| Due to the tremendous spike in the demand for portable electronics,electric vehicles and stationary electric grid stations,high-energy and high-power lithium and sodium ion batteries are gaining more limelight than ever.Carbon materials,owing to the high physicochemical stability,good electrochemical performance,as well as low cost,have been widely used in the Li+and Na+storage.However,these materials which are unable to cope with the requirements of commercial demand as sluggish kinetics entails with charge storage have limited their capacity by offering large overpotential and polarization at high current densities.It is vital important to explore novel carbon materials with high-rate performance.Heteroatom doping results in higher electrochemical activity and rapid and robust ion adsorption behavior of carbon.Nitrogen possesses some unique properties（e.g.,similar atom radius with carbon,multiplex configuration）thus has attracted tremendous attention for energy storage.Up to now,the main challenge for N-doped carbon-based materials is how to achieve a high N-doping level（above 10 at.%）.There are usually two ways to prepare nitrogen-doped carbon materials:in situ and post-treatment.However,there are shortcomings existing,such as limited nitrogen content of precursor,complex experimental operation,high cost,uncontrollable product morphology and safety risks.Therefore,finding precursors with high nitrogen content,or developing a simple preparation method,is the challenge of current work.Therefore,in this paper,melamine,a small organic molecule with ultra-high nitrogen content（66.7 at.%）and low cost,is selected as the nitrogen-rich precursor.By forming coordination compounds with metal ions and polymerizing with formaldehyde solution,highly nitrogen-doped carbon materials with different morphologies and nitrogen content were synthesized,then,a series of characterization were used to reveal the relationship between morphology,structure and electrochemical properties,which will provide a new idea and approach for the design,preparation and application of nitrogen-doped carbon materials.The details are as follows:（1）2D highly nitrogen-doped carbon nanosheets with a maximum nitrogen content of 29.49 at.%were prepared using Cu-Melamine coordination framework as precursor.The nitrogen content and configuration were regulated by changing different carbonization temperatures.As an anode material for lithium-ion batteries,the optimized NDCS shows high-rate performance(299.6m Ah g-1 at 10 A g-1)and long cycle life(375.5 m Ah g-1 after 1000 cycles at 5 A g-1).As an anode material for sodium-ion batteries,it also delivers high specific capacity of 66.7 m Ah g-1 at 10 A g-1 and 135.5 m Ah g-1 capacity after 1000cycles at 1 A g-1,indicating the great application value.（2）Three N-doped carbon（NDC）materials form Cu-,Zn-and Fe-Melamine coordination frameworks were comparatively investigated in terms of their morphology,structure,nitrogen content and configuration,and lithium/sodium storage performance.The results show that Cu-NDC with nanosheet morphology possesses higher nitrogen content,better rate and cycle performance.Zn-NDC with bulk shape achieves more defect structure and higher reversible capacity,while Fe-NDC with the bamboo shape exhibits rapid ion diffusion.（3）Melamine resin spheres were prepared by precipitation polymerization of melamine and formaldehyde solution.Ammonium chloride was used as curing agent to achieve a short time curing at low temperature（150 oC,6 h）.Nitrogen doped carbon microspheres with smooth surface,uniform particle size distribution,good spherical shape and monodisperse were successfully prepared at different heat treatment temperatures.The particle size is 2.56-1.6μm and nitrogen content is 23.86 at.%-0.22 at.%.As an anode material for sodium ion battery,it shows excellent cycle stability.The capacity retention rate can reach99%after 500 cycles at 0.1 A g-1,and 74%after 1500 cycles at 0.5 A g-1,respectively. |
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