Reply to the ‘Comment on “Negative effective Li transference numbers in Li salt/ionic liquid mixtures: does Li drift in the “Wrong” direction?”’ by K. R. Harris, Phys. Chem. Chem. Phys., 2018, 20, DOI: 10.1039/C8CP02595A

Abstract

We have recently directly measured the electrophoretic mobilities of the three species in “Li salt in-ionic liquid” electrolytes with a common anion by electrophoretic NMR. From the mobilities of the three species, we deduced the transference numbers and found a negative value for Li. We attributed this to a vehicle mechanism of ⁷Li transport, where Li is drifting in net negatively charged Li-anion clusters. In our work, as also in previous eNMR studies, ion drift velocities and thus electrophoretic mobilities are determined in the laboratory frame of reference. In his comment, Harris claims that in this frame conclusions on ion correlations are not deducable, but rather an internal reference frame, defined by the drift of one of the ions, needs to be used and that in such a reference frame no negative transference numbers would occur. While agreeing with Harris on the general relevance of reference frames in eNMR, we argue here that a reference frame of one of the ions leads to loss of information. We propose to use the reference frame of the center of mass of the electrolyte if an internal frame is required. An interesting observation is that the center of mass of an ionic liquid is drifting during the application of the short electric field pulse in eNMR. To account for this drift, we convert our data, determined from drift velocities in the laboratory frame into the center of mass frame. The results show that, independent of the reference frame chosen, using either a laboratory frame as an external frame or a center-of-mass frame as an internal frame, drift velocities of ⁷Li and anions are identical within error, such that the interpretation of a correlated motion of Li and anions holds.