Abstract
Classical polarizable molecular dynamics simulations have been performed for LiTFSI solutions in the EMIM-TFSI ionic liquid. Different temperature or pressure values and salt concentrations have been examined. The structure and dynamics of the solvation shell of Li+ cations, diffusion coefficients of ions, conductivities of the electrolytes, and correlations between motions of ions have been analyzed. The results indicated that regardless of the conditions, significant correlations are present in all systems. The degree of correlations depends mainly on the salt fraction in the electrolyte and is much less affected by temperature and pressure changes. A positive correlation between motions of Li+ cations and TFSI anions, leading to the occurrence of negative Li+ transference numbers, exists for all conditions, although temperature and pressure changes affect the speed of anion exchange in Li+ solvation shells.
Highlights
One of the key technological challenges of modern society is to meet the increasing demand for environmentally friendly, safe, cheap, and easy-to-manufacture energy storage devices
Analysis of correlations and their effect on transference numbers or ion transport was reported for Li salt/glyme solutions,[36−40] ionic liquids (ILs)/molecular solvent blends,[41,42] ILs, or salt solutions in ILs.[43−47] Recently, negative effective transference numbers of Li+ cations were determined for LiTFSI solutions in EMIM-TFSI IL48 and attributed to Li+-anion correlations
Subsequent molecular dynamics (MD) studies on NaFSI solutions in EMIM-FSI IL,[49] a series of electrolytes based on lithium salts dissolved in EMIM ILs with different anions,[50] or MeTFSI (Me = Li, Na) solutions in EMIM-TFSI51 demonstrated that the effect of negative transference numbers of metal cations emerges from strong correlations in ILs and is expected to appear in certain range of salt concentrations
Summary
One of the key technological challenges of modern society is to meet the increasing demand for environmentally friendly, safe, cheap, and easy-to-manufacture energy storage devices. Experimental works and computational modeling cover a range of temperatures of possible operating conditions between the melting and boiling points of the IL.[15−20] Studies on pressure effects on ILs focus mainly on structural and conformational changes or glass transitions,[21−31] but there are several works investigating transport properties diffusion coefficients or conductivity.[21,26,32−35]. Subsequent molecular dynamics (MD) studies on NaFSI solutions in EMIM-FSI IL,[49] a series of electrolytes based on lithium salts dissolved in EMIM ILs with different anions,[50] or MeTFSI (Me = Li, Na) solutions in EMIM-TFSI51 demonstrated that the effect of negative transference numbers of metal cations emerges from strong correlations in ILs and is expected to appear in certain range of salt concentrations. We performed simulations for several values of temperature and pressure as well as for increasing salt load to compare the effects of pT changes with those observed for the increase of Li+ concentration
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