First Principles Investigation Of The Interaction Between The He And H Atoms In 3C-β SiC And Li⁺/Ni²⁺ Exchange In LiNi_0.8Co_0.2O₂

It’s very urgent to develop nuclear fusion energy so as to solve the problems of energy shortage and environmental pollution.Phsma facing materials（PFMs）plays fairly important roles in the nuclear fusion process,because they directly face the plasma core and thus are under strong electromagnetic、High-energy particles irradiation.SiC materials,possessing low induced-activation/low after-heat properties,creep resistance,especially exhibiting remarkable stability in high radiation environments,have been proposed as one of the prime candidates for plasma-facing materials in future fusion reactors.On the other hand,energy conversion and storage is also an import problem.Lithium ion betteries are experiencing the large applications in this aspect,because of their high energy density、high power density、long cycle life and no memory effect.In this paper,we investigate some special performance of 3C-P SiC material and lithium ion betteries by first principles calculations.In Chapter 1,we give an introduction to the background of Plasma Facing Materials（PFMs）and the lithium ion battery materials,then put forward the problems needed to be solved.In Chapter 2,we first introduce the basic theory—Density Functional Theory,and the Vasp codes often used in our calculations.In the third chapter,site preference and diffusion behaviors of H in pure 3C-βSiC and in He-implanted 3C-β SiC are investigated,on the basis of the first-principles calculations.We find that the most stable sites for H in the pure 3C-βSiC is the anti-bond site of C（ABc）in Si-C,while it becomes the bond-center（BC）site of Si-C bonds in the He-implanted 3C-β SiC.Analysis on the electronic structures reveals that such change is attributed to the reduction of hybridization of C-Si bonds induced by the implanted He.Moreover,the presence of He strongly affects the vibrational features in the high frequency region,causing a blue shift of 25 cm-1 for C-H stretch mode with H at ABc site and a red shift of 165cm-1 for that at BC site,with respect to that in the pure system.In pure 3C-β SiC,H is diffusive with an energy cost of about 0.5 eV,preferring to rotate around the C atom in a Si-C tetrahedron with an energy barrier of just about 0.10 eV.In contrast,in the He-implanted 3C-β SiC,the energy barriers for H migration goes up to be about 0.95 eV,indicating the implanted-He blocks the diffusive H to some extent.Our calculations also show that the influence of He on H diffusion is effective in a short range,just covering the nearest neighbor.In the fourth chapter,we first investigate the trapping of Multiple Helium or Hydrogen Atoms within a 3C-β SiC vacancy.We find that a Si vacancy can accommodate up to 11 He atoms or 8 H atoms;while a C vacancy can accommodate up to 14 He atoms or 4 H atoms.We find the Hydrogen atoms cluster can push the Helium atoms cluster out in a Si/C vacancy.In the fifth chapter,we explore the stability and crystallographic parameters of the LiNi0.8Co0.2O2 system with different exchange ratio of Ni and Li ions,and discuss the influence of Li’/Ni²⁺ exchange on Li diffusion behaviors in LiNi0.8Co0.2O2.We find that the c-axis crystallographic parameter increase evidently with the increment of the ratio of Li+/Ni²⁺ exchange,and a-axis and b-axis crystallographic parameters change slightly.Moreover,the relative energy of the system goes up significantly,when the ratio of Li+Ni²⁺ exchange become large.Especially,the relative energy increases distinctly at the ratio of Li+/Ni²⁺ exchange above 16%.This means high ratio of Li+/Ni²⁺ exchange in LiNi0.8Co0.2O2 is energetically unfavorable.So the possibility of the system with Li+/Ni²⁺ exchange above 16%is very small,which is in agreement with the experimental observation that the ratio of Li+/Ni²⁺ exchange above 10%is hard to achieve.Then our calculations reveal that the activation barriers of the Li+ ion diffusion further decrease with the increment of the ratio of Li+/Ni²⁺ exchange.