Transference Numbers and Ion Coordination Strength for Mg²⁺, Na⁺, and K⁺ in Solid Polymer Electrolytes

In solid polymer electrolytes, the transference number (<i>T</i> _<i>i</i> ) is fundamental for performance as it defines the transport efficiency of the electrochemically active species. For systems "beyond Li", a lack of suitable methods to determine the <i>T</i> _<i>i</i> is limiting the understanding of the ion transport in such systems. In this study, a method based on electrophoretic NMR (eNMR) and electrochemical impedance spectroscopy (EIS) is used to determine the <i>T</i> ₊ for Na⁺, K⁺, and Mg²⁺ in poly(ε-caprolactone) (PCL), comparing this to previous results in poly(ethylene oxide). In common for all cations, a distinct correlation between strong coordination and a low <i>T</i> ₊ is observed. Paradoxically, however, the divalent Mg²⁺ in PCL displayed a <i>T</i> ₊ of 0.86 compared to ∼0.5 for the monovalent cations. Persistent clustering in this system suggests that it is better treated as a monovalent [MgTFSI]⁺ + TFSI^- system, resulting in a <i>T</i> ₊ of the [MgTFSI]⁺ cation of 0.43. These results serve as a door opener for the wide applicability of the eNMR/EIS method in order to provide an understanding of charge transport in multivalent systems.