Radical polymers are gaining increasing interest as active materials for batteries owing to their great potential for fast charging, and the fact that no critical elements are involved. PTMA [poly(2,2,6,6- tetramethylpiperidinyloxy-4-yl methacrylate)] is one of the best-investigated radical polymers for lithium batteries, providing high redox activity, fast kinetics and great reversibility during cycling.¹ However, the electrochemically active nitroxyl radicals are embedded in a six-membered ring with four methyl groups attached to stabilize the radical, which limits its theoretical capacity due to the rather high molecular weight of the overall functional moiety. To increase the theoretical capacity, a decrease of the “unnecessary” mass is therefore desirable. Smaller redox-active molecules, however, commonly show a high solubility in liquid electrolytes, which prevents stable long-term cycling.

Herein, we present a new redox-active polymer based on low molecular weight amides, for which the solubility issue has been overcome by proper design. We show that the electrolyte composition (salt concentration and the choice of solvents as well as their ratio) plays a decisive role for (i) the redox behavior and kinetics (broad vs. sharp redox peaks and the redox peak separation), (ii) the eventual redox potential, which is greater than for PTMA, and (iii) the long-term stable cycling at high dis-/charge rates. As a result, the rational optimization of the electrolyte composition enables achieving excellent electrochemical performance, rendering this new class of polymers highly promising for high-performance rechargeable batteries. Finally, we unveil the underlying phenomena by means of complementary ex situ and in situ characterization techniques to provide an in-depth understanding and, thus, allow for the tailored design of suitable electrolyte compositions for this new class of redox-active polymers and, potentially, polymer electrode materials in general.

(1) Kim, J.-K.; Kim, Y.; Park, S.; Ko, H.; Kim, Y. Encapsulation of Organic Active Materials in Carbon Nanotubes for Application to High-Electrochemical-Performance Sodium Batteries. Energy Environ. Sci. 2016, 9 (4), 1264–1269. https://doi.org/10.1039/C5EE02806J.