Design and Regulation of Anthraquinone’s Electrochemical Properties in Aqueous Zinc-Ion Batteries via Benzothiadiazole and Its Dinitro Derivatives

Abstract

Organic cathode materials are widely considered as highly promising for aqueous zinc-ion batteries (AZIBs) due to their tunable properties, low cost, and ease of processing and synthesis. Benzothiadiazoles have demonstrated significant potential as organic electrode materials in AZIBs, owing to their strong electron-accepting capabilities and the presence of multiple reversible redox sites in anthraquinone. In this study, we designed a polymer, poly(2-methyl-6-(7-methyl-5,6-dinitrobenzo[c][1,2,5]thiadiazol-4-yl)anthracene-9,10-dione) (PBDQ), with multielectron transfer capability through a copolymerization approach. Additionally, we synthesized another polymer, poly2,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)anthracene-9,10-dione(PBDQ-N), by introducing two electron-withdrawing nitro groups on the aromatic ring of benzothiadiazole. The introduction of nitro groups, with their unique electronic properties, enhances electron delocalization and increases the number of electrochemically active sites, thereby promoting faster zinc-ion insertion/extraction reactions. Experimental results show that both PBDQ and PBDQ-N exhibit excellent electrochemical properties due to the abundance of active sites and extended π-conjugation. Among them, PBDQ-N demonstrates outstanding performance, including an ultrahigh specific capacity of 446.2 mAh g^–1 at 0.1 A g^–1 and excellent cycle life exceeding 20,000 cycles at 10 A g^–1. Moreover, the lower lowest-unoccupied molecular orbital (LUMO) energy level and improved conductivity of PBDQ-N provide a fast electron transfer rate, resulting in a higher Zn²⁺ diffusion coefficient (3.47 × 10^–11–2.6 × 10^–8 cm² s^–1) and exceptional rate performance (234.6 mAh g^–1 at 10 A g^–1). Theoretical calculations and ex situ characterizations confirm that C═O, C═N, and N═O groups all participate as active sites in Zn²⁺ storage. This work highlights how molecular design and the introduction of functional groups, such as nitro groups, can effectively regulate the electrochemical properties of organic polymers in AZIBs. It also demonstrates the impact of these strategies on the electrochemical performances of these materials when they are used as cathodes in aqueous zinc-ion batteries.