Electrode Materials Of Lithium Ion Battery And Supercapacitor By First-Principles Simulation

The popularity of portable energy storage devices and electric vehicle stimulates the development of higher power and energy density for energy storage devices such as lithium ion batteries and supercapacitors. The electrode is the core component of lithium battery and supercapacitor. Thus, the electrode material is the key factor of overall performance quality. Therefore, exploitation of high performance electrode material has great significance for the research and application for lithium battery and supercapacitor.First, the chemical diffusion coefficient is an important parameter of ion transport properties for the cathode material of Li ion battery. Li insertion/extraction in cathode material usually accompany with phase transformation of host crystal. Cathode intercalation compound is a temporary storage of lithium in battery. It is desirable to select the intercalation compound with higher potential in order to obtain higher voltage. Hence, the crystal structures, reversible potentials and activation energies of LiMnxCoyNi1-x-yO2solid solutions are studied by means of density functional theory (DFT) calculations within generalized gradient approximation (GGA) and projector-augmented-wave (PAW) method. The general trends for the synergistic effects of TM ions are discussed. By analyzing Li diffusion energy barrier in various local circumstances of the multi-component solid solutions of lithium TM oxides, we predict that the real environment of LiMnxCoyNi1-x-yO2solid solutions affect the ability of ion conduction. These may help optimize compositions in future experiments.Lithium sulfur batteries have attracted much attention due to the high theoretical specific capacity as well as abundance of raw materials. However, to the best of our knowledge, there was no theoretical study on the Li storage behavior of Li2S materials, though Li2S-based materials have been intensively investigated in experiments. In particular, the microscopic mechanism for the effect of transition metal doping on the performance of Li2S remains puzzling. These facts motivate us to perform DFT calculations on the effects of transition metal (TM=Fe, Co, Ni, Cu) doping on lithium extraction/insertion behavior and the electrode potential of Li2S.Next, seeking appropriate anode materials is crucial for lithium ion batteries with sufficient lithium storage and excellent lithium reversibility to meet the demand of high voltage, large capacity and long cycle life. Nowadays, graphite is a commercially used anode material for lithium batteries. Therefore, the study of anode graphite materials has very practical significance. Using DFT calculation we have systematically investigated the atomic structures, electronic properties and saturation Li capacity in graphite materials with different interlayer spacing and different contents of substitutional boron dopants. Oue results not only provide valuable insights into the microscopic mechanism of lithium-ion batteries but also help design new anode materials with improved performance.Finally, compared to the traditional lithium ion battery, supercapacitor has high power density and long cycle life, but lower energy density. In order to improve the energy density of supercapacitor, we investigate the graphene sheets as supercapacitor electrode material with different geometries of hole defects and/or nitrogen doping at the atomistic scale by first-principles methods and non-equilibrium Green’s function technique. We mainly examine the following critical issues:(1) Will these holes as structural defects reduce the thermodynamic stability and electrical conductivity of the graphene electrode?(2) Will the graphene sheets incorporated with holes still retain their excellent mechanical properties?(3) What is the optimal shape and size of these holes for ion diffusion?(4) Which kind of C-N bonding configurations is responsible for the enhanced electrocatalytic activity of the N-doped graphene material? For revealing these issues, we calculate the formation energies, mechanical properties, diffusion behaviors and electrical conductance, aiming to enhance the overall performance of the supercapacitors by appropriate incorporation of holes.