Abstract
Charge transport in ionic liquids is a phenomenon of utmost interest for electrochemical (e.g. battery) applications, but also of high complexity, involving transport of ion pairs, charged clusters and single ions. Molecular understanding is limited due to unknown contributions of cations, anions and clusters to the conductivity. Here, we perform electrophoretic NMR to determine electrophoretic mobilities of cations and anions in seven different ionic liquids. For the first time, mobilities in the range down to 10(-10) m(2) V(-1) s(-1) are determined. The ionic transference number, i.e. the fractional contribution of an ionic species to overall conductivity, strongly depends on cation and anion structure and its values show that structurally very similar ionic liquids can have cation- or anion-dominated conductivity. Transference numbers of cations, for example, vary from 40% to 58%. The results further prove the relevance of asymmetric clusters like [CationXAnionY](X-Y), X ≠ Y, for charge transport in ionic liquids.
Highlights
To overcome this lack of information, the electrophoretic mobility m of each ionic species needs to be known in order to calculate transference numbers directly and gain model-free information about charge transport
The results further prove the relevance of asymmetric clusters like [CationXAnionY]XÀY, X a Y, for charge transport in ionic liquids
The results of cation and anion mobilities in seven Ionic liquids (IL) at 295 K are shown in Fig. 2 and are given in Table S1 in the Electronic supplementary information (ESI).† The determined mobilities vary for different IL within one order of magnitude
Summary
To overcome this lack of information, the electrophoretic mobility m of each ionic species needs to be known in order to calculate transference numbers directly and gain model-free information about charge transport. Calculation of apparent dissociation values becomes feasible for cations and anions separately, and yields large differences, which is a direct proof for larger, charged ion clusters in the IL. While such clusters had already been documented by spectroscopic methods, their relevance in charge transport can be quantified. The achieved availability of eNMR ion mobilities even for viscous ionic liquids opens up interesting perspectives for studies of more complex ionic systems, like Li salts in IL or IL mixtures, which are proposed for optimization of electrolyte materials In all these systems experimental information about ionic clustering and correlated motions is sparse, but urgently required
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