Charge Storage Mechanism of Redox Polymers for Energy Storage

Organic redox polymers have recently attracted significant interest for energy storage applications due to their high capacity, resource abundance, and environmental friendliness. While most studies focused on electrochemical performance, their redox mechanisms are not well-studied, especially for n-type polymers. The objective of this dissertation is to investigate the charge storage mechanism of three n-type polymers by electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D). The solvation structures of charge carriers and swelling behaviors of polymers were investigated, and their implications for battery performance were discussed. The charge storage mechanism for a π-conjugated n-type polymer P(NDI2OD-T2) was investigated in non-aqueous electrolyte solutions. The solvated ion clusters served as charge carriers for polymer redox reactions, whose structure varied with solvents. The rate performance of the polymer electrodes was determined by their degree of initial swelling in electrolytes. A non-conjugated polyimide-based polymer was investigated in both aqueous and non-aqueous electrolytes. The charge carriers are hydrated cations in aqueous electrolytes, whose hydration numbers vary with the electrolyte concentration. In non-aqueous electrolytes, charge carriers are less solvated by organic solvents. The reversible capacity of the polymer electrodes is determined by the degree of polymer swelling, while their cycle stability is affected by the ion cluster size. The charge storage mechanism of poly(benzoquinonyl sulfide) was investigated for zinc-ion battery applications. Non-hydrated zinc ions are found to be the cation species associated with the quinone-related redox reaction, while the counter anions also participate in the reaction probably due to the unique p-dopable linker present in the polymer. The volume change of the polymer electrode is the root cause of the capacity decay, which is related to the solvation of anions. Finally, zinc metal deposition process was studied in the presence of separators. Porous separators such as commercial glass fiber and polypropylene membranes promote directional pore-filling by deposited zinc. The thus-formed porous zinc forms ‘dead zinc’ upon stripping. In contrast, a nonporous poly(dimethylsiloxane-ethylene glycol polymeric) network separator confines zinc deposition beneath the separator and prevents the formation of dead zinc.