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

Abstract Ionic liquids (ILs) are low-temperature molten salts, where ion transport is primarily governed by ion–ion interactions. Yet, the impact of organic IL cations on critical 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 ¹ H, ⁷ Li, and ¹⁹ F pulsed-field gradient nuclear magnetic resonance (NMR) and electrophoretic NMR (eNMR) with those measured by electrochemical impedance spectroscopy. 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.