First-Principles Study Of Lithium Ion Conductors

Solid electrolytes open up a new way for renewable and green energy applications.For solid-state batteries and solid oxide fuel cells(SOFCs),solid electrolytes may be the leading technology of the future.Solid electrolyte materials are very important for the development of safe and high performance lithium ion batteries.They play an important role in modern electrochemical energy storage technology.The organic liquid electrolyte is flammable and the electrochemical stability needs to be improved,so it may be replaced by a non-flammable solid electrolyte.Despite growing interest in solid electrolytes,there are still many challenges in achieving high ionic conductivity.In this dissertation,we use a density functional theory(DFT)and ab initio molecular dynamics(AIMD)simulations study to predict the structural stability and ionic conductivity of solid electrolytes for lithium-ion and solid oxide fuel cells.The first chapter of the dissertation focuses on the background of lithium cathode materials,anode materials and ionic conductors.This chapter provides an overview of the ion transport of selected cathodes,anodes,and ionic conductors based on DFT and AIMD simulations in lithium-ion batteries.Chapter 2 is about methodology,namely DFT and AIMD simulations.In Chapter 3,we report an increase ionic conductivity in the intrinsic vacancy-induced in the recently synthesized Li3OBr inverse spinel structure compared to the anti-perovskite phase.The intrinsic defect structure of the anti-spinel Li3OBr contains a high concentration of octahedral vacancies with high stability.Ab initio molecular dynamic simulations confirm that octahedral vacancies can lead to an extraordinarily high Li mobility(0.136 mS/cm at room temperature)in anti-spinel Li3OBr.This ion conductivity enhancement mechanism opens up new avenues for the design of solid electrolyte.In Chapter 4,we studied a promising solid electrolyte material,LiTa2PO8(LTPO),which has been reported in experiments for high room temperature ionic conductivity(1.6 mS/cm).In order to understand its Li transport mechanism and find its theoretical performance limit,we systematically studied the properties of LTPO using density functional theory(DFT)and ab initio molecular dynamics(AIMD)simulations.The results show that LTPO has a wide electrochemical window.Ta,P and O are substantially immobile during Li diffusion,indicating high stability of the material.The Li ion diffusion channel forms a quasi 2D honeycomb structure.Theoretical simulations predict that the intrinsic ionic conductivity of LTPO is as high as 35.3 mS/cm at room temperature.The diffusion activation energy is very low(0.16 eV),which is consistent with the low energy barrier calculated from the minimum energy path on the potential energy surface.These results encourage further experimental studies of this promising superionic solid electrolyte material.In Chapter 5,we discuss the effects of NaWand O2-on high conductivity in Na-doped SrSiO3.The origin of high O2-ion conductivity in alkali metal-doped strontium silicate Sr3-3xNa3xSi3O9-1.5x(x = 0.45)(SNS)remains controversial.Therefore,AIMD simulations were performed to investigate the effects of Na+and O2-kinetics and the role of Na+on the conductivity in Na-doped SrSiO3.Our AIMD simulations show that the perfect SrSiO3 is an insulator,while the SNS exhibits high ionic conductivity(2.5×10-2 S/cm)with low activation energy(0.37 eV).Our research shows that Na+ is an excellent promoter of high ion conductivity,however O2-is local in their equilibrium position.This work highlights the underlining role of Na+in SNS ion conductivity.