Mixed electron-ion-water transfer in macromolecular radicals for metal-free aqueous batteries

Metal-free aqueous batteries offer environmentally friendly energy storage without depleting the global reserves of strategic elements. These batteries consist of solid, redox-active polymeric anodes and cathodes with a water-based electrolyte. However, the performance of available active polymers remains inferior, and design principles to guide future development are lacking. Here, we report on the coupled mass, electron, and water transfer behavior for a series of non-conjugated nitroxide radical polymers in a water-based electrolyte-containing organic salt for guiding future design. In situ monitoring of the charge-discharge process reveals the important role of polymer-water interactions. Specifically, improved polymer-water interactions manifest in accelerated kinetics, which promote capacity retention at higher discharge rates. The most hydrophilic polymer, poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl acrylamide) (PTAm), yielded a capacity of 115 mAh/g, or 97% of its theoretical value, at 0.05 mA/cm². At 5 mA/cm², the PTAm capacity remained relatively high at 50 mAh/g, whereas comparable polymer capacities were much lower.