Alternant P-Type/N-Type Conjugated Bipolar Copolymer Cathodes for Advanced Aqueous Zinc-Ion Batteries

To synergistically combine the features of high specific capacity of n-type organic materials with the high voltage of p-type organic materials, bipolar phenolthiazine-based cathodes with p-type/n-type structures are gaining increased attention for aqueous zinc-ion batteries (AZIBs). Nevertheless, their small molecular structure often leads to dissolution during cycling. To address the challenge, a bipolar poly(phenothiazine iodide-3,3'-diaminobenzidine), abbreviated as P(PTD-DAB), was synthesized as a cathode material. Through comprehensive spectroscopic analyses, we elucidated a cooperative p-type/n-type dual-ion storage mechanism. Specifically, the N atom in the phenothiazine core stores Zn2+ or H+ cations via n-type reactions, while the N atoms connecting phenothiazinium tetraiodide and 3,3'-diaminobenzidine monomers, along with S atoms, act as p-type active sites for storing OTF- anions. The material delivers an average operating voltage of 1 V and achieves a high specific capacity of 211.8 mAh g-1 at a low current density of 0.1 A g-1. Additionally, it shows 90% capacity over 3600 cycles at a high current density of 3 A g-1 while sustaining robust stability at a high mass loading of active cathode (10 mg cm-2). Furthermore, flexible belt-shaped cells maintain 88% capacity retention over 550 cycles at 3 A g-1 even when subjected to repeated bending conditions. This work represents a great advancement in developing bipolar polymers for high-performance AZIBs and offers valuable insights into their design principle and functionality.