Organic radical batteries (ORBs), in which the organic radical polymer is used as the cathode and/or anode, have been considered a promising alternative to conventional lithium-ion batteries, as the organic cathodes allow rapid charge transport and high cycling stability. One commonly studied stable nitroxide radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), is capable of a two-electron transfer process between its n-doped and p-doped states. The reported specific capacity of PTMA cathodes is between 77 to 220 mAh/g, depending on the charge/discharge conditions. Most previous research on PTMA is aimed at improving the electrode capacity and cyclability either by adding highly conductive materials or by optimizing the radical polymer synthesis. However, the charge storage process and the electrode/electrolyte interface in such systems are not understood on the molecular level. Here, we present the application of <i>in situ </i>electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) towards understanding the charge storage process in PTMA cathodes. 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. To further understand these systems, we investigate conjugated radical polymers (see Figure), or polymers that bear a conjugated backbone and a pendant nitroxide radical group. These exhibit multi-electron transfer and the conjugated backbone potentially enhances electronic conductivity. With a firmer understanding of how charge is transferred in organic radical polymers, a clearer path forward the molecular design of these macromolecules is formed.

Unraveling charge storage in organic radical polymers for battery electrodes

253rd National Meeting (2017)

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