Disentangling cation effects on ion mobility and structure in ionic liquid electrolytes

Ionic liquids (ILs) are low-temperature molten salts, and therefore the
transport of ions within ILs is dominated by ion-ion interactions.
However, the influence of organic IL cations on key electrolyte
properties, such as ion dissociation and overall transport behavior, in
lithium-salt-doped ILs remains poorly understood. Moreover, despite their
critical role in designing IL-based electrolytes for energy storage
applications, ion-ion interactions and ion-specific transport under an
applied electrical potential are seldom quantified, largely due to the
unique experimental and computational challenges involved. Herein, we
compare transport properties obtained using 1H, 7Li, and 19F pulsed-field
gradient nuclear magnetic resonance (PFG NMR) and electrophoretic NMR
(eNMR) with those measured by electrochemical impedance spectroscopy
(EIS). Non-equilibrium molecular dynamics (MD) simulations and eNMR
confirm the presence of negatively charged [Li(TFSI)n](1-n) aggregates
that migrate towards the positive electrode, resulting in negative lithium
transference numbers. Equilibrium MD simulations reveal a vehicular Li ion
transport mechanism facilitated by long-lived aggregates with Li+ cations
strongly bound to multiple TFSI– anions. Finally, we observe an inverse
relationship between the apparent charge of the TFSI– anion in the neat
IL, which is dictated by the IL cation, and Li+ transport in the
salt-doped systems. This highlights the opportunity to tune electrolyte
performance by tailoring cation chemistry.