Investigation On The Correlation Between Electronic Structure And Properties Of Li₂MnSiO₄ Cathode Material For Li-ion Battery

In this dissertation,using the first-principles calculations based on density functional theory,we perform a systematic investigation on the structural characteristics,electronic structure and electrochemical performances of Li₂MnSiO₄ compounds with Pmn2₁ space group.Then partial substitution of Mn/Si by V,Ti was applied to investigate the modification of Li₂MnSi0₄ property.At the last,the electronic properties and Li ion diffusion behavior are studied when Na subsititution at Li site when different concentrations in Li₂MnSiO₄.In the first part,The crystal structure,electronic and electrochemical properties of Li₂MnSiO₄ were studied in detail by using the generalized gradient approximation adding Hubbard-like correlation（GGA+U）method.The calculation results showed that Li₂MnSiO₄ has a low conductivity because of its large band gap（3.2eV）,in the process of lithium ion extraction,the y angle of LiMnSiO₄ changed significantly and the phase transition may occur.However the structure of Li₀.5MnSiO₄ was stabled,meaning that it was possible to achieve more than one Li⁺ per formula unit in reversible cycling.Li⁺ ion diffusion has two major diffusion channels predicted by the nudged elastic band（NEB）method.The first one was along a axis direction and another one was parallelling c axis with’ a zigzag trajectory respectively,however the diffusion rate of Li ion was slow.In the Second part,the influences on the performances of Li₂MnSiO₄ were investigated after partial substitution of Mn/Si by V.The calculation showed that electronic conductivity of the materials increased,The volume change after V doped at Mn site would increase during the Li intercalation and de-intercalation process,which is harm for the reversible cycling and hinders the lithium ion diffusion.On the other hand,the volume just shows insignificant change after V subsitituion at Si site in Li₂MnSiO₄.The broadening of diffusion channel and the reduction of the activation barrier results in the improvement of the Li diffusion coefficient,which will induce a positive effect on the cycling stability.In the third part,the impacts induced by Ti substitution at Mn site with different doping concentration on the performances of Li₂MnSiO₄ were investigated.Our results suggested that the band gap reduces after Ti doping,thus the electronic conductivity was improved.The negligible volume change during the Li intercalation and de-intercalation processe would provide better structural stability and cycling stability.Owing to the widening of the channel of Li diffusion and the weakening of the Li-O bond resulting from the increase of b axis,the Li⁺ diffusion was promoted.In our discussed range,the compound with doping concentration x=0.125 showed the best performance on Li diffusion.In the fourth part,the effect induced by the Na substitution at Li site with different concentrations on the electronic structure and Li+ diffusion of Li₂MnSiO₄ was studied.Our results showed that the Na doped Li₂MnSiO₄ with different doping concentrations have narrower band gap,which indicate that the electronic conductivity will be improved.The volume expansion,the increase of Li hopping distance and the widening of the ion diffusion channel make the Li diffusion barrier decrease.Especially when doping concentration x=0.125,Na substitution can improve the Li diffusion rate effectively.