Feature-based Attention On The Ionic Transport Mechanisms And The Conductive Properties Of Ionic Liquids

Regarded as new green solvents,room-temperature ionic liquids(RTILs)have been attracting considerable interest because of their many unique properties including the high ionic conductivity,wide thermal stability,low viscosity,negligible vapor pressure,wide liquid range and electrochemical windows.These attribute to their use in a number of electrochemical and technological applications in chemical synthesis,industrial processing,and energy storage.However,whether these applications are effective and practically valuable depends on the basic theoretical and experimental studies.The key lies in understanding the ionic transport mechanisms and the conductive properties of ionic liquids accurately.Particularly,due to the structural diversity and the complexity of the ionic interaction the present studies are still not enough.Therefore,in this paper the transport mechanism and the conductive properties of ionic liquids are analyzed from the thoughts of ion movement correlations and new phenomenological model dealing with electrode effect in broadband dielectric spectroscopy.The above work has comparatively higher theoretical and practical value.Firstly,based on the applications and progress of broadband dielectric spectroscopy method on RTILs,in this paper the model of equivalent electric circuit considering the electrode effect is established.Both the dielectric properties and the basic transport properties of the ionic liquids are obtained effectively which provides a new phenomenological method based on broadband dielectric spectroscopy.In this method the electric double layer is converted into an equivalent electric capacity which simplifies the ionic liquid and electrode system.Simultaneously,a complete set of characteristic frequencies are calculated from the dielectric and impedance spectroscopy and the corresponding physical meaning is analyzed which clarifys the physical mechanism of the parasitic electrode effect.Through the analysis of the characteristic frequencies and electric circuit parameters,high precision fitting especially for data at low and middle frequencies is archieved.Both the dielectric properties of electric double layer and bulk and the basic transport properties of the ionic liquids such as diffusion coefficient,ion mobility and effective ion concentration are also acquired.Secondly,based on the Green Kubo linear response theory,the basic transport properties such as a.c.conductivity and the self-diffusion coefficient are calculated from the concept of ion movement correlation which provides micro theoretical basis.The relationship between microscopic ionic movement correlations and macroscopic transport properties is derived in the mass-fixed frame.Not relying on the guess of ion association,the reasons about the deviation of the conductivity from the Nernst-Einstein equations are analyzed.Moreover,based on the conservation of momentum the relationship between the microscopic ion velocity cross-correlations and the macroscopic transport properties is also established.Thirdly,based on the features of the ion movement self-correlations,calculation methods of self diffusion coefficient of ionic liquid are compared and the method with lower cost is found via molecular dynamics simulations in the all-atom force field.Through the statistical calculation of the velocity autocorrelation function(VACF)and mean-square displacements(MSDs)for the centers of the ions,the effect and the properties of temperature on the ion micro motion are analyzed.Additionally,the self-diffusion coefficients are obtained both from the integration of VACF function and from the linear slope of MSDs.By the analysis of the calculation error of those two methods how to improve the accuracy of calculation is concluded.The Arrhenius fitting results extrapolating to room temperature show relatively high precision by several ns simulation times and the computational costs are obviously lower than those of other research methods.Finally,the physical significance of fitting parameters in the empirical equation of Vogel-Fulcher-Tamman(VFT)for ionic liquids is discovered and explained by the ion motion cross-correlations.Compared with the separate analysis of each transport properties,analysis of the temperature dependence of velocity cross-correlation coefficients(VCCs)should be able to get more comprehensive and accurate microscopic information of RTILs.The VCCs are significantly influenced by variation of anionic and cationic structures.The detail variation of the VCCs reflects the complex interactions between ions and the normalized VCCs could reasonably explain the fluctuations in the temperature dependence of the Nernst-Einstein deviation parameter.The VCCs also exhibit temperature dependency following the Vogel-Fulcher-Tammann equation with high precise.The fitting results show that the best-fit parameter T0 closely coincides with the glass transition temperature Tg and the VFT parameters(B/T0)is generally inversely proportional to fragility of the liquid.