Phenothiazine-based copolymer with redox functional backbones for organic battery cathode materials

Abstract Organic battery materials are receiving increasing attention owing to their elemental abundance, environmental sustainability, and structural diversity. Challenges including the solubility in electrolytes, moderate redox potentials, and inactive molecular fragments prevent organic materials from being ideal cathodes for practical implementation. Although polymerization, salification and alike solutions are effective in lowering the solubility in electrolytes, they unavoidably bring additional inactive fragments into the molecular structures, resulting in the sacrifice of theoretical galvanometric capacity compared with the pristine molecule. In this study, we use two redox-active fragments to construct an insoluble copolymer as a cathode material for organic batteries. The copolymer avoids the use of unnecessary molecular weight while maintaining high specific capacity and cycling stability. We find that a twisted geometry between the two redox-active fragments leads to a cross-conjugation effect that further consolidates the low stability of the individual fragment and enhances the flexibility of copolymer chains by forming mesopores that accelerate ion diffusion. The copolymer shows a high capacity of 142.5 mAh/g with energy / power density of 577 Wh/kg / 1,685 W/kg and a decent capacity retention of 87% after 500 cycles. Our strategy demonstrates the feasibility of designing organic battery materials that are qualified for taking solubility, capacity, and stability into consideration.