Synthesis and Charge Transport Properties of Redox-Active Nitroxide Polyethers with Large Site Density

To maximize the theoretical redox capacity of polymers containing cyclic nitroxides as redox-active pendant groups for high-density charge storage application, a compact five-membered ring with the smallest equivalent weight among the robust cyclic nitroxides was directly bound to a poly(ethylene oxide) chain. 2,2,5,5-Tetramethyl-3-oxiranyl-3-pyrrolin-1-oxyl was synthesized and polymerized via anionic coordinated ring-opening polymerization utilizing diethyl zinc/H2O as an initiator. The unpaired electron in the monomer survived during the polymerization, giving rise to a high density redox polymer with a weight-specific theoretical capacity of 147 mA h/g. Cyclic voltammetry of the polymer layer confined at the surface of an electrode revealed a large redox capacity comparable to the theoretical capacity, which was ascribed to the efficient swelling and yet insoluble properties of the polyether in electrolyte solutions by virtue of the high molecular weight of >105 and adhesive properties allowing immobilization of the layer on the electrode surface. The redox capacity also indicated that the ionophoric polyether matrix accommodated electrolyte anions through the polymer/electrolyte interface to neutralize positive charges produced by the oxidation of the neutral radicals at the polymer/electrode interface. The diffusion coefficient for the redox gradient-driven charge hopping process corresponded to a large second-order rate constant in the order of 107 M−1 s−1, which suggested an efficient electron self-exchange reaction throughout the polymer layer due to the large redox site population and hence to the small intersite distance. Test cells fabricated with a polymer/carbon fiber composite layer on an aluminum current collector as the cathode and a Li anode sandwiching an electrolyte layer were capable of charging and discharging as a secondary battery with an output voltage near 3.7 V and were durable for more than 103 charging−discharging cycles without substantial degradation.