Simulation Study On The Influence Of Molecular Crowding Effect On Aqueous Electrolyte

The rational utilization of electric energy plays a vital role in promoting the energy revolution and the development of new energy formats.Supercapacitor is a new type of fast energy storage device.It has many advantages such as high power density,fast charge and discharge speed,wide temperature adaptability,long cycle life,and environmental friendliness.However,the shortcomings of supercapacitors are as prominent as their advantages.For example,the energy density is relatively low.Fish and bear ’s paw can not have both,power density and energy density seems to be only reconciled rather than bimodal.Therefore,there are different opinions on the outcome between supercapacitors and battery energy storage.The two great powers gradually became a strong joint force,but both still maintained their leading advantages.The research object of this paper is supercapacitors,more accurately,the electrolyte part of supercapacitors.At present,the commonly used electrolytes are mainly divided into three categories : organic electrolyte electrolyte,ionic liquid electrolyte and aqueous electrolyte.This paper mainly studies the aqueous electrolyte.Compared with ionic liquids and organic electrolytes,the biggest advantage of aqueous electrolytes is high conductivity,high-speed ion conduction in the electrolyte,showing ultra-high power density.And it has many advantages such as high safety,simple assembly environment and low cost.Molecular dynamics simulation is one of the commonly used theoretical tools to study the energy storage mechanism of supercapacitors,which can well reflect and prove the relationship between microstructure and macroscopic physical properties.In this paper,a series of studies on the performance of aqueous electrolyte were carried out by molecular dynamics simulation.The results are as follows.(1)The effects of temperature,graphene size and electrolyte concentration on the electrolyte diffusion and structure of ethylene glycol graphene supercapacitors were studied by molecular dynamics simulation.By comparing the average number of hydrogen bonds,number density distribution,mean square displacement and diffusion coefficient,we found that temperature and concentration had a significant effect on the properties of the system,while the size of graphene had little effect on the properties of the system.With the increase of ethylene glycol concentration,the hydrogen bond network between water molecules is gradually broken,the average number of hydrogen bonds between water molecules decreases,and the average number of hydrogen bonds between water and ethylene glycol molecules increases.Ethylene glycol is more capable of forming hydrogen bonds than water molecules.Therefore,the addition of ethylene glycol has a good protective effect on water molecules.(2)Through the study of(1)system,we have locked the research direction is the influence of temperature and electrolyte concentration on the performance of aqueous electrolyte.The effects of temperature and methylurea concentration on the performance of aqueous electrolyte were studied by molecular dynamics simulation.By adjusting the temperature and electrolyte concentration,we found many interesting phenomena,which have great reference significance for the performance of electrolyte system.As the number of methylurea molecules increases,the diffusion coefficients of water,lithium ions,bis(trifluoromethylsulfonyl)imide anions and methylurea decrease significantly.When the concentration of methyl urea in the system is low,the diffusion coefficient of each component increases with the increase of temperature.When the concentration of methylurea in the system is low,the average number of hydrogen bonds between water molecules decreases monotonically with increasing temperature.However,it is interesting that with the increase of the concentration of methyl urea,the average hydrogen bond number of water and bis(trifluoromethyl)sulfonylimide anion does not decrease monotonously with the increase of temperature,but fluctuates and increases in some temperature sections.With the increase of temperature,the average number of hydrogen bonds between water molecules and between water and methyl urea will decrease,but the decrease will slow down with the increase of temperature.The number density distribution of each component in the electrolyte is greatly affected by temperature and methyl urea concentration.After adding methylurea,it is no longer evenly distributed even at high temperature,and irregular cluster groups appear under the hydrogen bonding of methylurea.Under low temperature conditions,water molecules are aggregated by methylurea and bistrifluoromethylsulfonylimide anions to form stable clusters.The single free water is reduced,the freezing point of water is reduced,and the adaptability of the electrolyte to the low temperature environment is enhanced.The temperature resistance of the system is enhanced,and the stability is also enhanced.(3)Through the interesting findings in(2),we have carried out more detailed research.We replaced methyl urea with sulfolane,and increased the number of model portions of sulfolane concentration,so that the number of sampling portions of the data is more.We have obtained a hydrogen bond change trend similar to that of the methyl urea system.When the concentration of sulfolane continues to increase,the average hydrogen bond number between water molecules tends to increase abnormally monotonously,which is interesting and meaningful.The effects of temperature and sulfolane concentration on aqueous electrolyte were studied by molecular dynamics simulation.When the concentration of sulfolane is low,the average number of hydrogen bonds between water molecules decreases monotonously with the increase of temperature.However,when the concentration of sulfolane reaches a certain value,the average number of hydrogen bonds between water molecules is no longer monotonically decreasing,but increases first and then decreases.When the concentration of sulfolane continues to increase,the average number of hydrogen bonds between water molecules shows an abnormal monotonic increase.This work provides theoretical support for the in-depth study of molecular crowding in aqueous electrolytes over a wide temperature and concentration range at the molecular level.