Insights into the Structure and Transport of the Lithium, Sodium, Magnesium, and Zinc Bis(trifluoromethansulfonyl)imide Salts in Ionic Liquids

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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 ...