Synthesis of Fused Biimidazole-Containing Polymers and Their Application as High-Potential Organic Cathode Materials for Lithium-Ion Batteries

Abstract

The development of sustainable cathode-active materials for lithium-ion batteries requires organic redox systems that combine high reaction potential, sufficient capacity, and cycling stability. High-potential n-type redox-active motifs are particularly attractive because smaller-sized cation insertion generally enables smoother ion diffusion than in p-type systems; however, such materials remain limited. In this study, we report the synthesis of a fused biimidazole-containing polymer, poly(biimidazo[4,5-b]-1,4-benzoquinone) (PBIBQ), and investigate its applicability as a high-potential organic cathode material. PBIBQ was synthesized from tetraamino-1,4-benzoquinone and oxalic acid, and its structure was characterized by solid-state NMR and X-ray photoelectron spectroscopy. Electrochemical measurements revealed that PBIBQ exhibits reversible redox behavior with an average discharge potential of 3.25 V vs Li/Li+ and a specific capacity of 133 mAh g–1, demonstrating one of the highest energy-density levels among nonbenzoquinone/hydroquinone-type organic cathodes. Ex situ XPS analyses and density functional theory calculations support that the biimidazole/tetraazafulvalene-type unit functions as the redox-active center and suggest that Li+ insertion between nitrogen atoms is the dominant charge-storage process. However, PBIBQ suffered a gradual capacity decrease during multicycling due to dissolution of the active material and/or its reduced derivatives. To address this issue, a cross-linked derivative, NW-PBIBQ, was synthesized using a tetrafunctional carboxylic acid linker, and the composite of NW-PBIBQ and multiwalled carbon nanotubes was further prepared to improve charge transport. These approaches successfully suppressed dissolution and lowered charge-transfer resistance, improving rate capability and cycle stability. Overall, these results demonstrate that a fused biimidazole-based polymer is a promising molecular motif to realize high-voltage n-type organic cathode.