First-principles Studies On Secondary Lithium Battery Related Materials

To reduce the usage of non-renewable energy resources such as fossil fuel,developing electrochemical power sources is a promising solution.Among different electrochemical devices,secondary lithium batteries drew most of the attention in recent years due to their excellent performances.The further increases of batteries'energy density relies on the development of materials.Density functional theory?DFT?based first-principles calculation methods are widely applied to the material study.They can help researchers to understand the mechanism of experimental phenomenon at the atomic scale.In this thesis,several key materials in high energy density secondary lithium batteries are studied by first-principles calculation methods:solid electrolytes and high energy density cathode materials.The materials are studied from the aspects of crystal structure,electronic structure and ionic transportation.The mechanism of improving effects by dopants and elemental substitution is illustrated.To combine the high ionic conductivity of sulfide solid electrolyte and the high stability of oxide solid electrolyte,We started from the investigation on the effects of oxygen doping on the sulfide materials.Taking?-Li₃PS₄ as a model material,We studied the influences of oxygen doping on its properties by first-principle calculations.The forms and lattice positions of oxygen dopants are determined.The results from electronic structure calculation show that oxygen dopants do not reduce the original electrochemical window of?-Li₃PS₄.With bond-valence based force-field methods and Nudged Elastic Band method,we find that oxygen dopants would activate a new ionic migration pathway around themselves and transform the original two-dimensional ionic transportation feature into three-dimensional.The results from bulk phase and interface simulations show that oxygen dopants can also help stabilizing the?-Li₃PS₄ to room temperature,and the oxygen on the surface of?-Li₃PS₄ can suppress the interfacial reaction between electrolyte and Li metal.Further simulations of zinc-oxygen co-doping reveal that,at low doping contents,dopants can also drive the transformation of ionic transportation behavior into three-dimensional and enhance the ionic conductivity.The effects of zinc-oxygen co-doping are verified by experiments.Based on the improving effects of oxygen dopants on the properties of sulfide material?-Li₃PS₄,we further explore the possibility of using oxysulfide materials as solid electrolyte material by using a DFT based crystal structure prediction software.A brand-new oxysulfide material,LiAlSO,is designed from scratch.The crystal structure of this material is orthorhombic.The results from simulation of ionic migration pathways show a one-dimensional channel with low activation energy.The results of molecular dynamic simulation and NEB calculation reveal that the lithium ions hopping mode with lowest barrier is a collective“kick-off”mode making this material a potential superionic conductor.Thermodynamic property calculations indicate that the material has a structural phase transition at elevated temperature and the fast ionic transportation channel preserves during the phase transition.The electronic structure calculation results show that the material has a huge band gap and potentially has a wide electrochemical window.Doping and elemental substitution are also commonly used strategies for tuning the propertiese of high energy density cathode materials.I utilized the the simulation methods of ionic migration to illustrate the mechanism of improving effects of dopants and substitutions on cathode materials.For the study on the high-voltage spinel cathode material LiNi_0.5Mn_1.5O₄,experimental results show that Al³⁺dopants at the vacant octahedral sites on the surface of particle can effectively suppress the dissolution of transition metal ions into electrolyte and increase the coulombic efficiency.With first-principles calculation,we find a significant shrinkage of transition metal ions migration channels with a certain content of Al³⁺dopants at the octahedral sites.Furthermore,Al³⁺dopants increase the migration barriers of transition metal ions and hinder their dissolution kinetically.On the contrary,Al³⁺dopants would not block the overall transportation of Li ions.For the study of layer structured lithium-rich cathode material Li₂MoO₃,experimental results show that the Fe²⁺substitution can effectively suppress the drop of delithiation voltage during cycling.Simulation finds that Fe²⁺substitution can affect where Li ion vacancy forms and drive a uniform migration of transition metal ions into the lithium layer.With the presence of Fe²⁺ions,more transition metal ions tend to migrate into lithium layer at low level of delithiation.These findings suggest that Fe²⁺ions thermodynamically promote the homogeneous mixing between transition meal ions and Li ions at early state of charge.The non-layer structure with cations mixing stays stable in the following cycles and attenuates the voltage fading.Elemental substitution affects not only the ionic migration but also the electronic structure.We study the effects of Mn⁴⁺substitution on Li-Co-O system.Electrochemical measurements show that the substitution by Mn⁴⁺ions obviously increases the delithiation voltage.Synchrotron X-ray absorption spectroscopy measurements suggest that Mn⁴⁺ions weaken the covalency between Co and O at delithiated state.Calculation results show that the substitution by Mn⁴⁺causes significant localization of electrons in the material.Upon delithiation?charging?,the influence of“inductive effect”is lessened as the oxidized Co becomes more electronegative,leading to smaller electronegativity difference between Co and Mn.However,electron localization is not influenced by delithiation and contributes unwaveringly to the decrease of the Co-O covalency throughout the whole charging process,which affects the delithiation voltage.