Electrophoretic NMR Measurements of POSS-based Multivalent Electrolytes

An outstanding goal in the development of next-generation Li-ion battery electrolytes is fine-tuned control over ion and solvent motion, in particular, chemistries that enable faster cation mobility relative to that of the anion, yielding greater cationic transference numbers. Polymer electrolytes are a promising approach, as anionic species can be coupled to the polymer backbone, but for full characterization, techniques that can pinpoint changes in ionic diffusivities and velocities are required. In this work, we perform pulsed-field-gradient (PFG) NMR and electrophoretic NMR (eNMR) measurements on a novel polyhedral oligomeric silsesquioxane (POSS)-based polyanionic electrolyte dissolved in a carbonate-based solvent. PFG measurements clearly indicate slower anion self-diffusion as compared to the cation, seeming to imply large transference numbers. However, electrophoretic NMR data, which comprise the electric-field-induced velocities of the cation, anion, and solvent, reveal a more complex picture in which the cation velocity is negative in the laboratory frame. We interpret this result as a demonstration of strong clustering between the cations and the polyanions, such that Li⁺ cations migrate the “wrong way” under application of an electric field. Thus the resulting transference number determined by electrophoretic NMR in these multivalent electrolytes is consistently negative, in contrast to the naïve expectation from the self-diffusion results. This work highlights the importance of quantifying cation, anion and solvent motion under an electric field with electrophoretic NMR, and extends the approach to multivalent systems.