Molecular Dynamics Simulation Of The Effect Of Adding Acetonitrile On Ionic Liquid-Ni₃（HITP）₂ Supercapacitor

Supercapacitors play an important role in energy storage due to their advantages such as high-power density,long cycle life and wide operating temperature range.However,its development is restricted by its low energy density.The energy of supercapacitors is mainly stored at the electrode/electrolyte interface,which is several nanometers thick.The selection of electrode and electrolyte material is very important for the energy storage performance of supercapacitors.On the one hand,the conductive Metal-Organic Framework（MOF）presents a broad development prospect in energy storage due to its advantages of high porosity,highly ordered structure,and easy adjustment of properties.On the other hand,as a new type of green electrolyte,ionic liquid has excellent thermal stability,very low volatility,non-flammability,and other advantages,and can withstand higher operating voltage,which can greatly improve the energy density of supercapacitor.Therefore,the combination of conductive MOF and ionic liquid is expected to provide higher energy density and power density for supercapacitors.In this paper,a cross-scale simulation framework based on constant potential molecular dynamics simulation and transmission line model is developed,and the theoretical solutions of the transmission line model under different boundary conditions are systematically given.Then,the energy storage mechanism and charging characteristics of the supercapacitor composed of conductive MOF electrode and ionic liquid are analyzed by using this framework.The results show that the capacitive and impedance characteristics of the system under the condition of pure ionic liquid[Bmim][PF₆]as an electrolyte are not good.This is because there is a strong interaction between cation and anion in the system,which makes it difficult to separate cation and anion when applying potential to the electrode.At the same time,there is a large energy barrier for ions to enter the electrode pore.To improve its performance,adding organic solvent acetonitrile（ACN）to the system is considered.Simulation results show that the addition of ACN can greatly improve the impedance and capacitance performance of ionic liquid-conductive MOF supercapacitors,which is mainly due to the change of coordination structure of cation and anion in the ionic liquid system and the weakening of the interaction between cation and anion.According to the potential of mean force of ions entering the electrode pore obtained by the umbrella sampling algorithm,it can be found that ACN can also effectively reduce the energy barrier of ions entering the electrode pore.The electrochemical performance curves obtained by the cross-scale analysis framework based on the transmission line model are in good agreement with the results obtained by the electrochemical experiment,which also reflects the correctness of the cross-scale analysis framework and the accuracy of the simulation results.