First-Principles Study Of Ions Transport Mechanism And Electrochemistry Window Of Solid State Electrolytes

Nowadays,with the speedy developments of the electric vehicles,batteries with high energy density are urgently demanded.However,the volatile,flammable and explosive chemical substances in the conventional organic electrolyte have safety issues for the battery system.Therefore,it is imperative to develop all-solid battery technology to break through the limit of the application of organic electrolyte.However,the study of the electrochemical window of solid-state electrolyte is just limited to the thermal dynamics aspect,and the theoretical study results hardly meet with experimental results;Besides,the ionic conductivity is mainly optimized through ions doping,however,the effects of the dopant content and the dopant type on the electrolytes performance is still unclear.Based on the first-principles calculations,this thesis studied the electrochemical window or ion transport mechanism of three typical materials in all-solid electrolyte:Li₇La₃Zr₂O₁₂(LLZO),which is the representative of garnet system,LiTi₂P₃O₁₂(LTP),which is the representative of NASICON system,and Li_0.3La_0.567TiO₃(LLTO),which is the representative of perovskite system.And the following important research results were obtained:(1)Through DFT calculation and experimental verification,the electrochemical decomposition mechanism of cubic phase LLZO was comprehensively studied.The kinetic energy barrier of electrochemical oxygen evolution was revealed to be the major contribution to the high electrochemical stability of LLZO electrolyte,and the volume and stiffness(q/r³)model of the polyhedron model was built to screen high performance electrolyte materials and improve the electrochemical stability.It is found that the electrolyte of Nd³⁺etc.substitution obtained a wide electrolyte window(>5.5 V),which meets the experiment results very well.(2)Through DFT and AIMD simulation,cubic LLZO with high valence cations substitution of Zr site can effectively simulate Li ions migration,and with 12.5%Ta substitution was predicted to have a low activation energy of 0.19 e V.The statistical analysis showed that this effect can be attributed to the sub-balanced configuration“Li_24d-Li_48g-v”,in which one Li24d ion is only connected with one 48g Li vacancy,and it can be a guidance to further improve Li ions conductivity.(3)Combining DFT calculation and AIMD simulation,the NASICON electrolyte Li_1.5Al_0.5Ti_1.5P₃O₁₂ with 25%Al substitution was predicted to have a low activation energy of 0.20eV.The analysis in point of structure shows that the low activation energy mainly because of the framework polyhedron distortion,which simulates Li ions migration.