| The development and progress of human society have accelerated energy consumption,and the existing energy storage technologies can no longer meet the growing demands of electronics,electric vehicles,energy storage power stations and other fields.The development of green and efficient energy storage technologies is crucial to the sustainable development of economy and society.The energy density of traditional lithium ion battery is approaching the theoretical limit.Due to the cheap and abundant properties of sodium,sodium ion battery becomes the most promising alternative energy storage technique in the postlithium ion battery era.Carbon-based materials are considered as the best anode materials for sodium ion batteries due to their low cost,stable chemical properties and abundant renewable resources.From the perspective of graphitization,carbon materials are mainly divided into graphite,soft carbon and hard carbon.Because the size of sodium ion does not match the limited spacings between the graphite layers,soft carbon and hard carbon are more suitable for sodium ion batteries.However,the capacity and cycling stability of pure soft carbon and hard carbon in the application of sodium ion anode materials cannot meet the requirements of practical applications.Moreover,compared with hard carbon,the storage mechanism of soft carbon materials has not been well reported.In this work,the classical soft carbon materials are modified by oxygen functionalization with liquid phase oxidation method.Subsequently,the oxygen functional groups were regulated by nitrogen atoms to evaluate and investigate the sodium storage mechanism of soft carbon.As comparision,hard carbon materials were treated and researched using the same method.The main research contents and conclusions are as follows:(1)Glucose and 3,4,9,10-perylene tetraacetic dianhydride(PTCDA)was used as the precursors of hard and soft carbon,respectively.The oxygencontaining functional groups were treated with concentrated sulfuric acid and potassium permanganate,and oxygen functional groups and defects were introduced to prepare the oxygen-rich functional group modified products,which were used in sodium ion batteries.The results showed that the defects and oxygen functional groups(especially the anhydride and quinone functional groups)of the two carbon materials were significantly increased after oxidation treatment,which enriched the active sites of sodium storage.(2)The kinetic analysis showed that the oxidized soft carbon has a higher diffusion coefficient of sodium ions,which makes it have a reversible capacity of322mAhg-1 at a current density of 50mAg-1,which is higher than that of the oxidized hard carbon(257mAhg-1).(3)Nitrogen-regulated oxygen functionalized soft carbon(N-SC)and hard carbon(N-HC)materials were prepared by one-step pyrolysis with urea as the doping source,which were applied to the sodium ion half cell.The results show that the decomposition of urea destroys the original structure of carbon materials,and nitrogen atoms are successfully introduced and abundant defects are exposed during the pyrolysis process.Through the analysis of the contents of different nitrogen species,it was found that the content of pyridine nitrogen on the surface of N-HC was higher than that of N-SC,but the content of C=O functional group with fast sodium storage activity was much lower than N-SC,it was inferred that the pyridine nitrogen could inhibit the formation of C=O functional group.(4)The capacity of N-SC decreases slowly with the increase of current density,and has a reversible capacity of 148mAhg-1 at 2Ag-1,which is 1.5 times that of N-HC.Abundant of C=O functional groups make the surface adsorption contribution capacitance reach more than 90%,which is conducive to the carbon materials to maintain excellent electrochemical performance during fast charge and discharge,confirming the the mechanism of sodium storage mainly by adsorption. |
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