First-principles Calculations On New Cathode Materials For Lithium-ion Batteries

Lithium-ion batteries have been widely used in portable electronics,electric vehicles and energy storage systems.However,the present materials cannot meet the requirements of the fast-developing applications on the energy density and fast charge and discharge performances of the Li-ion batteries.As a novel research method,the first-principles calculations can be applied not only to explain the observed experimental phenomena,but also to provide improvement schemes,design new materials and predict their properties,speed up the rate of discovery of new materials.In this thesis,first-pricinples calculations were performed to understand the distinct structural stability of the monoclinic NaMnO₂ and LiMnO₂ during de-intercalation,to select substitutional elements for suppressing the Mo migration in Li₂MoO₃,and to design fluorocarbonate cathode materials,AMCO₃F?A = Li/Na,M = Fe,Co,Ni,and Mn?and predict their physical and electrochemical properties.The reason for the distinct cycling performances of the NaMnO₂ and Li MnO₂ upon de-intercalation was explored.Calculations show that it is difficult for the Mn-ion in NaMnO₂ to migrate into the Na layer.However,when triple-vacancies are generated in the Li layer,the Mn-ion in Li MnO₂ can migrate into the Li layer and impede the diffusion of the Li-ion.The migration of Mn-ion in LiMnO₂ is a found to follow the double-vacancy mechanism and there is an obvious metastable state in the interstitial tetrahedral sites in the O layer.The differences of these two materials in charge distribution and in the strength of the Mn-O bonds are supposed to be responsible for their distinct peroformances.Calculations were conducted to study Mo-ion migration in Li₂MoO₃,and select substitutional transition metals for the material.Calculations show that,when four vacancies were generated in the Li layer,the Mo-ion migrate into the 3b position.is Mo migaration into the Li layer hinders the diffusion of the Li-ion after and degrade the dynamic properties of the material.Based on the calculations,we found that Sbsubstitution can confine the Mo-ion migration and stabilize O ions and,therefore,improve the kinetic performance of Li-ion in Li₂MoO₃.New fluorocarbonate materials,AMCO₃ F,were designed by replacing the Cu and Ca in KCuCO₃ F and KCaCO₃ F with transition metals?Fe,Mn,Co,Ni?and substituting the K with Li and Na,respectively.First-principles calculations were performed to relax the structure of of AMCO₃ F and analyze the valence state of each element.Taking account of the kinetic?the diffusion coefficient of Li-ion and electronic conductivity?and thermodynamic?phase stability and electrochemical potential?properties,we believe that LiCoCO₃ F and LiFeCO₃ F are two promising cathed materials for secondary batteries.