NMR investigation of multi-scale dynamics in ionic liquids containing Li+ and La3+: from vehicular to hopping transport mechanism

The dynamics in mixtures of ionic liquid and monoatomic cations has been studied at different time scales ranging from the nanosecond up to the second. The mixtures were composed of cholinium bis(trifluoromethanesulfonyl)imide ([Chol][TFSI]) and LiTFSI, with LiTFSI mole fraction, LiTFSI, spanning from 0 to 0.5 (saturated solution), and [Chol][TFSI] and La(TFSI)3 with 0< x{La(TFSI)3}} < 0.12. The translational self-diffusion coefficients of Chol+, TFSI- and Li+ have been measured, along with NMR their relaxation times at various magnetic fields, in order to decipher the intertwined dynamics between the ions, and to reveal how the local dynamics impact the long range translational diffusion. When the concentrations of lithium and lanthanum are increased in the liquid, the long range dynamics of all the ions drop. In the case of LiTFSI, the self-diffusion coefficient of lithium becomes higher than the one of TFSI at high concentration, revealing a change in lithium transport mechanisms. The NMR relaxation data confirm this change, showing a clearer transition at x{LiTFSI}=0.15. It is interpreted as a change from a vehicular transport mechanism of the lithium below x{LiTFSI}=0.15 to a hopping mechanism above. A similar crossover seems to occur in the lanthanum solutions. It is suspected that the hydroxyl group of the organic cation is involved in this phenomenon.