Organic radical polymers were studied from two aspects – electron/ion transfer during the reduction-oxidation reaction and its application as a battery electrode. The former revealed fundamental reaction mechanisms that govern electrochemical behaviors of the organic radical polymers, while the latter tackled practical issues of electrode dissolution using a synthetic approach.
The doping mechanism of nitroxide radical polymer PTMA was quantitatively investigated using quartz crystal microbalance with dissipation monitoring (EQCM-D) during electrochemical processes. Results showed that two doping mechanisms exist – doping by lithium expulsion and anion uptake. The relative dominance of one over the other was controlled by anion type, electrolyte concentration, and timescale. These results could apply in any scenario in which electrolyte is in contact with a non-conjugated redox-active polymer and present a means of quantifying doping effects.
A one-step post-synthetic, carbon-compatible crosslinking method was developed to effectively crosslink the PTMA electrode and prevent its dissolution. The highest electrode capacity of 104 mAh g-1 (vs. a theoretical capacity of 111 mAh g-1) was achieved by introducing 1mol% of the crosslinker, whereas the highest capacity retention (99.6%) was obtained with 3mol% crosslinker. Both lithium expulsion and anion uptake were observed in doping, and the dominance was related to crosslinking density (i.e. free volume) in the electrode. This study indicated the importance of forming a network using a minimum amount of crosslinker, persevering radical content during crosslinking, and allowing enough free volume for electrolyte penetration.
The electron and ion transfer mechanism of conjugated radical polymers (CRPs) with intentionally varied radical loadings (0, 25 or 100%) was studied to understand their inferior capacity compared to their non-conjugated partners. Results showed that the electron transfer shifted from delocalized electron transfer to localized electron hopping under higher radical loading. The extent of internal charge transfer between the conjugated backbone and the pendant radical was dominated by the radical loading. Doping occurred by exchanging one anion and one solvent molecule for every electron transferred in the CRP with 100% radical loading. For future design, the trade-off between radical loading and electronic conductivity need to be balanced
