Study Of Energy Storage Mechanisms Of Supercapacitors With Oligomeric Ionic Liquids

Supercapacitors own many unique advantages such as instantaneous charged-discharged ability,good cycle stability,and environment-friendliness,thus,they have become important electrochemical energy storage devices in new energy fields.The key to advancing the large-scale application of supercapacitors is to increase energy density and maintain high power density.Oligomeric ionic liquids(OILs),newly emerging electrolytes,owing to high working voltage,wide liquid temperature range,and more flexible and adjustable structure and properties,have a good application prospect in increasing the energy density of supercapacitors.However,the energy storage performance and mechanism of supercapacitors with OILs remain to be revealed.In addition,the high viscosity and low conductivity of OILs will degrade the excellent power performance of supercapacitors.Therefore,it is very necessary to carry out further research on the energy storage mechanism and power performance of supercapacitors with OILs,In this work,using molecular dynamics simulation(MD),the microstructures and properties of the solid-liquid interfaces of electrodes and OILs were studied.The main research results are as follows:(1)By studying the capacitance and microstructures of the electrical double layers in OILs near graphene electrodes,it is found that the differential capacitance of OILs is different from that of monocationic ionic liquids(MILs).The differential capacitance-voltage(C-V)curves of OILs are camel-shaped.OILs have larger capacitance under higher electrode voltages,thereby,higher energy density.This is because the accumulation of cations in OILs on the solid-liquid interfaces leads to a higher concentration of ionic charges,resulting in the lattice saturation effect.This work reveals the energy storage mechanisms of supercapacitors with OILs and promotes the application of OILs in the field of electrochemistry.(2)By studying the effect mechanism of organic solvents on the dynamic performance of OILs,it was revealed that the low-viscosity solvent can improve the conductivity and energy storage performance of OILs.The increase in conductivity is ascribed to the fact that the organic solvent strongly weakens interactions between ions.The improvement of the energy storage performance is because the polar organic solvent weakens the overscreening effect of charges,thus,more ions are adsorbed on the electrode surfaces,which helps to improve the capacitance.In addition,Reducing the polarity of the organic solvent can promote capacitance more effectively.This work enables supercapacitors to have both high energy density and high power density.(3)The work designs a nonflammable ternary mixed electrolyte including humid hydrophilic ionic liquid,acetonitrile,and water to improve the safety of organic solvents and the energy density,power density,and economy of supercapacitors.Although water is added into the mixed electrolyte of humid ionic liquid and acetonitrile,the adsorption of water at the solid-liquid interfaces becomes weakened,which helps to widen the operating voltage window of supercapacitors and improve the energy density.The reduced adsorption of water on the electrode surface can be attributed to the decrease of the water-electrode and water-particles interaction energy and the increase of the energy barrier of the adsorption of water on the electrode.This work can promote the practical application of high-performance supercapacitors.(4)By studying the energy storage performance of the electrical double layer in OILs near the carbon nanotube electrode,it was found that the electric field intensity increases as the electrode curvature decreases,and the number of ions adsorbed on the electrode increases,increasing the capacitance..Unlike graphene electrodes and MILs,the capacitance of the electrical double layer in OILs fails to depend on the electrode voltage.The stability of energy storage performance is attributed to the fact that the electrostatic shielding effect of charges leads to the electrical double layer of similar thickness,resulting in similar capacitance.In addition,The electrical double layer in OILs near the carbon nanotube electrode owns a stronger energy storage capacity under higher voltages.This work provides theoretical guidance for the design of high-performance supercapacitors.In conclusion,in this work,the energy storage mechanisms of supercapacitors with OILs have been reviewed and the power and safety performance has been optimized,which provides a reasonable scheme for designing supercapacitors with high energy and power density,safety,and low cost.