Ah g-1 at a current density of 50 m A g-1 and a capacity retention rate of 91.9%after 300 weeks of cycling at 500 m A g-1.However,when the time reached 50 min,the electrochemical properties of hard carbon showed a decreasing trend due to excessive nitrogen doping and the formation of inactive sites as some nitrogen atoms did not combine effectively with the carbon layers. |
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