Defect Control And Energy Storage Behavior Of Two-dimensional Carbon Nanomaterials Based On Ball-milling Strategy

Carbon materials have been widely used in various fields due to the merits of wide resources,low cost,and stable properties.Electrochemical energy storage is one of the most commonly explored fields.Typically,active carbons and graphene have become the most representative capacitive energy storage materials at present.However,the energy storage efficiency of carbon surface is very low,resulting in limited capacitance.The active sites such as functional groups and heteroatoms usually show pseudocapacitive energy storage behavior with relative sluggish electrochemical kinetics,which may decrease the power performance of the devices.Therefore,developing new energy storage sites and inducing the synergistic effect between different active sites have become the key point for preparing high performance energy storage carbon materials.Besides,miniaturization,lightweight,and high energy have become the development trend of supercapacitors.Therefore,exploring dense carbon electrode materials to improve the volumetric performance become research focus of future supercapacitors.But the common densification methods such as mechanical compression and vacuum filtration inevitably reduce the energy storage sites and ion transport channels,further decreasing the capacitance and rate performance of carbons.Thus,it is crucial to develop simple and effective densification methods to improve the volumetric performance without the sacrifice of capacitance and rate capability.To solve the above-mentioned problems,we propose ball-milling strategy to control the defect structure of two-dimensional carbon nanomaterials.The ball-milling treatment has significant influence on the morphology,structure,and chemical compositions of carbon materials.Moreover,ball-milling strategy also holds advantages of simple operation,low equipment requirements,and environmental protection.Therefore,it is widely used in both basic research and industrial production to achieve the purposes of structural transformation,and performance control of carbon materials.This work studied the evlution of the defect structure of the two-dimensional carbon nanomaterials during ballmilling.We prepared a series of dense carbons with different defect degree by changing the ball-milling time,and further explore the influence of the defect content on the electrochemical properties of the carbon materials.The work contains the following results:(1)Graphene prepared by rapid thermal expansion was used as the research object and ball-milled under the protection of argon to obtain a series of defective graphene blocks(DGB-x,where x stands for the ball-milling time).During ball-milling,the graphene obviously stack and agglomerate,and transform from two-dimensional layers to compact blocks.The thickness is increased and the size is decreased with ball-milling.The composition of materials,especially the oxygen content and distribution,has no change after ball-milling,while the defect density,sp3 structure,and free electron of DGB-x gradually increase with ball-milling due to the destruction of carbon skeleton.In addition,both the surface area and pore volume of DGB-x are greatly reduced,and the density is improved with the prolong of ball-milling time.(2)When used as capacitive energy storage material,DGB-8 exhibits higher capacitance and good rate performance compared with graphene.The DGB-8 shows gravimetric capacitance of 235 F g-1 and volumetric capacitance of 215 F cm-3 at 1 A g-1,41%capacitance retention at 100 A g-1,and superb cycling stability.According to the electrochemical analysis,we propose that the intrinsic defects in DGB-x show high capacitive activity.The capacitance provided by intrinsic defects is approximately linear with the defect density.One unit defect density can provide a remarkably high capacitance of 114 F g-1 with quasi double-layer energy storage behavior.(3)When used as the sodium storage material,DGB-3 exhibits the reversible gravimetric and volumetric capacities of 507 mAh g-1 and 397 mAh cm-3 at 50 mA g-1,respectively,high initial coulomb efficiency of 84.7%,good rate performance,and excellent cycle stability.The electrochemical tests and simulation results show that the intrinsic defects in DGB-x also have high sodium storage activity.The intrinsic defects can bind more sodium ions compared with ideal graphene structure,and they also show the quasi doublelayer sodium storage behaviors.In addition,with the same coating thickness(volume),DGB-3 can provide similar volumetric capacities but nearly half weight compared with graphite,which is of great significance for the device miniaturization and lightweight.(4)We adjusted the oxygen content,oxygen distribution,and sp2 network of graphene oxide under different heat treatment temperature to obtain reduced graphene oxide(RGO)with different oxidation degree.Then the intrinsic defects are introduced into RGO by ball-milling to prepare dense reduced graphene oxide(DRGO-x,where x stands for the heat treatment temperature).The DRGO-300 exhibits gravimetric capacitance of 303 F g-1.And it also shows high volumetric capacitance of 278 F cm-3 at 1 A g-1,as well as good rate capability.In addition,the assembled asymmetric device shows better electrochemical performance than the most reported aqueous capacitors.We propose that there is synergistic effect between functional groups and intrinsic defects,and the oxygen groups can promote the full utilization of the energy storage activity of intrinsic defects.(5)We prepared porous carbon nanosheets with high nitrogen content using copper nitrate and melamine as raw materials.The layered structure of carbon nitride nanosheets can be formed taking advantage of the coordination effect and intermolecur forces between copper ions and amino-groups of melamine.Then the intrinsic defects are introduced and transformation of nitrogen species is induced through ball-milling to obtain dense carbon materials with high capacitive and sodium storage performance.We propose a simple "bottom-up" strategy is proposed for the preparation of nitrogen-doped carbon nanosheets,and broaden the application of melamine in the field of energy storage.