An organic imidazolium derivative additive inducing fast and highly reversible redox reactions in zinc-bromine flow batteries

In zinc-bromine redox flow batteries (ZBBs), the weak molecular structure and stability of bromine-complexing agent (BCA) can sometime negatively affect battery's performance. To address this issue, this paper introduces a 1,2-dimethyl-3-ethylimidazolium bromide (DMEIm∙Br, C 7 H 13 BrN 2 ), comprising planar molecular structure with strong molecular-polarizability and low steric hindrance. The effectiveness of the DMEIm∙Br is compared and verified with those of two popular BCAs through various electrochemical experiments including full-cell tests for 200 cycles. Experimental results show that the DMEIm∙Br significantly contributes to apparently enhancing reaction kinetics and reversibility of Zn 2+ /Zn (s) and Br − /Br 2 redox couples by inducing highly reversible zinc-plating/stripping (by strong electrostatic shielding effect) and bromine-capture/release (along with strong bromine-binding strength) in anolyte and catholyte solutions, respectively. The superior chemical and electrochemical properties are clearly demonstrated by the fact that the DMEIm∙Br-supported solution in ZBBs exhibits 5.53 (24.19) and 7.29 (16.99) % higher current and voltaic efficiencies than the pristine solution at the temperature of 25 (60) °C, respectively. It also exhibits remarkably improved discharge-capacity retention of averagely 99.17% for 200 cycles along with a slight discharge-capacity loss of only 1.46% vs. 1st cycle at 200th cycle, even at the high temperature of 60 °C. • Imidazolium derivative is a bromine salt for high-efficiency Zn–Br RFB, C 7 H 13 BrN 2 . • The additive causes fast and highly reversible reactions of Zn 2+ /Zn and Br − /Br 2 /Q∙Br 3 . • Zn (Br 2 ) diffusion coefficient is 4.9 (15.1) times higher than the pristine one. • Charge-transfer and Warburg-diffusion resistances of 2.4 and 3.1 times are lower. • Discharge capacity retention is averagely 99.7% for 200 cycles even at 60 °C.