High N-doped Carbon Materials Derived From Graphitic Carbon Nitride And Their Li/Na Ions Storage Performance

Under the background that traditional fossil energy sources are gradually being replaced by new energy sources,energy storage devices represented by lithium ions batteries are widely applied in various fields due to their advantages of higher energy density and cycle stability and so on.As the critical component of alkali metal ion batteries,the anode has a decisive influence on the capacity,energy density,and cycle performance of batteries.At present,anode materials of commercial lithium ions battery is mainly carbon materials,and the typical graphite theoretical ratio The capacity is only372 m Ah/g.As the most modified carbon material,nitrogen-doped carbon presents good specific capacity,cycle stability,and cost performance,but there are still difficulties in increasing the content of nitrogen and adjusting the configuration of nitrogen defects.In order to obtain N-doped carbon with high N content and controllable N-doping configuration,this paper proposes to use N riched（57.1 at.%）graphitic carbon nitride（g-C₃N₄）as the precursor,employing the reductive denitrification strategy,by using different types of reducing agents synthesized N-doped carbons with tunable N-doping content and N-doping configurations in a certain level under different synthesis conditions.By characterizing and testing the properties and performance of these synthesized N-doped carbons,analyzing the structure and morphology and composition,as well as the relationship between the properties and the electrochemical performance,clarifying the Li⁺/Na⁺storage mechanism of synthesized N-doped carbons.This paper raises a novel route to synthesize N-doped carbons,and provides reference and guide for the research and application of N-doped carbon materials in the field of electrochemical energy storage.Achieve the following research results:（1）Using aluminum powder（Al）as the reducing agent,employing the metal thermal reduction,g-C₃N₄ as the precursor,the nitrogen-doped carbon material was prepared,which proved the fesible of reductive denitrification for N-doped carbons fabrication.The relationship between the morphology,structure,N-doping content,N-doping configuration of fabricated N-doped carbons and the fabricating tempetaure,and their electrochemical performance were studied detailedly.In the fabricating temperature of600-800℃,as the heating treatment temperature increases,the structure of product changes from g-C₃N₄ structure to amorphous carbon structure under the metal thermal reduction denitrification.With the increase of the heating treatment temperature,the content of nitrogen decreases gradually,and the proportion of graphitic-N increases gradually,besides,the electrochemical performance of fabricated N-doped carbons increase first and then decrease.The N-doped carbon fabricated at 700℃delivered an N content of 23.5 at.%,as well as presented a Li⁺storage capacity of 977 m Ah/g after 100cycles under a current density of 0.05 A/g.（2）Employing template catalysis,annealing the precursor of mixture of cobalt acetate pyrolyzing into Co as template and melamine polymerizing into g-C₃N₄ at 550℃to fabricated free-standing N-doped CNTs as anode for flexible lithium ions battery.The successful fabrication of free-standing N-doped CNTs demonstrated the morphology and shape of carbons derived from g-C₃N₄ could be designed by employing different templates.The physical and chemical properties,and electrochemical performance of the fabricated sample as free-standing anode were studied in detail,exhibiting the fabricated N-doped CNTs have good free-standing performance,and presented 4.98 at.%N-doping content and areal capacity of 2.14 m Ah/cm² after 100 cycles under a current density of 1m A/cm².（3）Using g-C₃N₄ as the precursor,employing polypyrrole（PPy）assist to synthesized N-doped carbons with N-doping content of higher than 24 at.%were obtained,indicating that N-doped carbons with ultra high N content could be reached by using g-C₃N₄ as precursor.N-doped carbons with ultra high N content were fabricated by annealing the mixture of g-C₃N₄ and a little of PPy,besides they exhibited higher specific surface area and porosity than N-doped carbons derived from thermal metal reduction.During the annealing,g-C₃N₄ was reduced by PPy and transformed into high N-doped carbons.When the dosage of g-C₃N₄ and PPy is 2:0.2（wt.）,the fabricated N-doped carbon nanosheets contain 27.4 at.%N heteroatoms,and exhibited an stable Li⁺storage specific capacity of 1639 m Ah/g after 200 cycles under 0.05 A/g,as well as presented outstanding rate performance and Li⁺diffusion rate.（4）Using g-C₃N₄ as precursor,polyvinylpyrrolidone（PVP）assist to synthesized high N-doped carbons with N-doping content up to 33.7 at.%.Thermogravimetric analysis confirmed that g-C₃N₄ transformed into N-doped carbons under the assistance of PVP.The N-doped carbon nanosheets fabricated at 750℃have an N content of 31.4 at.%and a stable specific capacity of～1236 m Ah/g under 0.05 A/g for lithium ions battery.The N-doped carbon nanosheets fabricated at 700℃have an N content of 33.7 at.%,and a stable specific capacity of～300 m Ah/g under 0.05 A/g for sodium ions battery.In addition,the analysis of the diffusive capacity and the capacitive capacity shows that the prepared N-doped carbon nanosheets have obvious pseudocapacitive performance,so they exhibited excellent Li⁺storage specific capacity of 1163 m Ah/g and Na⁺storage specific capacity of 140 m Ah/g after 1000 cycles under a big current density of 1 A/g.（5）Sodium borohydride（NaBH₄）was employed to reductively denitrify g-C₃N₄ into N-doped carbons,and the in-situ presodiation was done when g-C₃N₄ is transforming into N-doped carbons,so the initial coulombic efficiency of fabricated N-doped carbons has been improved.The g-C₃N₄ and NaBH₄ were mixed well and heated to 450-650°C for thermal treatment,and then the by-products were removed to obtain N,B co-doped carbons.In the process,Na⁺preferentially react with high adsorption energy sites such as graphitic-N（causing irreversible capacity）,there by reducing the irreversible capacity,improving the initial coulombic efficiency of carbon as anode materials for sodium ions batteries.The N-doped carbon fabricated at 650℃has an N content of 27.8 at.%and graphitic-N content of 1.1 at.%,and exhibited an improved initial coulombic efficiency of 96.1%,and Na⁺storage specific capacity of 308 m Ah/g after 500 cycles under a current density of 0.05 A/g.（6）N-doped carbon dirived from reductive denitrification of g-C₃N₄ was carried out with lithium metal powder（Li）,and the in-situ prelithiation was done when g-C₃N₄ is transforming into N-doped carbons,improving the initial coulombic efficiency of fabricated N-doped carbons.Concretly,g-C₃N₄ and Li were mixed well and then annealing at different temperatures to obtain N-doped carbons with different N content,N-doping configurations,and prelithiation level.Due to the prelithiation,the initial coulombic efficiency of fabricated N-doped carbon was improved up to 95.6%.When the ratio of g-C₃N₄ to Lsi is 4:1（wt.）,the fabricated N-doped carbon shows an improved initial coulombic efficiency of 93.7%,and Li⁺storage specific capacity of 1499.9 m Ah/g after 100 cycles under 0.05 A/g.