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.