Theoretical Study On Polymer Electrode Materials For Lithium Ion Batteries

Lithium-ion batteries play an important role in the power supply of electronic equipment,power grid storage,tools and vehicles.The performance requirements of lithium-ion batteries are getting higher and higher,and the electrode material is one of the most important composites to improve the performance of lithium-ion batteries.At present,common inorganic electrode materials,such as LiNiO₂^[1],LiMnO₄^[2],LiFePO₄^[3,4],LiCoO₂^[5]have been deeply studied and widely applied.However,inorganic materials have two shortcomings:one is limited mineral resources,especially Co and Ni;the other is that production or recovery requires high temperature and releases a large amount of carbon dioxide,which is harmful to the environment.In recent years,organic electrode materials have attracted considerable attention from researchers because of their environmental friendliness,rich raw materials and diverse structures.So far,many organic electrode materials have been reported.However,organic compounds also have an extremely serious disadvantage in that they are easily dissolved in an organic electrolyte.In order to effectively overcome the dissolution of the electrode material,we have selected an organic conjugated carbonyl polymer having a low solubility.In recent years,the single molecule model with DFT calculation^[6-8]has been widely used as a small organic molecular electrode material for lithium ion batteries,but its potential can not be accurately predicted.High throughput screening method and organic crystal structure based on dispersion correction density functional theory（DFT-D）can accurately calculate the potential of organic electrode materials.Therefore,the research focus of this paper is to establish a precise theoretical screening method to select the best organic polymer electrode materials,which will guide the experiment.The main research content of this paper are as follows:In chapter 1,The development,principle,prospect and composition of lithium ion battery are introduced Furthermore,the components of lithium ion batteries,including the electrode materials concerned in this paper,are introduced,and the advantages and disadvantages of the existing inorganic and organic electrode materials are discussed and compared.The development history of organic polymer electrode materials,especially organic polymer with conjugated structure,is introduced.Then,the application of density functional theory in the development of lithium-ion batteries is introduced.In chapter 2,The first-principles and the application of the modified density functional theory（DFT）for dispersion force are introduced,and the research on the electrode materials of batteries based on DFT is also introduced.Finally,the software package used in the calculation process of this paper is explained.In chapter 3,According to the known cell parameters of polyimide,the crystal structure was constructed,and the crystal structure,theoretical potential,band gap,charge distribution and lithium ion migration path of unknown polyimide were predicted.Compared with DFT-D3 and DFT calculation,the theoretical potential value of DFT-D2 method is obviously closer to the experimental results,indicating that DFT-D2 method is more suitable for the study of polyimide electrode materials.By calculating the transport barriers of lithium ions along the b-axis,c-axis and ab-plane,it is found that lithium ions migrate along the b-axis more probabilistically,and the band gap after lithium intercalation is less than 1.0 eV,which proves that the material has good conductivity and is expected to be a candidate for electrode materials.In chapter 4,Density functional theory was used to study eleven organic polymer electrode materials that have been reported.By fitting the theoretical potential value and the experimental value,a semi-empirical theoretical screening method was established to predict the potential and electrochemical performance of a large number of organic polymer electrode materials.