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

In solid polymer electrolytes, the transference number (T-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 T-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 T+ for Na+, K+, and Mg2+ in poly(epsilon-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 T+ is observed. Paradoxically, however, the divalent Mg2+ in PCL displayed a T+ of 0.86 compared to similar 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 T+ 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.