| In recent years,lithium-ion batteries(LIBs)have been widely used in portable electronic equipment,electric vehicles and miscellaneous power devices due to its increasing energy density.However,there are still some problems of low coulombic efficiency,cycle lifetime and poor safety in commercialized LIBs.All these problems can be attributed to Li atom plating and formation of lithium dendrite on the surface of graphite anode during charging process.So far,people have put forward a variety of strategies to inhibit recrystallization growth from the view of lithium metal.Carbon materials,especially graphene,dominate the anode of LIBs due to their multifunctional roles,such as,high tunable porosity,chemical stability,and good conductivity.They can lower the local current density and the ion polarization to realize Li deposition uniform.However,the detailed absorption behavior and electronic properties of Li atoms at the surface of carbon materials from the microscopic level are very hard to be investigated experimentally because of the high activity of Li.There is no clear explanation for the nucleation mechanism and inhibition principle of lithium dendrite in experiments.In this paper,double-layered graphene was used as the research object to explore the effects of different defects and doping types of graphene surfaces on the nucleation of lithium ions.The inhibition mechanism of lithium dendrite was analyzed theoretically.The main research contents and results are as follows:(1)Effect of different vacancy defects on graphene surface on Li ions nucleation.Different numbers of Li atoms were adsorbed on the surface of graphene with single vacancy defect(SV)and double vacancy defect(DV),respectively.The adsorption process,geometric configuration,energy stability and electronic properties were analyzed with the increase of lithium concentration.The results show that the SV defect tends to promote the nucleation of lithium dendrite.With the increase of lithium concentration,the adsorption energy decreases gradually,and the distance between of Li atoms becomes smaller,which is easy for Li ion to aggregate into clusters.However,the existence of divacancy gap promotes diffusion of lithium ions to the interlayer and prevents accumulation continuously of lithium ion on the graphene surface.In addition,the growth of lithium dendrites is inhibited to a certain extent by the high adsorption energy.(2)The inhibition mechanism of lithium dendrite after nitrogen-doping defective graphene.the initial Li nucleation on the basal plane of graphene with both N doped SV and DV defects can be suppressed.The difference is that Li ions can accumulate on the top surface of N-doped graphene with SV defect(SV-3N)in the near 2D structure,and the mechanism of inhibition is called “adsorption mechanism”.For the N-doped graphene with DV defect(DV-4N),the Li ions not only deposit uniformly on its top surface,but also can enter easily the interlayer space to increase the reversible capacity of Li-ions batteries,which is called “adsorption-intercalation” mechanism.(3)Prediction of lithium dendrite inhibition by co-doped defective graphene with N、B、S、P.Through the adsorption of single Li atom on co-doped graphene surface,the adsorption energy,diffusion barrier and the structure change are analyzed.It is predicted that the inhibition mechanism of B-N co-doped system is similar to the N-doping mechanism.It can be used as a substitute material for nitrogen element;S-N co-doping,especially DV defect doping,can not only inhibit dendrite by adsorption and intercalation on the surface,but also promote entry of Li ions from the edge of graphene by increasing layer spacing.Improving ratio performance and increasing lithium storage capacity;however,the positive adsorption energy is not conducive to the stable existence of Li ions on the surface of graphene for P-N co-doped graphene,and the larger diffusion barrier has little effect on dendrite inhibition. |
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