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

The chlorine-based redox couple (Cl – /Cl 2 ) 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 (Cl 2 ), 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 Cl 2 fixer. The produced Cl 2 via electrochemical oxidation from Cl – reacts with SF to form soluble N -chlorosulfamide (Cl-SF, Cl + ), effectively mitigating Cl 2 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 Cl 2 in the electrolyte remains below 0.2 mM, and the Cl 2 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.