Structural Design And Electrochemical Performance Of Carbon Materials By Ball Milling Strategy

Carbon materials are widely used in energy storage due to their abundant sources,stable physicochemical properties,as well as low cost.In order to meet the increasing performance requirements of energy storage devices,the structural design of carbon materials to improve energy storage capacity has become a research focus,and the regulation and optimization of pore,defect,and crystal structure are usually emphasized.Recently,mechanical ball milling has been preliminarily applied in the preparation and modification of carbon materials.It is widely used in the laboratory and industrial production thanks to low equipment requirements,room temperature and solvent-free working environment,high efficiency and eco-friendliness.An in-depth understanding of the structure evolution of carbon materials during ball milling and its influence on the energy storage performance will be helpful to open up a new way for the controllable preparation of high-performance carbon electrode materials.Based on this,in this work,a activated carbon with typical porous and disordered structure was selected as a model,and the influence of ball milling on the structure and function and the underlying mechanism were explored in detail.Thereafter,the influence of ball milling on the capacitive energy storage performance of activated carbon was studied.The structure of ball milled activated carbon after high temperature treatment was also explored and analyzed,which provided ideas for the preparation of high-performance electrode materials in lithium and sodium ion batteries.As a comparison,simple and adjustable graphene carbon models were used to reveal the effect of physicochemical properties of carbon materials on their structural transformation during ball milling.Subsequently,defective and dense carbon materials were obtained by ball milling highly crystalline graphene nanosheets,which were further used as the models to investigate the energy storage behavior of intrinsic defects in ion capacitors（taking Zn-ion capacitors as an example）.Specific works are as follows:（1）A activated carbon（AC,YP-50F）with porous and disordered structure was chosen as a research object,and BAC-x was obtained by ball milling in air atmosphere for 1-48 hours（x represents ball milling time）.Ball milling is a common method to shatter crystals or introduce defects in carbon materials,but herein,we found that the porous and disordered carbon lattice of activated carbon are gradually converted to order structure during ball milling,leading to repaired defects and increased crystallinity.Simultaneously,a large number of oxygen components are introduced into the bulk phase.Along with the ball milling time from 1 to 48 hours,the particles are continuously broken,reassembled and cold-welded.The particle size of BAC-24 increases by 2 times and the oxygen content increases by 4 times.Correspondingly,the developed pore structure is first exposed and then gradually eliminated.The specific surface area of BAC-24 decreases from 1364 m² g^-1 of AC to 156 m² g^-1,and the compaction density increases from 0.573 to 1.423 g cm^-3.The simultaneous crystallinity improvement and oxygen functionalization are hardly to achieve by other methods.The mechanism is that the carbon bonds in carbon skeleton is broken by mechanical energy,and the formed highly active carbon free radicals and defects promote the carbon atom rearrangement to form ordered carbon lattice structure or oxidization to form oxygen-containing functional groups.（2）By investigating the capacitive properties of AC and BAC-x,we found that with the structural transformation of AC during ball milling,the capacitive energy storage behavior gradually changes from a double electric layer on the carbon surface to a pseudocapacitance reaction of oxygen-containing functional groups.Among them,BAC-24 shows the highest capacitance performance with specific capacitances of 184 and 138 F g^-1 at 1 and 100 A g^-1,respectively,and a capacitance retention rate of 75%.And thanks to the high compaction density,its volumetric capacitance is about three times higher than AC’s.In addition,the high crystalline structure and the surface electrolyte wettability enhanced by oxygen-containing functional groups enable fast electron and ion transport rate,leading to a robust electrochemical kinetics of BAC-24.This provides a preparation thought of high-performance carbon materials for compact capacitive storage.（3）AC and BAC-x were used as precursors for heat treatment at 1500to prepare AC-1500 and BAC-x-1500.We found AC-1500 still has well-developed pore structure with high specific surface area of 1152 m² g^-1and micropore volume of 0.49 cm³ g^-1.While these of BAC-x-1500 are significantly decreased to only 154 m² g^-1 and 0.04 cm³ g^-1 for BAC-12-1500,respectively.What’s more,the closed pore volume increases from 0.01 cm³ g^-1of AC-1500to 0.12 cm³ g^-1 of BAC-12-1500.The mechanism would be that the carbon lattice rearrangement of AC induced by ball milling promotes the further development,rearrangement and stacking of the crystal microdomains during high temperature treatment,forming a large number of closed pores.Correspondingly,the electrochemical sodium storage behavior changes from a simple"sloping"of AC-1500 to a"sloping-plateau"of BAC-x-1500 with a low potential filling of closed pores.The plateau capacity of BAC-12-1500 reaches123.8 m Ah g^-1.This provides a way to modulate the structure of carbon materials by ball milling to prepare long-plateau sodium storage materials.Similarly,the carbon lattice rearrangement of AC by ball milling promotes the development of highly crystalline graphite structure after graphitization at2800.GBAC-24 shows the highest graphitization degree,corresponding to the longest lithium storage plateau(183.3 m Ah g^-1).This provides a reference for structure design of non-graphitized carbon materials such as AC by ball milling to prepare high crystalline graphite for lithium storage.（4）Graphene oxide（GO）,expanded graphene nanosheets（EG）,and thermal reduction EG at 1500（EG-1500）with different oxygen content were selected as milling precursors.After ball milling,the most of oxygen in GO lost;EG-1500 with low oxygen content is oxidized,which is similar to AC;Oxygen content of EG is at an equilibrium point,which is basically unchanged during the milling process.In addition,high-crystallized EG possesses introduced intrinsic defects after ball-milled,and dense carbons with different intrinsic defect content（BG-x）were obtained,which is contrary to the result of defect repairs of AC.Therefore,the oxygen content and crystallinity evolutions of carbon materials during ball milling are related to their intrinsic physicochemical properties.EG and BG-x were used as positive electrodes of Zn-ion capacitors to investigate the electrochemical energy storage behavior of intrinsic defects.By electrochemical analysis,we found that the intrinsic defects exhibit capacitive storage ability for Zn²⁺,H⁺and CF₃SO₃^-with robust charge transfer kinetics.One unit defect density provides a capacitance of 90 F g^-1.The simulation results show that the energy storage mechanism is that the ion adsorption energy at the defect is much lower than that on the pure carbon surface.After that,we prepared defective and porous carbon（BSG）by Na Cl assisted ball milling,which exhibits a high compaction density of 0.83 g cm^-1.The abundant pores and microcrystalline structures provide developed ion and electron transport channels,respectively,promoting the efficient utilization of intrinsic defects as active sites.The BSG exhibits high specific capacitance(224 F g^-1/186 F cm^-3at 0.5 A g^-1),high capacitance retention(52.2%at 20 A g^-1),and good cyclic stability.The defect design of high crystalline carbon by ball milling opens up a new way to prepare high performance Zn-ion capacitor cathode materials.