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

QW Huang and TC Lourenco and LT Costa and Y Zhang and EJ Maginn and B Gurkan, JOURNAL OF PHYSICAL CHEMISTRY B, 123, 516-527 (2019).

DOI: 10.1021/acs.jpcb.8b08859

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 (PYR13TFSI) and 1-ethyl-3-methylimidazolium dicyanamide (EMIMDCA), and a lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt (0-1 M). A 1:9 volumetric mixture of PYR13TFSI/EMIMDCA 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 LiTFSI 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.

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