π-Conjugation-extended benzoimidazoisoindolone-based n-type ladder polymers for highly air-robust aqueous sodium-ion electrochromic energy storage

Electro-active conjugated polymers have shown prominent advantages in the field of electrochromic energy storage materials (EESMs) in recent years. However, achieving highly air-stable n-type conjugated polymer EESMs in aqueous electrolytes remains an enormous challenge. The present study reports the first investigation of benzoimidazoisoindolone-based n-type fully conjugated ladder polymers (BBL-EC and BAL-EC) for EESMs, featuring high electron affinity, efficient and reversible electrochemical redox activity, high electron/ion conductivity, and an extended conjugated backbone structure. These polymers demonstrate enhanced electrochromic performance using aqueous sodium ions as an electrolyte under air atmosphere, alongside high optical contrast and exceptional cycling stability. The optical contrast of BAL-EC was 53.8% at 740 nm, which remained unchanged after 1200 cycles owing to the extended π-conjugated structure. Furthermore, BAL-EC exhibited a high specific capacitance of 257.22 mAh g^-1. The electrochromic energy storage mechanism of benzoimidazoisoindolone-based conjugated polymers involves sequential electron transfer with preferential carbonyl reduction, followed by imidazole ring reduction, which enables synergistic regulation of color switching and energy storage. Benzoimidazoisoindolone-based n-type fully conjugated ladder polymers demonstrate promising potential for highly air-stable aqueous sodium-ion electrochromic energy storage systems.