Homogeneous Chlorine-Based Redox Reaction for High Energy Density Aqueous Flow Batteries

The chlorine-based redox couple (Cl–/Cl2) offers significant advantages, including low cost, high redox potential, and solubility, making it a promising candidate for the design of aqueous batteries. However, its application is hindered by the volatilization of chlorine gas (Cl2), which not only reduces the cycle life of the battery but also poses a risk of environmental pollution. To address this challenge, we introduce sulfamide (SF) into the electrolyte as a Cl2 fixer. The produced Cl2 via electrochemical oxidation from Cl– reacts with SF to form soluble N-chlorosulfamide (Cl-SF, Cl+), effectively mitigating Cl2 volatilization. Furthermore, Cl-SF exhibits exceptional electrochemical reversibility with the two-electron transfer process (Cl–/Cl+), significantly enhancing the specific capacity of the redox couple. Experimental results show that after charging, the concentration of Cl2 in the electrolyte remains below 0.2 mM, and the Cl2 concentration in the gas phase above the electrolyte is maintained less than 6 ppm. Batteries assembled with the designed catholyte exhibit stable operation for over 380 cycles, with a Coulombic efficiency of 98% and a maximum discharge capacity of 97.5 Ah L–1. This study presents a viable solution for the safe and sustainable large-scale deployment of chlorine-based flow batteries.