Computational Insight Of Interfacial Structures And Their Electric-Double-Layer Charge Storage Mechanism For 2D Nanolayer Electroactive Materials Of Supercapacitor By Molecular Dynamics Simulation

In the development of present society,human’s energy consumption and demand are growing,how to fully and effectively collect,store and utilize new energy sources is of vital importance.Electrochemical energy storage is one of the most important methods of large-capacity energy storage.Achieving high energy/high power density and long working cycle is to determine the ideal electrode material and find the suitable electrolyte material.Selecting appropriate experimental and theoretical analytical testing techniques to conduct detailed research and exploration on the underlying mechanism of electrochemical energy storage systems will help to provide basic guidance for the material selection and electrode design of high-performance energy storage equipment.In this dissertation,we will focus on using of molecular dynamics simulation methods and combined with experimental analysis techniques and first principles calculation based on density functional theory.Respectively,constructing two-dimensional Graphene and MXene supercapacitor simulation model,systematically simulating and analyzing the characteristic surface structure of the Graphene and MXene electrodes,the structure change of the electric double layer during the charge and discharge process,the dynamic energy storage mechanism and the ion diffusion and migration characteristics of the electrolyte.The research work in this paper will be carried out in the following order:Firstly,the surface characteristics of electrode materials,such as morphology and surface functional groups,have an important influence on the interface double-layer and energy storage characteristics.In this chapter,graphene electrode materials with two-dimensional characteristics are selected as the research object to study the changes of interface double-layer structure and energy storage characteristics with surface functionalization.Analysis of the surface structure characteristics of functionalized graphene by the first-principles calculation method showed that local charge redistribution would occur around oxygen-containing functional groups,resulting in a strong polarization on the surface of graphene electrode.The results of molecular dynamics simulation show that the surface polarity of functionalized graphene changes the interface dielectric layer structure and the dipole orientation of water molecules on the electrode surface,and then induces the net charge redistribution and the change of zero charge potential on the graphene electrode surface.On the surface of the hydroxyl functionalized electrode,the potential of zero-charge increased from 0.28 V to 0.71V,while on the surface of the carboxyl functionalized electrode the potential of zero-charge decreased to-0.41 V.Surface polarity due to surface functionalization can limit the orientation arrangement of electrolyte molecules and weaken the mobility of electrolyte molecules.This limitation will reduce the dielectric constant of the electrolyte at the interface of the double-layer,and lead to the reduction of integral specific capacitance on the surface of the functionalized electrode when two kinds of functional groups are modified,and the specific capacitance gradually decreases with the increase of oxygen-containing functional group concentration.Subsequently,the dynamic characteristics of electrolyte ions during charging and discharging will directly determine the power characteristics of supercapacitor devices.In order to study the dynamic migration characteristics and dynamic energy storage mechanism of double-layer capacitors in nano-restricted environment,MXene,a two-dimensional layered electrode material with intrinsic two-dimensional slit nanorods,is selected as the object of study in this chapter.Using the first principles calculation based on density functional theory,the local atomic model and surface charge distribution of MXene are determined.Then the potential function parameters were fitted,and the parameters of the potential function fitted by MXene for molecular dynamics simulation were obtained.On this basis,the simulation model of MXene electrode supercapacitors in ionic liquid electrolyte was constructed by using molecular dynamics simulation method.The simulation results show that,the diffusion characteristics of electrolyte ions in 0.7 nm layers are¹.3-2 times higher than that of other layers（1.0nm,1.4nm）and bulk in nano-limited environments.This will help the MXene electrode to show better power density during charging and discharging.In addition,we can observe the periodic changes of the number of anions between the electrode layers,the structure of the double-layer and the arrangement of polar molecular dipoles during the dynamic charge-discharge cycle.Finally,the state of the electrode material,such as the dynamic change of the volume of the electrode,is an important and important factor which cannot be ignored in the charge-discharge process.The state of electrode material will directly affect the circulation life and safety of energy storage devices.Non-fixed multi-layer MXene nanopore electrode models,with different surface functional group,were designed and constructed.We present a molecular dynamics simulation study achieved on two-dimensional Ti₃C₂T_x MXenes in an[EMIM]⁺[TFSI]^-ionic liquid electrolyte.The molecular dynamics simulation reproduces the volume change pattern of different features of the negative electrode and the positive electrode during the charge-discharge process observed in the experiment:the electrode expands at the negative electrode side and contracts at the positive electrode side.By tracking the number and rearrangement of ions in the electrode holes,we give a quantitative description of the different charge storage mechanisms in the negative electrode and the positive electrode.In the negative electrode,charge storage is mainly realized through the heterogeneous charge ion implantation process（69%-94%,varying with the variety of surface functional groups）,while in the positive electrode hole,it is mainly realized through the co-ion/counter-ion exchange process（65%-88%）.This asymmetric energy storage mechanism results in an increase in the total number of ions in the negative electrode and a decrease in the total number of ions in the positive electrode during charging.The variation patterns of the ions in the two electrodes are in good agreement with the dynamic volume change trend of the electrode observed in the experiment.