Transference Numbers\nand Ion Coordination Strength\nfor Mg<sup>2+</sup>, Na<sup>+</sup>, and K<sup>+</sup> in Solid Polymer\nElectrolytes

In solid polymer\nelectrolytes, the transference number\n(<i>T</i><sub><i>i</i></sub>) is fundamental for\nperformance\nas it defines the transport efficiency of the electrochemically active\nspecies. For systems “beyond Li”, a lack of suitable\nmethods to determine the <i>T</i><sub><i>i</i></sub> is limiting the understanding of the ion transport in such\nsystems. In this study, a method based on electrophoretic NMR (eNMR)\nand electrochemical impedance spectroscopy (EIS) is used to determine\nthe <i>T</i><sub>+</sub> for Na<sup>+</sup>, K<sup>+</sup>, and Mg<sup>2+</sup> in poly(ε-caprolactone) (PCL), comparing\nthis to previous results in poly(ethylene oxide). In common for all\ncations, a distinct correlation between strong coordination and a\nlow <i>T</i><sub>+</sub> is observed. Paradoxically, however,\nthe divalent Mg<sup>2+</sup> in PCL displayed a <i>T</i><sub>+</sub> of 0.86 compared to ∼0.5 for the monovalent cations.\nPersistent clustering in this system suggests that it is better treated\nas a monovalent [MgTFSI]<sup>+</sup> + TFSI<sup>–</sup> system,\nresulting in a <i>T</i><sub>+</sub> of the [MgTFSI]<sup>+</sup> cation of 0.43. These results serve as a door opener for\nthe wide applicability of the eNMR/EIS method in order to provide\nan understanding of charge transport in multivalent systems.