Molecular Simulations Of Electrode-Electrolyte Interface Structures And Energy Storage Mechanisms In Ionic-Liquid-based Supercapacitors

Driven by the demand of the efficient utilization of intermittent renewable energies and the growth of electric vehicles,the development of electrochemical energy storage(EES)devices with extraordinary performance has been at the limelight of energy technologies.As a new type of green energy storage device,electrical double layer(EDL)capacitor,also known as supercapacitor,whose performance is controned by interfacial structures of EDL and their formation processes,has attracted great attention owing to the extraordinary power density,long cycle life,and wide operating temperature.Nevertheless,the development of supercapacitors with high energy density remains a huge challenge.Since the energy density of supercapacitors goes with the square of applied voltage,choosing an appropriate electrolye with high working voltage is essential.Among electrolytes used in supercapacitors,room temperature ionic liquids(RTILs),with unique characteristics including especially wide electrochemical windows,excellent thermal stability,and nonflammability,are an emerging class of candidates for such devices.Therefore,the theoretical study on the EDL of RTILs-based supercapacitors is the key to designing and optimizing the energy storage performance of such devices.In this dissertation,molecular dynamics(MD)simulation and theoretical model are joint to explore the transport properties of RTILs,and then to investigate the effects of the micro-characteristic of electrode-electrolytes interfaces on the performance of supercapacitor.Additionally,these results provide the microcosmic mechanism and technical guarantee for the sake of the rational utilization of such ionic liquids-based supercapacitor.The main results of this dissertation are highlighted as follows:We investigated the ion state and ion transport mechanisms in typical RTILs with MD simulation.It has been shown that ions may reside in two states,free and bound states with an inter-state exchange.Based on the ion trajectory density,we proposed a dynamic criterion for classifying free and bound ions,and demonstrated that this criterion is consistent with the static one.From these criteria,the average portion of free ion is estimated as 15?25%,increasing with temperature in range of 300 to 600 K.Moreover,we proposed a two-dynamic-state model to characteristic the dynamic properties of RTILs.Based on the such model,we then revealed the ion transport mechanism and modified the Nernst-Einstein equation with the concentrations and the diffusion coefficients of free ions,which is able to predict the conductivity of ionic liquid accurately.To obtain underlying energy storage mechanisms of supercapacitor,the effects of ion configuration and the operating temperature on the behaviors of EDL microstructure are investigated by MD simulations.The electrodes show the specific adsorption for ions.With the applied voltage,the EDL structures alter with ions rearrangement,resulting in the change of differential capacitance.In addition,the simulated results showed a transition of the differential capacitance-potential curve from camel to bell shape with increasing temperature.The origin of the temperature dependence is consistent with the charge density distribution.This study reveals a general rule of the influence of ion configuration and operating temperature on the double-layer structure,which provides theoretical guidance for the design and optimization of the supercapacitor.Aiming at the problem that the impurity water in the hydrophobic RTILs will narrow the electrochemical window of such electrolytes,the effects of adding salt on the surface-adsorbed water and the electrochemical stability of humid ionic liquids have been explored with MD simulation and density functional theory calculations.Strongly bounded with Li~+ions,water molecules are largely excluded away from both negatively and positively charged electrodes.The water molecules remaining adsorbed on the electrode surface are almost associated with Li~+,leading to lower activity.The Li-bonding and the re-arranged orientation of the adsorbed water inhibit the water electrolysis,thus preventing the reduction of electrochemical windows of humid hydrophobic ionic liquids.Besides,this finding has been verified by experiments.Our work provides the underlying mechanism and a direct approach for the rational utilization of hydrophobic ionic liquid.