Insights into the Structure and Transport of the Lithium, Sodium, Magnesium, and Zinc Bis(trifluoromethansulfonyl)imide Salts in Ionic Liquids
Details of the lithium (Li+), sodium (Na+), magnesium (Mg2+), and zinc (Zn2+) cation coordination and electrolyte transport properties are examined using molecular dynamics (MD) simulations for the N-butyl-N-methylpyrrolidinium bis(trifluoromethansulfonyl)imide (pyr14TFSI) ionic liquid (IL) doped with LiTFSI, NaTFSI, Mg(TFSI)2, and Zn(TFSI)2 salts. MD simulations are performed as a function of temperature using a polarizable force field (APPLE&P) that yields the Li+, Na+, Mg2+, and Zn2+ cation binding energies to the TFSI- anions in excellent agreement with quantum chemistry results. At 333 K, 4.7-4.8 TFSI- oxygen atoms from approximately three TFSI- anions coordinate Li+ and Na+, while Zn2+ and Mg2+ cations are instead coordinated by approximately six TFSI- oxygen atoms. Significant Na+ coordination with the fluorine atoms of the TFSI- anions is observed, unlike for Li+, Mg2+ and Zn2+. The cation-TFSI- binding motifs and the propensity of the salts to form large aggregates are temperature dependent with opposite trends noted for the electrolytes containing the Li and Na salts vs Mg salts. The MD simulations accurately predicted electrolyte transport properties including ionic conductivity, viscosity, and self-diffusion coefficients. A connection between the metal cation coordination, transport properties, and transport mechanisms is established for the different cations. The much longer cation-anion residence times for the divalent Zn2+- and Mg2+-containing electrolytes, as compared to those with monovalent Na+ and Li+, indicate the significantly slower desolvation kinetics of the divalent salts and the dominance of the vehicular cation transport mechanism relative to the anion exchange mechanism.