Surface Modification And Electrochemical Properties Of Commercial Graphite Anodes By Plasma Technology

With the wide application of chemical power,lithium-ion batteries have the advantages of high volume energy density and mass energy density,long cycle life,environmental friendliness and high safety performance.But the demand of lithium-ion batteries for development is that higher energy density.Among them,the negative materials have a great influence on energy density.At present,the main commercial graphite negative material is graphite,but its mass specific capacity of372 m Ah g^-1can not meet the needs of production and life.However,due to its preferential price and good cycling performance,it is still the first choice of negative material.Therefore,we need to find a modification method to improve the specific capacity of graphite.Plasma technology is a technology which uses plasma to obtain high temperature heat source,it can produce high energy instantaneously and change the molecular structure of materials.It has been widely applied to the preparation of catalysts,but less used in the electrode materials.In this paper,three commercial graphite anodes,natural graphite,artificial graphite and mesophase carbon microspheres,were modified by plasma technology to observe the effect of plasma on their structure and properties.The aim is to improve the electrochemical performance of commercial graphite anode by plasma technology and to study its reaction mechanism.XRD,SEM,Raman and other characterization methods were used to test the structural changes.The prepared materials were assembled into button batteries to measure their CV,impedance,charge-discharge curves and other electrochemical properties.The reaction mechanism was studied by TEM,XPS,IR and other characterization methods.The results were as follows.（1）The surface of MCMB was modified by plasma technology,the morphology of MCMB was studied under different plasma conditions.The optimum plasma conditions were plasma power 100 W,treatment time 40 min,reaction pressure 50 Pa,oxygen to argon ratio 1:4,and PMCMB-100 was formed.Compared with MCMB,PMCMB-100 surface becomes rough,graphitization degree decreases and defect degree increases.The first discharge specific capacity is 671.4 m Ah g^-1.After 250cycles,the discharge specific capacity is 466.7 m Ah g^-1,and the charge-discharge efficiency is 99.5%.This shows that the system has good cycle performance.In the high current charging and discharging of 0.4,0.6,0.8 and 1 C,the discharge specific capacities are 406.9,367.9,307.3 and 357.7 m Ah g^-1,which are always larger than MCMB.The first effect of lithium supplementation on PMCMB-100 was 96.75%when the time of lithium supplementation was 15 minutes.The first charge specific capacity of the battery is 191.1 m Ah g^-1,the first full charge efficiency is 87.2%,the capacity is 101.3 m Ah g^-1after 100 cycles.（2）The electrochemical properties of MCMB materials improved significantly after plasma treatment,and the mechanism of reaction compatibilization was discussed.The spacing of graphite layer of PMCMB-100 is 0.347 nm,which is larger than that of graphite layer 0.3354 nm.This indicates that plasma opens the spacing of graphite layer on the surface,and the chaos of material surface is increased,the degree of isotropic orientation（OI）decreases;the shorter the ionic migration path,the higher the ionic migration rate,and the larger the interlayer spacing can also provide a larger channel for the transmission of lithium ions and promote the diffusion rate of lithium ions.The oxygen atom content of PMCMB-100 increased from 6.38%to 19.45%.The content of C=O and COO-increased,while the content of C-O decreased.Carbon-oxygen double bonds in carbonyl（C=O）and carboxyl（COO-）could adsorb lithium ions extra,thus increasing the capacity of materials.As the specific surface area of PMCMB-100 increases,more lithium ions can be adsorbed,and the pore size of PMCMB-100 is larger,which accelerates the diffusion of ions,so that PMCMB-100 has better capacity and rate performance.（3）Natural graphite（NG）and artificial graphite（AG）were modified by plasma with a ratio of argon to oxygen of 4:1 at 100 W,40 min,50 Pa,and two kinds of anode materials,NG-100 and AG-100 were obtained.The surface graphite layer of NG-100material became cracked,the surface was rough from smooth,the interlayer spacing increased,the graphitization degree decreased,the isotropy increased,and the surface functional groups increased.The first discharge specific capacity of NG-100 was1453.7 m Ah g^-1,and after 100 cycles,the discharge specific capacity was still at 428.4m Ah g^-1,the charge-discharge efficiency was 99.2%.The carbon layer coated on AG-100 surface is oxidized,the surface becomes rough,the crystal spacing of 002becomes larger,the graphitization degree decreases,the OI value decreases,the isotropy becomes larger,and the surface functional groups increase.The first discharge specific capacity of AG-100 is 584.7 m Ah g^-1,after 100 cycles,the discharge specific capacity is 413.5 m Ah g^-1,the charge-discharge efficiency is as high as 99.6%,and the cycle performance of AG-100 has been greatly improved.The rate performance has also been significantly improved.