Bridging Electrostatic Screening and Ion Transport in Lithium Salt-Doped Ionic Liquids

Abstract: Alkali salt-doped ionic liquids are emerging as promising electrolyte systems for energy applications, owing to their excellent interfacial stability. To address their limited ionic conductivity, various strategies have been proposed, including modifying the ion solvation environment and enhancing the transport of selected ions (e.g., Li$^+$). Despite the pivotal role of electrostatic interactions in determining key physicochemical properties, their influence on ion transport in such systems has received relatively little attention. In this work, we investigate the connection between ion transport and electrostatic screening using atomistic molecular dynamics simulations of 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([pyr${14}$][TFSI]) doped with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) at molar fractions x${LiTFSI}$ $\le$ 0.3. We find that the charge-charge and density-density correlation functions exhibit oscillatory exponential decay, indicating that LiTFSI doped [pyr${14}$][TFSI] is a charge- and mass-dense system. The electrostatic screening length decreases with increasing LiTFSI concentration, whereas the decay length of the density-density correlation functions remains nearly unchanged. Notably, we find that the x${LiTFSI}$-sensitive screening length serves as a central length scale for disentangling species-specific contributions of ion pairs to collective ion transport upon LiTFSI doping. This framework provides a unifying perspective on the interplay between structure and transport in ionic liquid systems.