Real-Time Insight into the Doping Mechanism of Redox-Active Organic Radical Polymers
Authors: Wang Shaoyang , Li Fei , Easley Alexandra Danielle , Lutkenhaus Jodie L. 
Published: February 2020

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 cyclic voltammetry. 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. This work is reported in Wang, S., Li, F., Easley, A.D., & Lutkenhaus, J.L., Nature Materials 18 (1), 69-75 (2019).

Here, we quantify the real-time mass transfer of anions and solvent during the reduction and oxidation of PTMA, which allows for the first-ever separation of lithium expulsion and anion uptake in an organic radical polymer. This is accomplished using in situ electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) during cyclic voltammetry, a method that yields the instantaneous electrode mass change and shear modulus via viscoelastic modeling. We present an analysis method for PTMA by which the mass exchanged per electron transferred is quantified, and, in some cases, the number of solvent molecules transferred can also be calculated.

Our results show that two doing modes dominate in oxidation: doping by lithium-ion expulsion and doping by anion uptake. The effect of dopant anion type (i.e. CF3SO3
-, ClO4
-, and BF4
-), the degree of solvent participation, and the effect on charge storage behavior with mass transport is correlated to polymer-dopant and dopant-solvent interactions. Understanding the detailed mass and charge transport process is a leap forward in describing the redox reaction doping mechanism in organic radical polymers and adds a new facet of discussion in the nature of electron transport, as the two are intimately coupled. In turn, this may impact the future design of organic radical polymers for any application in which the polymer is in contact with electrolyte (organic batteries, electrochromics, sensors).