Microstructure And Lithium Storage Properties Of Graphite Modified By High Current Pulsed Electron Beam

Lithium ion batteries have been extensively used in many energy storage fields due to their green,high energy efficiency,long life,portability and light weight.The anode material of lithium ion battery is one of its principal components,and its structure and properties play a paramount function in the performance of the battery.Graphite,as the most successfully commercialized anode material,has excellent conductivity,a satisfied layered structure adapted to the intercalation and de-intercalation of lithium,low cost and environmentally friendly.However,the theoretical specific capacity of graphite anode material is only 372m Ah/g and the regular laminated structure hinders its rating performance.This remains a challenge in terms of further enhancing the lithium storage capacity and rate performance of graphite anode material.Up to now,there are primarily two methods referring to the microstructure modification of graphite anode material.One is to expand the graphite layer spacing and reduce the dimension of graphite,and the other is to construct defects on the graphite structure.In this study,graphite is modified by high current pulsed electron beam direct irradiation for the first time.Through microsecond pulse energy deposition,a non-uniform dynamic temperature field is induced on the surface of the material,resulting in severe thermal stress effect to enlarge the layer spacing and increase the structural defects of graphite,so as to effectively improve the lithium storage capacity and rate performance of graphite anode.In accordance with the development requirements of the high current pulsed electron beam technology,a domestic carbon fiber cathode for the emission of high current pulsed electron beam was developed,the structure of penning discharge plasma anode was optimized to achieve a uniform and stable plasma channel.On this foundation,artificial graphite and natural graphite were irradiated by HOPE-I high current pulsed electron beam.The effects of high current pulsed electron beam irradiation on the microstructure of graphite were investigated by means of laser confocal microscope,scanning electron microscope,transmission electron microscope,X-ray diffraction and Raman spectroscopy.The microstructure variation mechanism of graphite was explored,and the electrochemical lithium storage performance of modified graphite was analyzed,so as to provide necessary theoretical basis and process technology for improving the lithium storage performance of graphite anode material.The main research contents and results are as follows:High current pulsed electron beam carbon fiber cathode was prepared,in which the carbon fiber was a blend of short carbon fiber and carbon fiber powder.A stable support structure can be realized through the intersection and overlapping of short carbon fibers.The addition of carbon fiber powder can boost the emission tips of the cathode.The experiments show that a uniform beam spot can be acquired by the cathode with the addition of 1.5 vol.%carbon fiber powder,the size of the beam spot was consistent with the cathode emission surface,and the working performance of the cathode is stable.Aiming at the problem of unstable operation and uneven plasma caused by the arc spot phenomenon in the working process of penning discharge plasma anode,the structure of penning discharge plasma anode was optimized by introducing grid electrode.The results show that a stable and uniform plasma channel can be formed under the conditions:grid electrode with grid wire width 0.3mm,working pressure 7×10^-2 Pa,magnetic field 0.18 T,anode voltage 4.8 k V and adjustable current limiting resistance 20Ω.Artificial graphite（particle size 14μm,specific surface area 2.1 m²/g）was irradiated by high current pulsed electron beam with different energy density.The results demonstrate that graphite particles were transformed into graphite sheets under electron beam irradiation with an energy density of 2 J/cm².When the energy density increased to 2.5 J/cm²,the graphite particles were transformed into defective graphene nanosheets（specific surface area 125.8m²/g）.The graphite surface was ablated if the electron beam energy was raised to 3 J/cm².Natural graphite with particle sizes（12μm、5μm、1μm）were irradiated by high current pulsed electron beam with energy density of 2.5 J/cm².It is observed that the degree of change in the microstructure of graphite decreased with the redution of particle size,where the graphite microstructure with particle size 12μm changed most significantly,that is,it was transformed into defective graphene nano-sheets.Combined with the simulation results of temperature field and stress field of electron beam interaction with graphite,the temperature field on the surface of graphite changes abruptly under the action of the electron beam of 2.5J/cm²,the heating rate reached～10⁸℃/s,the maximum temperature was located at the sub-surface of graphite,up to～3100℃,the induced stress amplitude was～110 MPa,and expansion deformation of the graphite always occurred along the c-axis.The high current pulsed electron beam has unique energy characteristics when interacting with materials,namely,the deposited energy reaches the maximum at 1/3 of the maximum range of the electron beam.In the process of irradiating graphite by electron beam,it will induce violent expansion of graphite particles,leading to the expansion of the graphite layer spacing instantly,as a result,causing exfoliation of graphite by overcoming the weak interlayer van der Waals force.At the same time,the high temperature generated by irradiation will induce defects.The types of defects are mainly consisted of stone-wales and divacancies,and then form the defective graphene nanosheets structure.The degree change in the graphite microstructure was determined by thermal stress distribution and amplitude of electron beam interaction with graphite determine.The reversible specific capacity of artificial graphite electrode was 360 m Ah/g at a current density of 0.2 C.The lithium storage capacity of modified graphite reached 450.5 m Ah/g after100 cycles,with almost no attenuation after 500 cycles.When discharged at a high rate of 5 C,the reversible lithium storage capacity of the modified artificial graphite was 2.3 times greater than that of the artificial graphite electrode,resulting in exceptional lithium storage and rate performance.The lithium storage capacity of the natural graphite（12μm）was 345.5 m Ah/g under a current density of 0.2 C,and the lithium storage capacity decayed to 140 m Ah/g after 500cycles.The lithium storage capacity of modified natural graphite was 420.4 m Ah/g,and the capacity retention rate was 94.4%after 500 cycles.The capacity attenuation of natural graphite electrode was caused by the poor stability of solid electrolyte interface（SEI）due to the co embedding of solvent molecules.Comparatively,the SEI film of the modified natural graphite electrode was relatively stable,owing to the existence of graphene nanosheets in its structure,the increased active sites for lithium storage and the enlarged interlayer spacing,which were beneficial to the formation of more unobstructed ion diffusion channels and paths,improving the cycling performance of electrodes.Based on the research work of this paper,the graphite was converted into defective graphene nanostructure by high current pulsed electron beam irradiation.When used as anode material for lithium-ion battery,large specific surface area and rich defective structure increase lithium storage active sites.The expanded layer spacing was favorable to alleviate the volume change in the process of charge and discharge and promote the transport dynamics of lithium ions and electrons.The lithium storage performance of graphite anode material was signficantly improved.