Structural and Dynamic Characterization of Li–Ionic Liquid Electrolyte Solutions for Application in Li-Ion Batteries: A Molecular Dynamics Approach

Abstract

Pyrrolidinium-based (Pyr) ionic liquids (ILs) have been proposed as electrolyte components in lithium-ion batteries (LiBs), mainly due to their higher electrochemical stability and wider electrochemical window. Since they are not naturally electroactive, in order to allow their use in LiBs, it is necessary to mix the ionic liquids with lithium salts (Li). Li–PF6, Li–BF4, and Li–TFSI are among the lithium salts more frequently used in LiBs, and each anion, namely PF6 (hexafluorophosphate), BF4 (tetrafluoroborate), and TFSI (bis(trifluoromethanesulfonyl)azanide), has its own solvation characteristics and interaction profile with the pyrrolidinium ions. The size of Pyr cations, the anion size and symmetry, and cation–anion combinations influence the Li-ion solvation properties. In this work, we used molecular dynamics calculations to achieve a comprehensive view of the role of each cation–anion combination and of different fractions of lithium in the solutions to assess their relative advantage for Li-ion battery applications, by comparing the solvation and structural properties of the systems. This is the most-comprehensive work so far to consider pyrrolidinium-based ILs with different anions and different amounts of Li: from it, we can systematically determine the role of each constituent and its concentration on the structural and dynamic properties of the electrolyte solutions.

Density and Dynamic Viscosity of Aqueous Electrolytes with Increased Li-Salt Solubility Due to Addition of Ionic Liquids

“…The dynamic viscosity measurement has a relative standard uncertainty u r (η) of 0.05. The experimental values observed by us were found to be in good agreement with the literature data. − Slight discrepancies may arise due to differences in the instrumentation used for the physical property measurement and/or the differences in the purity of the chemicals used.…”

“…A comparison at T = 298.15 K and at LiTFSI molality = 21 mol kg –1 reflects the inbuilt higher density of EMIm.TFSI containing water-in-(salt + RTILs) electrolyte as compared to EMIm.OTf, Pyr 14 .TFSI, and N 4111 .TFSI containing water-in-(salt + RTILs) electrolytic systems that show statistically similar values. The results are in line with the densities of neat RTILs: EMIm.TFSI (1.53 g cm –3 ), EMIm.OTf (1.39 g cm –3 ), Pyr 14 .TFSI (1.40 g cm –3 ), and N 4111 .TFSI (1.40 g cm –3 ), respectively, highlighting the relatively stronger ionic interactions between EMIm + and TFSI – in comparison to the ions of the other RTILs. Clearly, the density values of ternary electrolytes are higher in comparison to the density of neat RTILs; however, surprisingly, they are lower as compared to the density of LiTFSI (21 mol kg –1 ; 1.724 g cm –3 ) dissolved in water at T = 298.15 K. These observations hint at the weakening of ionic interactions between Li + and TFSI – with the addition of RTIL, but the overall increase in density highlights the enhancement of attractive interactions within the water-in-(salt + RTILs) electrolytic mixtures. ,, …”