(669d) Real-Time Insight into the Doping Mechanism of Redox-Active Organic Radical Polymers

In recent years, organic radical polymers have received great attention as active materials for fast-charging battery electrodes. Organic radical polymers are electrochemically active owing to the reversible reduction-oxidation (redox) reaction of pendant radical groups and offer a vast synthetic landscape for customization. Interest in these polymers as battery electrodes has grown due to its high theoretical capacities, fast electron transfer kinetics, and long cyclability. One commonly studied stable nitroxide radical polymer is poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA). This electroactive material consists of stable nitroxyl radical groups attached to a polymethylacrylate backbone. Upon oxidation or charging, the neutral nitroxide radical transfers an electron to the current collector, resulting in an oxoammonium cation. Simultaneously, anions dope the polymer cathode to maintain charge neutrality. Upon reduction or discharging, the polymer is de-doped and the neutral nitroxide radical is retrieved. Electronic charge transfer within the organic radical polymer occurs by an electron hopping mechanism with Brownian motion of the redox centers.

Although there is much more understood about electron transfer in organic radical polymers, there is significantly less understood regarding mass transfer and the doping mechanism, which is equally important for understanding the overall redox mechanism. Here, we specifically examine ion transport in PTMA cathodes for nonaqueous batteries. Using in situ electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D), we quantitatively observe the ion transport (or doping) process in organic radical polymers during the redox process for the first time. EQCM-D monitors changes in frequency and dissipation of a PTMA-coated quartz crystal during controlled electrochemical interrogation (cyclic voltammetry and chronoamperometry). The change of mass and shear viscosity can be further obtained from viscoelastic modeling of the raw data, leading to a quantitative view of mass transport associated with the doping process.

Our results show that there are two different doping mechanisms. Specifically, ions can dope internally vs externally depending on whether the ion is transporting from within the electrode or externally from the bulk electrolyte. The nature and sequence of doping is controlled by anion type and concentration. These results indicate the importance of electrolyte design for not only organic radical batteries, but also redox flow batteries to maintain rapid kinetics.