Solvation Structure and Dynamics of Li⁺ in Ternary Ionic Liquid-Lithium Salt Electrolytes

The structural and dynamical changes in the solvation shell surrounding Li⁺ in a multianion environment are studied by Raman spectroscopy and molecular dynamics (MD) simulations. The ternary electrolyte is composed of a mixture of two ionic liquids (ILs), n-methyl- n-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([PYR13][TFSI]) and 1-ethyl-3-methylimidazolium dicyanamide ([EMIM][DCA]), and a lithium bis(trifluoromethanesulfonyl)imide ([Li][TFSI]) salt (0-1 M). A 1:9 volumetric mixture of [PYR13][TFSI]/[EMIM][DCA] formed an eutectic that exhibited a lower melting point than that of either parent IL. The local structure of Li⁺ in this eutectic is found to be heterogenous and preferentially solvated by [DCA], which is the smaller and more abundant anion. Whereas [TFSI] is able to bridge multiple Li⁺ at high salt concentrations and form both monodentate and bidentate conformations through its oxygen atoms, [DCA] is capable of forming only monodentate coordination with Li⁺ through either of its end nitrogen atoms. The Raman and MD analyses suggest a wide distribution of solvation structures in the form of [Li(TFSI) _m(DCA) _n]^{( m+ n-1)-} where m = 0-1 and n = 3-4. The computations showed increased ion pair lifetime for Li⁺-[DCA] and decreased lifetimes for Li⁺-[TFSI] in the ternary mixture with the increase in the [Li][TFSI] concentration. These results show that the solvation and transport properties of charge carriers in ILs can be modified via the presence of multiple ions with varying degree of coordination, which provides an approach to impact the performance in electrochemical processes.